# The NuSTAR Hard X-ray Survey of the Norma Arm Region

Francesca M. Fornasini, John A. Tomsick, Jaesub Hong, Eric V. Gotthelf, Franz Bauer, Farid Rahoui, Daniel Stern, Arash Bodaghee, Jeng-Lun Chiu, Maïca Clavel, et al.

► To cite this version:

Francesca M. Fornasini, John A. Tomsick, Jaesub Hong, Eric V. Gotthelf, Franz Bauer, et al. The NuSTAR Hard X-ray Survey of the Norma Arm Region. The Astrophysical Journal Supplement, 2017, 229 (2), pp.33. 10.3847/1538-4365/aa61fc. hal-01554857

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# The NuSTAR Hard X-Ray Survey of the Norma Arm Region

Francesca M. Fornasini<fnref n="1" />,<fnref n="2" />, John A. Tomsick<fnref n="2" />, JaeSub Hong<fnref n="3" />, Eric V. Gotthelf<fnref n="4" />, Franz Bauer<fnref n="5" />,<fnref n="6" />,<fnref n="7" />, Farid Rahoui<fnref n="8" />, Daniel Stern<fnref n="9" />, Arash Bodaghee<fnref n="10" />, Jeng-Lun Chiu<fnref n="2" />, Maïca Clavel<fnref n="2" />, Jesús Corral-Santana<fnref n="5" />, Charles J. Hailey<fnref n="4" />, Roman A. Krivonos<fnref n="2" />,<fnref n="11" />, Kaya Mori<fnref n="4" />, David M. Alexander<fnref n="12" />, Didier Barret<fnref n="13" />, Steven E. Boggs<fnref n="2" />, Finn E. Christensen<fnref n="14" />, William W. Craig<fnref n="2" />,<fnref n="15" />, Karl Forster<fnref n="16" />, Paolo Giommi<fnref n="17" />, Brian W. Grefenstette<fnref n="16" />, Fiona A. Harrison<fnref n="16" />, Allan Hornstrup<fnref n="14" />, Takao Kitaguchi<fnref n="18" />, J. E. Koglin<fnref n="19" />, Kristin K. Madsen<fnref n="16" />, Peter H. Mao<fnref n="16" />, Hiromasa Miyasaka<fnref n="16" />, Matteo Perri<fnref n="17" />,<fnref n="20" />, Michael J. Pivovaroff<fnref n="15" />, Simonetta Puccetti<fnref n="17" />,<fnref n="20" />, Vikram Rana<fnref n="16" />, Niels J. Westergaard<fnref n="14" />, and William W. Zhang<fnref n="21" />

Received 2016 October 12; revised 2017 February 6; accepted 2017 February 19; published 2017 April 6

## Abstract

We present a catalog of hard X-ray sources in a square-degree region surveyed by the Nuclear Spectroscopic Telescope Array (NuSTAR) in the direction of the Norma spiral arm. This survey has a total exposure time of 1.7 Ms, and the typical and maximum exposure depths are 50 ks and 1 Ms, respectively. In the area of deepest coverage, sensitivity limits of $5 \times 10^{-14}$ and $4 \times 10^{-14}$ erg s$^{-1}$ cm$^{-2}$ in the 3–10 and 10–20 keV bands, respectively, are reached. Twenty-eight sources are firmly detected, and 10 are detected with low significance; 8 of the 38 sources are expected to be active galactic nuclei. The three brightest sources were previously identified as a low-mass X-ray binary, high-mass X-ray binary, and pulsar wind nebula. Based on their X-ray properties and multiwavelength counterparts, we identify the likely nature of the other sources as two colliding wind binaries, three pulsar wind nebulae, a black hole binary, and a plurality of cataclysmic variables (CVs). The CV candidates in the Norma region have plasma temperatures of $\approx 10–20$ keV, consistent with the Galactic ridge X-ray emission spectrum but lower than the temperatures of CVs near the Galactic center. This temperature difference may indicate that the Norma region has a lower fraction of intermediate polars relative to other types of CVs compared to the Galactic center. The NuSTAR logN–logS distribution in the 10–20 keV band is consistent with the distribution measured by Chandra at 2–10 keV if the average source spectrum is assumed to be a thermal model with $kT \approx 15$ keV, as observed for the CV candidates.

**Key words:** binaries: general – Galaxy: disk – novae, cataclysmic variables – X-rays: binaries – X-rays: stars

## 1. Introduction

Hard X-ray observations of the Galaxy can be used to identify compact stellar remnants—white dwarfs (WDs), neutron stars(NSs), and black holes (BHs)—and probe stellar evolution in different environments. While a number of sensitive surveys of Galactic regions (e.g., Muno et al. 2009; Townsley et al. 2011; Fornasini et al. 2014) have been performed by the Chandra X-ray Observatory, its soft X-ray band (0.5–10 keV) is often insufficient for differentiating between different types of compact objects. The Nuclear Spectroscopic Telescope Array (NuSTAR; Harrison et al. 2013),

with its unprecedented sensitivity and angular resolution at hard X-ray energies above 10 keV, provides a unique opportunity to study the X-ray populations in the Galaxy. During the first two years of its science mission, NuSTAR performed surveys of the Galactic center (GC) and the Norma spiral arm in order to compare the X-ray populations in these regions of the Galaxy, which differ with regard to their star formation history and stellar density. The NuSTAR sources found among the old, high-density GC stellar population are described in Hong et al. (2016), and, in this paper, we present the results from the NuSTAR Norma arm survey.

<footnote n="1">¹ Astronomy Department, University of California, 601 Campbell Hall, Berkeley, CA 94720, USA; f.fornasini@berkeley.edu</footnote>

<footnote n="2">² Space Sciences Laboratory, 7 Gauss Way, University of California, Berkeley, CA 94720, USA</footnote>

<footnote n="3">³ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA</footnote>

<footnote n="4">⁴ Columbia Astrophysics Laboratory, Columbia University, New York, NY 10027, USA</footnote>

<footnote n="5">⁵ Instituto de Astrofísica and Centro de Astroingeniería, Facultad de Física, Pontificia Universidad Católica de Chile, Casilla 306, Santiago 22, Chile</footnote>

<footnote n="6">⁶ Millennium Institute of Astrophysics (MAS), Nuncio Monseñor Sótero Sanz 100, Providencia, Santiago, Chile</footnote>

<footnote n="7">⁷ Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, CO 80301, USA</footnote>

<footnote n="8">⁸ European Southern Observatory, K. Schwarzschild-Strasse 2, D-85748 Garching bei München, Germany</footnote>

<footnote n="9">⁹ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA</footnote>

<footnote n="10">¹⁰ Georgia College, 231 W. Hancock Street, Milledgeville, GA 31061, USA</footnote>

<footnote n="11">¹¹ Space Research Institute, Russian Academy of Sciences, Profsoyuznaya 84/32, 117997 Moscow, Russia</footnote>

<footnote n="12">¹² Centre for Extragalactic Astronomy, Department of Physics, Durham University, Durham, DH1 3LE, UK</footnote>

<footnote n="13">¹³ Université de Toulouse; UPS-OMP; IRAP; Toulouse, France & CNRS; Institut de Recherche en Astrophysique et Planétologie; 9 Av. colonel Roche, BP 44346, F-31028 Toulouse cedex 4, France</footnote>

<footnote n="14">¹⁴ DTU Space—National Space Institute, Technical University of Denmark, Elektrovej 327, DK-2800 Lyngby, Denmark</footnote>

<footnote n="15">¹⁵ Lawrence Livermore National Laboratory, Livermore, CA 94550, USA</footnote>

<footnote n="16">¹⁶ Cahill Center for Astrophysics, 1216 E. California Boulevard, California Institute of Technology, Pasadena, CA 91125, USA</footnote>

<footnote n="17">¹⁷ ASI Science Data Center (ASDC), via del Politecnico, I-00133 Rome, Italy</footnote>

<footnote n="18">¹⁸ Department of Physical Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan</footnote>

<footnote n="19">¹⁹ Kavli Institute for Particle Astrophysics and Cosmology, SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA</footnote>

<footnote n="20">²⁰ INAF—Astronomico di Roma, via di Frascati 33, I-00040 Monteporzio, Italy</footnote>

<footnote n="21">²¹ NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA</footnote>
In 2011, the Norma Arm Region Chandra Survey (NARCS) observed a 2° × 0.8 region in the direction of the Norma spiral arm (Fornasini et al. 2014, hereafter F14). The near side of the Norma arm is located at a distance of about 4 kpc, while the far Norma arm is at a distance of 10–11 kpc (Vallée 2008). The Norma region was targeted because its stellar populations are younger than those in the GC but older than those in the young Carina and Orion star-forming regions observed by Chandra (F14 and references therein). An additional goal of this survey was to identify low-luminosity high-mass X-ray binaries (HMXBs) falling below the sensitivity limits of previous surveys in order to constrain the faint end of the HMXB luminosity function; the evolutionary state of the Norma arm and the large number of OB associations along this line of sight (Bodaghee et al. 2012) make it an ideal place to search for HMXBs.

About 300 of the 1130 Chandra sources detected at $≥3σ$ confidence in the Norma region were found to be spectrally hard in the 0.5–10 keV band, with median energies >3 keV. The majority of these sources are expected to be magnetic cataclysmic variables (CVs) and active galactic nuclei (AGNs), although some could also be HMXBs, low-mass X-ray binaries (LMXBs), or colliding wind binaries (CWBs). Distinguishing between these types of sources is not possible based on Chandra data alone, especially since most of the Norma X-ray sources have low photon statistics.

Since Chandra's resolution enables the identification of unique optical/infrared counterparts, spectral identification of the counterparts has helped shed light on the physical nature of some of the Norma X-ray sources (Rahoui et al. 2014). However, not even this information is necessarily sufficient; for example, HMXBs and CWBs both have massive stellar counterparts in the optical/infrared, and it can be difficult to differentiate them spectrally in the Chandra band with <100 photon counts, as is the case for most NARCS sources. NuSTAR observations, due to their superior sensitivity above 10 keV in the energy range of iron $Kα$ and $Kβ$, provide critical information for differentiating hard X-ray sources. For example, CWBs can be distinguished from HMXBs because they have thermal spectra that fall off steeply above 10 keV and strong 6.7 keV Fe emission (Mikles et al. 2006 and references therein), and magnetic CVs can be distinguished from nonmagnetic CVs by their harder spectra, lower equivalent widths of the 6.7 keV line, and higher line ratios of 7.0/6.7 keV Fe emission (e.g., Xu et al. 2016).

The first set of observations of the NuSTAR Norma arm survey were carried out in 2013 February and improved the identification of three NARCS sources (Bodaghee et al. 2014, hereafter B14), discovered one transient (Tomsick et al. 2014, hereafter T14), and permitted the study of the disk wind of the LMXB 4U 1630–472 (King et al. 2014). In this paper, we present a catalog of all point sources detected in the NuSTAR Norma arm survey. The NuSTAR observations and basic data processing are described in Sections 2 and 3. Descriptions of our source detection technique, aperture photometry, and spectral analysis are found in Sections 4, 5, and 5.7, respectively. In Section 6, we discuss the physical nature of the NuSTAR detected sources and compare the Norma X-ray populations to those seen in the GC region.

## 2. Observations

### 2.1. NuSTAR

NuSTAR observations of the Norma arm region began in 2013 February and were completed in 2015 June. During this

period, NuSTAR performed 61 observations in the Norma region, shown in Figure 1. Every pointing consists of data from two co-aligned focal plane modules (FPMs), A and B, each of which has a field of view (FOV) of 13' × 13'.

The NuSTAR observations were planned to minimize contamination from stray light and ghost rays. Stray light is the result of zero-bounce photons reaching the detector from bright sources within a few degrees of the FOV, while ghost rays are single-bounce photons from bright sources within about 1° of the FOV. The pattern of stray light contamination is well understood and can be carefully predicted,<fnref n="22" /> while the patterns of ghost rays are more challenging to model (Koglin et al. 2011; Harrison et al. 2013; Wik et al. 2014; Madsen et al. 2015; Mori et al. 2015).

Therefore, rather than observing the whole region surveyed by Chandra, we performed simulations of stray light contamination and focused our observations on three areas of the sky that would be least affected by stray light. Even in these “cleaner” areas, at least one of the FPMs was often affected by stray light, so exposure times for more contaminated observations were lengthened to compensate for the fact that we would not be able to combine data from both modules. Seven additional pointings were specifically made at the locations of some of the brightest NARCS sources found to be hard in the Chandra band and for which optical or infrared spectra have been obtained (Rahoui et al. 2014; J. Corral-Santana et al. 2017, in preparation). Unfortunately, despite this adopted strategy, the first mini-survey of the Norma region was highly contaminated by ghost rays because a BH binary in the region, 4U 1630–472, serendipitously went into outburst while the NuSTAR observations were taking place (B14). Having learned about the spatial extent of ghost ray contamination, later observations in the proximity of 4U 1630–472 were timed to occur only when it was in quiescence.

Finally, in addition to the observations dedicated to the Norma survey as part of either the baseline NuSTAR science program or the NuSTAR legacy program, a series of observations were made to regularly monitor the pulsar associated with HESS J1640–465 (Gotthelf et al. 2014, hereafter G14), a very luminous TeV source that resides within the Norma survey area. Combining all such observations taken prior to 2015 March yields a total exposure of 1 Ms over a 100 arcmin² field, which we call the “deep HESS field.” While the detailed analysis of the pulsar’s braking index is discussed in Archibald et al. (2016), here we present the other NuSTAR sources detected in the deep HESS field.

Table 1 lists all the NuSTAR observations included in our analysis. Although the sources in the first mini-survey (King et al. 2014; B14; T14), HESS J1640–465 (G14), and IGR J16393–4643 (Bodaghee et al. 2016, hereafter B16) have been analyzed separately and in more detail by others, we include these sources in our analysis in order to measure the photometric properties of all sources in a consistent way, allowing us to calculate the number-flux (logN–logS) distribution of the NuSTAR Norma region (NNR) sources.

### 2.2. Chandra

In this study, we make extensive use of information from the NARCS catalog, as well as the soft (<10 keV) X-ray

<footnote n="22">²² Stray light constraints for new observations can be checked with the stray light simulation tool at http://www.srl.caltech.edu/NuSTAR_Public/NuSTAROperationSite/CheckConstraint.php.</footnote>
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Top panel: smoothed 3–40 keV count rate mosaic (units of counts s⁻¹). Bottom panel: 3–40 keV exposure map without vignetting correction (units of s). The mosaics have been cleaned of most contamination from ghost rays and stray light; some residual ghost ray contamination can be seen in the first mini-survey (top panel, upper right), while one wedge of stray light around (ℓ, b) = (338°, 0.08), which is due to GX 340+0, is not removed because a bright source, IGR J16393–4643, is embedded in it.</figcaption></figure>

spectra of some of the NARCS sources. The analysis of these
*Chandra* observations and the details of the spectral
extraction are provided in F14. We also use two other
archival *Chandra* observations that cover part of the area
surveyed by NuSTAR: ObsID 7591 provides an additional
epoch for a transient source (NuSTAR J164116–4632.2,

discussed in Section 5.5), and ObsID 11008 provides spatially
resolved observations of NARCS sources 1278 and 1279
(Rahoui et al. 2014), which are blended in the NARCS and
NuSTAR Norma observations. For reference, we provide
information about all of these relevant archival *Chandra*
observations in Table 2.
<table><caption>**Table 1**
NuSTAR Observations of the Norma Arm Region</caption><thead><tr><th rowspan="2">ObsID<br>(1)</th><th colspan="3">Pointing (J2000)</th><th rowspan="2">Start Time<br>(UT)<br>(5)</th><th rowspan="2">Exposure<br>(ks)<br>(6)</th><th rowspan="2">SL Removal<br>(FPM)<br>(7)</th><th rowspan="2">SL<br>Source<br>(8)</th><th rowspan="2">Other<br>Contamination<br>(9)</th></tr><tr><th>R.A.<br>(2)</th><th>Decl.<br>(3)</th><th>PA<br>(4)</th></tr></thead><tbody><tr><td colspan="9"><strong>Wide Shallow Survey</strong></td></tr><tr><td colspan="9"><strong>First Mini-Survey</strong></td></tr><tr><td>40014001001</td><td>248.4829</td><td>-47.7204</td><td>160.1494</td><td>2013 Feb 24 01:46</td><td>18.4</td><td>...</td><td>...</td><td>Ghost rays from 4Ub in AB</td></tr><tr><td>40014002001</td><td>248.3623</td><td>-47.6444</td><td>160.1471</td><td>2013 Feb 24 11:31</td><td>19.5</td><td>...</td><td>...</td><td>Ghost rays from 4Ub in AB</td></tr><tr><td>40014003001</td><td>248.2407</td><td>-47.5669</td><td>160.1266</td><td>2013 Feb 21 20:31</td><td>20.8</td><td>...</td><td>...</td><td>Ghost rays from 4Ub in AB</td></tr><tr><td>40014004001</td><td>248.5977</td><td>-47.6374</td><td>160.1231</td><td>2013 Feb 22 07:46</td><td>19.5</td><td>...</td><td>...</td><td>Ghost rays from 4Ub in AB</td></tr><tr><td>40014005001</td><td>248.4775</td><td>-47.5622</td><td>160.1304</td><td>2013 Feb 22 17:31</td><td>21.3</td><td>...</td><td>...</td><td>Ghost rays from 4Ub in AB</td></tr><tr><td>40014006001</td><td>248.3529</td><td>-47.4868</td><td>160.1393</td><td>2013 Feb 23 04:46</td><td>18.9</td><td>...</td><td>...</td><td>Ghost rays from 4Ub in AB</td></tr><tr><td>40014007001</td><td>248.7099</td><td>-47.5554</td><td>160.1350</td><td>2013 Feb 23 14:31</td><td>22.7</td><td>...</td><td>...</td><td>Ghost rays from 4Ub in AB</td></tr><tr><td>40014008002</td><td>248.5845</td><td>-47.4826</td><td>160.1196</td><td>2013 Feb 20 23:31</td><td>16.6</td><td>...</td><td>...</td><td>Ghost rays from 4Ub in AB</td></tr><tr><td>40014009001</td><td>248.4670</td><td>-47.4038</td><td>160.1198</td><td>2013 Feb 21 10:46</td><td>14.7</td><td>...</td><td>...</td><td>Ghost rays from 4Ub in AB</td></tr><tr><td colspan="9"><strong>Later Observations</strong></td></tr><tr><td>40014011002</td><td>250.0712</td><td>-46.4909</td><td>280.7063</td><td>2013 Jun 20 00:06</td><td>21.5</td><td>AB</td><td>4Ua</td><td>...</td></tr><tr><td>40014012001</td><td>250.0006</td><td>-46.4546</td><td>280.7266</td><td>2013 Jun 20 14:21</td><td>19.7</td><td>AB</td><td>4Ua</td><td>...</td></tr><tr><td>40014013001</td><td>249.9200</td><td>-46.4004</td><td>281.4251</td><td>2013 Jun 21 03:16</td><td>20.5</td><td>A</td><td>4Ua</td><td>...</td></tr><tr><td>40014014001</td><td>250.2358</td><td>-46.3989</td><td>285.7049</td><td>2013 Jun 21 17:46</td><td>16.4</td><td>...</td><td>...</td><td>1.5 streak in AB</td></tr><tr><td>40014015001</td><td>250.0770</td><td>-46.3706</td><td>285.7091</td><td>2013 Jun 22 08:21</td><td>19.2</td><td>...</td><td>...</td><td>...</td></tr><tr><td>40014016001</td><td>250.2620</td><td>-46.5441</td><td>285.6937</td><td>2013 Jun 22 21:21</td><td>19.6</td><td>AB</td><td>4Ua</td><td>...</td></tr><tr><td>40014017001</td><td>250.1762</td><td>-46.5238</td><td>285.6774</td><td>2013 Jun 23 11:51</td><td>24.2</td><td>AB</td><td>4Ua</td><td>...</td></tr><tr><td>40014018001</td><td>249.9326</td><td>-46.3469</td><td>286.8740</td><td>2013 Jun 24 00:51</td><td>23.8</td><td>...</td><td>...</td><td>...</td></tr><tr><td>40014019001</td><td>250.1520</td><td>-46.3873</td><td>286.8743</td><td>2013 Jun 24 15:21</td><td>25.6</td><td>...</td><td>...</td><td>...</td></tr><tr><td>40014021002</td><td>249.1106</td><td>-47.1553</td><td>168.0928</td><td>2014 Mar 09 21:56</td><td>29.1</td><td>AB</td><td>GX, 4Ua</td><td>...</td></tr><tr><td>40014022001</td><td>249.0348</td><td>-46.9577</td><td>168.0985</td><td>2014 Mar 10 15:31</td><td>28.4</td><td>AB</td><td>GX, 4Ua</td><td>...</td></tr><tr><td>40014023001</td><td>249.2029</td><td>-47.1072</td><td>168.1000</td><td>2014 Mar 11 07:41</td><td>28.8</td><td>AB</td><td>GX, 4Ua</td><td>...</td></tr><tr><td>40014024001</td><td>248.8388</td><td>-46.9903</td><td>168.1169</td><td>2014 Mar 11 23:46</td><td>28.1</td><td>AB</td><td>GX, 4Ua</td><td>...</td></tr><tr><td>40014025001</td><td>248.8796</td><td>-47.0734</td><td>168.1144</td><td>2014 Mar 12 17:36</td><td>29.1</td><td>AB</td><td>GX, 4Ua</td><td>...</td></tr><tr><td>40014026001</td><td>249.1206</td><td>-46.9073</td><td>168.0941</td><td>2014 Mar 13 11:26</td><td>30.2</td><td>AB</td><td>GX, 4Ua</td><td>...</td></tr><tr><td>40014027001</td><td>249.1610</td><td>-47.0090</td><td>168.1171</td><td>2014 Mar 14 03:31</td><td>30.2</td><td>AB</td><td>GX, 4Ua</td><td>...</td></tr><tr><td>40014028002</td><td>249.0277</td><td>-47.1640</td><td>168.1544</td><td>2014 Mar 18 12:36</td><td>29.6</td><td>AB</td><td>GX, 4Ua</td><td>...</td></tr><tr><td>40014029001</td><td>249.9367</td><td>-46.8984</td><td>168.2590</td><td>2014 Mar 19 04:41</td><td>29.2</td><td>AB</td><td>GX, 4Ua</td><td>...</td></tr><tr><td>40014030001</td><td>250.2174</td><td>-46.7179</td><td>168.3050</td><td>2014 Mar 19 20:46</td><td>27.3</td><td>B</td><td>4Ua</td><td>...</td></tr><tr><td>40014031001</td><td>250.5222</td><td>-46.7773</td><td>168.3521</td><td>2014 Mar 20 13:01</td><td>30.0</td><td>B</td><td>4Ua</td><td>...</td></tr><tr><td>40014032001</td><td>250.4317</td><td>-46.7896</td><td>168.4181</td><td>2014 Mar 21 05:01</td><td>30.9</td><td>B</td><td>4Ua</td><td>...</td></tr><tr><td>40014033002</td><td>250.4849</td><td>-46.6649</td><td>168.2038</td><td>2014 Mar 24 10:41</td><td>31.5</td><td>B</td><td>4Ua</td><td>...</td></tr><tr><td>40014034001</td><td>250.2701</td><td>-46.8265</td><td>168.1849</td><td>2014 Mar 25 02:46</td><td>31.2</td><td>AB</td><td>GX, 4Ua</td><td>...</td></tr><tr><td>40014035001</td><td>250.0454</td><td>-46.8714</td><td>168.1523</td><td>2014 Mar 25 18:56</td><td>39.2</td><td>AB</td><td>GX, 4Ua</td><td>...</td></tr><tr><td>30001008002</td><td>249.8301</td><td>-46.6567</td><td>295.0558</td><td>2014 Jun 26 02:21</td><td>50.4</td><td>...</td><td>...</td><td>...</td></tr><tr><td>30001012002</td><td>248.6712</td><td>-47.6364</td><td>171.9830</td><td>2013 Mar 23 08:31</td><td>16.3</td><td>A</td><td>GX</td><td>Ghost rays from 4Ub in AB</td></tr><tr><td>30001016002</td><td>248.5333</td><td>-47.3795</td><td>164.6452</td><td>2014 Mar 06 22:56</td><td>21.3</td><td>AB</td><td>GX, 4Ua</td><td>...</td></tr><tr><td>30001017002</td><td>248.8967</td><td>-47.3836</td><td>210.3881</td><td>2014 May 12 21:31</td><td>49.0</td><td>AB</td><td>GX, 4Ua</td><td>...</td></tr><tr><td>30001033002</td><td>249.4897</td><td>-46.9015</td><td>145.8254</td><td>2015 Jan 28 05:16</td><td>51.8</td><td>AB</td><td>GX, 4Ua</td><td>...</td></tr><tr><td>30160001002</td><td>249.3137</td><td>-47.5723</td><td>267.7851</td><td>2015 Jun 11 14:46</td><td>49.4</td><td>AB</td><td>GX, 4Ua</td><td>...</td></tr><tr><td>30160002002</td><td>248.9436</td><td>-47.5918</td><td>261.9558</td><td>2015 Jun 07 23:46</td><td>97.1</td><td>AB</td><td>GX, 4Ua</td><td>...</td></tr><tr><td>30160003002</td><td>249.0412</td><td>-47.8404</td><td>244.4634</td><td>2015 May 31 11:11</td><td>76.7</td><td>AB</td><td>GX, 4Ua</td><td>...</td></tr><tr><td>40001022002</td><td>249.5341</td><td>-47.2183</td><td>164.5577</td><td>2014 Mar 07 11:51</td><td>100.6</td><td>AB</td><td>GX, 4Ua</td><td>...</td></tr><tr><td colspan="9"><strong>Deep HESS Field</strong></td></tr><tr><td>30002021002</td><td>250.1049</td><td>-46.5763</td><td>353.7407</td><td>2013 Sep 29 06:56</td><td>62.8</td><td>AB</td><td>GX, 4Ua</td><td>SL of unknown origin in A</td></tr><tr><td>30002021003</td><td>250.1324</td><td>-46.5412</td><td>353.7551</td><td>2013 Sep 30 16:31</td><td>20.8</td><td>A</td><td>4Ua</td><td>SL of unknown origin in A</td></tr><tr><td>30002021005</td><td>250.2036</td><td>-46.5095</td><td>161.2653</td><td>2014 Feb 28 23:16</td><td>99.5</td><td>AB</td><td>4Ua</td><td>...</td></tr><tr><td>30002021007</td><td>250.2027</td><td>-46.5145</td><td>161.2702</td><td>2014 Mar 06 01:51</td><td>35.9</td><td>AB</td><td>4Ua</td><td>...</td></tr><tr><td>30002021009</td><td>250.2175</td><td>-46.5088</td><td>166.7254</td><td>2014 Mar 14 21:21</td><td>32.5</td><td>AB</td><td>4Ua</td><td>...</td></tr><tr><td>30002021011</td><td>250.2296</td><td>-46.5012</td><td>179.7925</td><td>2014 Apr 11 13:11</td><td>22.5</td><td>AB</td><td>4Ua</td><td>...</td></tr><tr><td>30002021013</td><td>250.1923</td><td>-46.5268</td><td>227.3736</td><td>2014 May 25 01:56</td><td>21.6</td><td>...</td><td>...</td><td>...</td></tr><tr><td>30002021015</td><td>250.1802</td><td>-46.5601</td><td>289.9801</td><td>2014 Jun 23 12:51</td><td>29.2</td><td>A</td><td>4Ua</td><td>...</td></tr><tr><td>30002021017</td><td>250.1814</td><td>-46.5644</td><td>295.1336</td><td>2014 Jun 25 13:31</td><td>22.0</td><td>...</td><td>...</td><td>...</td></tr><tr><td>30002021019</td><td>250.1913</td><td>-46.5447</td><td>295.1661</td><td>2014 Jun 27 05:41</td><td>19.5</td><td>...</td><td>...</td><td>...</td></tr><tr><td>30002021021</td><td>250.1762</td><td>-46.5687</td><td>295.1179</td><td>2014 Jun 30 01:41</td><td>19.8</td><td>...</td><td>...</td><td>...</td></tr><tr><td>30002021023</td><td>250.1892</td><td>-46.5586</td><td>311.5738</td><td>2014 Jul 11 02:21</td><td>22.1</td><td>...</td><td>...</td><td>...</td></tr><tr><td>30002021025</td><td>250.1569</td><td>-46.5398</td><td>330.9082</td><td>2014 Aug 10 05:36</td><td>21.9</td><td>A</td><td>IGR, 4Ua</td><td>6' streak in B</td></tr><tr><td>30002021027</td><td>250.1477</td><td>-46.5524</td><td>344.3607</td><td>2014 Sep 11 10:56</td><td>20.0</td><td>AB</td><td>GX, 4Ua</td><td>...</td></tr><tr><td>30002021029</td><td>250.1247</td><td>-46.5392</td><td>356.5397</td><td>2014 Oct 11 01:01</td><td>22.1</td><td>AB</td><td>4Ua</td><td>...</td></tr></tbody></table>
<table><caption>**Table 1**
(Continued)</caption><thead><tr><th rowspan="2">ObsID<br>(1)</th><th colspan="3">Pointing (J2000)</th><th rowspan="2">Start Time<br>(UT)<br>(5)</th><th rowspan="2">Exposure<br>(ks)<br>(6)</th><th rowspan="2">SL Removal<br>(FPM)<br>(7)</th><th rowspan="2">SL<br>Source<br>(8)</th><th rowspan="2">Other<br>Contamination<br>(9)</th></tr><tr><th>R.A. (°)<br>(2)</th><th>Decl. (°)<br>(3)</th><th>PA (°)<br>(4)</th></tr></thead><tbody><tr><td>30002021031</td><td>250.1400</td><td>-46.5260</td><td>15.7729</td><td>2014 Nov 05 07:56</td><td>4.3</td><td>AB</td><td>4Ua</td><td>...</td></tr><tr><td>30002021033</td><td>250.2058</td><td>-46.4950</td><td>129.7677</td><td>2015 Jan 08 04:46</td><td>4.2</td><td>...</td><td>...</td><td>...</td></tr><tr><td>30002021034</td><td>250.2115</td><td>-46.4858</td><td>129.7237</td><td>2015 Jan 12 18:16</td><td>16.7</td><td>...</td><td>...</td><td>...</td></tr><tr><td>30002031036</td><td>250.2188</td><td>-46.5018</td><td>154.4146</td><td>2015 Feb 14 18:26</td><td>31.8</td><td>AB</td><td>4Ua, 4Ub</td><td>...</td></tr></tbody></table>

**Notes.** (4) Position angle (east of north). (7) FPMs from which stray light background photons from sources in column 8 were removed. (8) Stray light background sources: GX = GX 340+0, 4Ua = 4U 1624–49, 4Ub = 1630–472, IGR = IGR J16318–4848. Although additional stray light from IGR J16320–4751 was present in some of the first mini-survey observations and stray light from 4U 1624–49 and GX 340+0 was present in observation 30001008002, this stray light background was not removed because real sources could be seen in the raw data residing in the stray light-contaminated regions. The contamination in observations 30002021002A, 30002021003A, 30002021036B, and 30001012002 was so extensive that these observations were not included in our analysis.

<table><caption>**Table 2**
Archival *Chandra* Observations Used in This Study</caption><thead><tr><th rowspan="2">Chandra<br>ObsID<br>(1)</th><th colspan="2">Pointing (J2000)</th><th rowspan="2">Start Time<br>(UT)<br>(4)</th><th rowspan="2">Exposure<br>(ks)<br>(5)</th><th rowspan="2">References<br>(6)</th></tr><tr><th>R.A. (°)<br>(2)</th><th>Decl. (°)<br>(3)</th></tr></thead><tbody><tr><td>7591</td><td>250.187126</td><td>-46.520108</td><td>2007 May 11 11:01</td><td>28.8</td><td>Lemiere et al. (2009)</td></tr><tr><td>11008</td><td>250.134287</td><td>-46.393394</td><td>2010 Jun 19 22:10</td><td>39.6</td><td>Rahoui et al. (2014)</td></tr><tr><td colspan="6"><strong>Norma Arm Region Chandra Survey (NARCS)</strong></td></tr><tr><td>12507</td><td>250.373201</td><td>-46.662951</td><td>2011 Jun 06 10:15</td><td>18.8</td><td>"..."</td></tr><tr><td>12508</td><td>250.155011</td><td>-46.530604</td><td>2011 Jun 06 15:57</td><td>18.5</td><td>"..."</td></tr><tr><td>12509</td><td>249.937805</td><td>-46.397816</td><td>2011 Jun 06 21:22</td><td>19.4</td><td>"..."</td></tr><tr><td>12510</td><td>250.180190</td><td>-46.812896</td><td>2011 Jun 09 12:29</td><td>19.9</td><td>"..."</td></tr><tr><td>12511</td><td>249.961646</td><td>-46.681456</td><td>2011 Jun 17 11:15</td><td>19.3</td><td>"..."</td></tr><tr><td>12512</td><td>249.743370</td><td>-46.550407</td><td>2011 Jun 27 04:52</td><td>20.5</td><td>"..."</td></tr><tr><td>12513</td><td>249.984947</td><td>-46.965904</td><td>2011 Jun 27 11:00</td><td>20.2</td><td>"..."</td></tr><tr><td>12514</td><td>249.767582</td><td>-46.829470</td><td>2011 Jun 10 16:07</td><td>19.8</td><td>"..."</td></tr><tr><td>12515</td><td>249.550110</td><td>-46.695978</td><td>2011 Jun 10 22:04</td><td>19.5</td><td>"..."</td></tr><tr><td>12516</td><td>249.790838</td><td>-47.111874</td><td>2011 Jun 11 03:46</td><td>19.5</td><td>"..."</td></tr><tr><td>12517</td><td>249.572205</td><td>-46.978413</td><td>2011 Jun 11 09:28</td><td>19.5</td><td>"..."</td></tr><tr><td>12518</td><td>249.354673</td><td>-46.844540</td><td>2011 Jun 11 15:10</td><td>19.5</td><td>"..."</td></tr><tr><td>12519</td><td>249.594334</td><td>-47.262081</td><td>2011 Jun 13 04:25</td><td>19.3</td><td>"..."</td></tr><tr><td>12520</td><td>249.375577</td><td>-47.128273</td><td>2011 Jun 13 10:13</td><td>19.0</td><td>"..."</td></tr><tr><td>12521</td><td>249.157932</td><td>-46.994022</td><td>2011 Jun 13 15:46</td><td>19.0</td><td>"..."</td></tr><tr><td>12522</td><td>249.396933</td><td>-47.410725</td><td>2011 Jun 13 21:20</td><td>19.0</td><td>"..."</td></tr><tr><td>12523</td><td>249.178061</td><td>-47.276529</td><td>2011 Jun 14 02:53</td><td>19.0</td><td>"..."</td></tr><tr><td>12524</td><td>248.960334</td><td>-47.141940</td><td>2011 Jun 14 08:27</td><td>19.5</td><td>"..."</td></tr><tr><td>12525</td><td>249.198427</td><td>-47.559064</td><td>2011 Jun 14 14:08</td><td>19.5</td><td>"..."</td></tr><tr><td>12526</td><td>248.979417</td><td>-47.424468</td><td>2011 Jun 14 19:50</td><td>19.0</td><td>"..."</td></tr><tr><td>12527</td><td>248.761625</td><td>-47.289491</td><td>2011 Jun 15 19:36</td><td>19.3</td><td>"..."</td></tr><tr><td>12528</td><td>248.998831</td><td>-47.707016</td><td>2011 Jun 16 01:24</td><td>19.0</td><td>"..."</td></tr><tr><td>12529</td><td>248.779750</td><td>-47.572056</td><td>2011 Jun 16 06:58</td><td>19.0</td><td>"..."</td></tr><tr><td>12530</td><td>248.561776</td><td>-47.436667</td><td>2011 Jun 16 12:31</td><td>19.3</td><td>"..."</td></tr><tr><td>12531</td><td>248.798050</td><td>-47.854617</td><td>2011 Jun 16 18:09</td><td>19.5</td><td>"..."</td></tr><tr><td>12532</td><td>248.578823</td><td>-47.719259</td><td>2011 Jun 16 23:51</td><td>19.5</td><td>"..."</td></tr><tr><td>12533</td><td>248.360823</td><td>-47.583518</td><td>2011 Jun 17 05:32</td><td>19.5</td><td>"..."</td></tr></tbody></table>

**Note.** (6) References in which archival observations were previously presented and analyzed.

Furthermore, in this study, we make use of *Chandra* observations that were triggered to follow up four transient sources discovered by NuSTAR. These *Chandra* observations were used to constrain their soft X-ray spectra and better localize their positions so as to be able to search for optical and infrared counterparts. The follow-up observations of one of these transients, NuSTAR J163433–4738.7, are discussed in T14, and the others are presented in Section 5.5 and listed in Table 3.

### 3. NuSTAR Data Processing and Mosaicking

The raw data of each observation were processed using CALDB v20150612 and the standard NuSTAR pipeline v1.3.1 provided under HEASOFT v6.15.1 to produce event files and exposure maps for both FPMs. We made exposure maps with and without vignetting corrections to be used in different parts of our analysis.
<table><caption>**Table 3**
Chandra Follow-up Observations of NuSTAR Transients</caption><thead><tr><th rowspan="2">Chandra<br>ObsID<br>(1)</th><th rowspan="2">Source<br>No.<br>(2)</th><th colspan="2">Pointing (J2000)</th><th rowspan="2">Start Time<br>(UT)<br>(5)</th><th rowspan="2">Exposure<br>(ks)<br>(6)</th><th rowspan="2">Delay between NuSTAR & Chandra<br>Observations (days)<br>(7)</th></tr><tr><th>R.A. (°)<br>(3)</th><th>Decl. (°)<br>(4)</th></tr></thead><tbody><tr><td>16170</td><td>19</td><td>250.315079</td><td>-46.540562</td><td>2014 Mar 17 05:44</td><td>4.9</td><td>3</td></tr><tr><td>16171</td><td>20</td><td>250.591644</td><td>-46.716049</td><td>2014 Oct 20 06:31</td><td>4.9</td><td>210</td></tr><tr><td>17242</td><td>25</td><td>248.999542</td><td>-47.807671</td><td>2015 Jul 04 10:26</td><td>9.8</td><td>34</td></tr></tbody></table>

**Notes.** (2) NNR source that triggered the *Chandra* observation. (7) Time elapsed between the *NuSTAR* observation in which the source is detected and the *Chandra* follow-up observation. These times vary significantly because some of these sources were obvious in the raw images, while others required mosaicking and careful photometric analysis to determine that they were significant detections.

Next, we cleaned the event files of stray light contamination by filtering out X-ray events in stray light-affected regions. Table 1 indicates whether stray light removal occurred in either FPMA or FPMB, as well as the source responsible for the stray light. In one exceptional case, we did not remove the stray light seen in the FPMA and FPMB of observation 30001008002, since a bright source, IGR J16393-4643, is located within the stray light regions caused by GX 340+0 and 4U 1624-49. We also excised the most significant ghost rays from observations from the first mini-survey, defining the ghost ray pattern regions in the same way as B14. One observation, 30001012002, was performed to follow up NuSTAR J163433-4738.7, a transient source discovered in the first mini-survey. This observation helped to characterize the outburst duration of this transient (T14), but it was so extensively contaminated by ghost rays that it was not included in our analysis. Finally, a few observations show additional contamination features, such as sharp streaks (listed in Table 1), which were also removed.

To improve the astrometric accuracy of the NuSTAR observations, we calculated the shifts between the positions of bright NuSTAR sources and their Chandra counterparts in NARCS observations that were astrometrically registered using infrared counterparts in the VISTA Variables in the Via Lactea (VVV; Minniti et al. 2010) survey (Fornasini et al. 2014). The positions of bright sources, which could be easily identified in raw images, were determined using the IDL gcntrd tool, which makes use of the DAOPHOT “FIND” centroid algorithm. This source localization was done independently for each FPM of each observation and was used to apply translational shifts to event files and exposure maps. In performing astrometric corrections, we limited ourselves to using sources with >100 net counts in each individual observation and FPM and located on-axis. For on-axis sources with this number of counts, we expect the statistical error on the centroid to be <6" based on simulations (B. Grefenstette 2014, personal communication). NARCS 999 is very bright, with > 10,000 net counts, and therefore the statistical uncertainties of the astrometric corrections derived from this source are <2" at 90% confidence. The other sources used for astrometric corrections have 100-300 net counts, and their associated statistical uncertainties are expected to be 5-6" at 90% confidence. Table 4 lists the applied boresight shifts and the bright sources used for astrometric correction. We were only able to apply these astrometric corrections to 23 out of 60 observations (43 out of 117 modules) due to the dearth of bright X-ray sources in our survey. Our inability to astrometrically correct all the observations does not significantly impact the results of our photometric and spectral

analysis, since the radii of the source regions we use are significantly larger than the expected shifts. The boresight shifts range from 1" to 14"; 20% of the shifts are larger than 8", which is more than expected based on NuSTAR's nominal accuracy of ±8" at 90% confidence (Harrison et al. 2013) but is not unexpected given that the statistical errors on the source positions may be as high as 6". Checking each shifted and unshifted image by eye and comparing the locations of NuSTAR sources with their Chandra counterparts in shifted and unshifted mosaic images, we confirm that these boresight shifts constitute an improvement over the original NuSTAR positions.

We reprojected the event files of each observation onto a common tangent point and merged all the observations and both FPMs together to maximize the photon statistics. We then generated mosaic images on the common sky grid in the 3-78, 3-10, 3-40, 10-20, 10-40, 20-40, and 40-78 keV bands. To create mosaic exposure maps, we combined the individual exposure maps by adding exposure values at the location of each sky pixel in the mosaic image; we made exposure maps both without vignetting corrections and with vignetting corrections evaluated at 8, 10, and 20 keV. We used the exposure maps without vignetting corrections when we calculated the source significance and net counts, since these calculations require comparing the exposure depth in the source and background region apertures and the background is dominated by nonfocused emission. Instead, when calculating sensitivity curves (Section 6.2), we used exposure maps with vignetting corrections, since the source emission is focused by the telescope mirrors. When calculating the source fluxes, vignetting corrections are taken into account through the ancillary response file (ARF). An exposure-corrected NuSTAR mosaic image in the 3-40 keV band and exposure map without vignetting correction are shown in Figure 1. As can be seen, the typical exposure depth of the Norma survey is 30-100 ks, while the exposure of the deep field is 1 Ms.

# 4. Source Detection

## 4.1. Generating Trial Maps

To identify sources in the NuSTAR Norma survey, we employed a technique that was specifically developed for the NuSTAR surveys. This technique, which we refer to as the “trial map” technique, is described in detail by Hong et al. (2016), so we only provide a brief explanation here. The NuSTAR GC region survey (Hong et al. 2016) and the NuSTAR extragalactic surveys (Civano et al. 2015; Mullaney et al. 2015) all use this technique as the basis for their detection method. As a result of
<table><caption>**Table 4**
Boresight Corrections</caption><thead><tr><th>ObsID<br>(1)</th><th>Total<br>shift<br>(")<br>(2)</th><th>R.A. Shift<br>(")<br>(3)</th><th>Decl.<br>Shift<br>(")<br>(4)</th><th>Reference<br>Source<br>(NARCS ID)<br>(5)</th></tr></thead><tbody><tr><td>30001008002A</td><td>5.5</td><td>-6.5</td><td>3.25</td><td>999</td></tr><tr><td>30001008002B</td><td>6.9</td><td>-0.1</td><td>6.7</td><td>999</td></tr><tr><td>30001033002A</td><td>5.6</td><td>-1.9</td><td>-5.4</td><td>750</td></tr><tr><td>30001033002B</td><td>3.2</td><td>-2.1</td><td>-2.8</td><td>750</td></tr><tr><td>30002021002B</td><td>4.2</td><td>6.0</td><td>0.0</td><td>1321</td></tr><tr><td>30002021003B</td><td>10.8</td><td>13.7</td><td>-5.2</td><td>1321</td></tr><tr><td>30002021005A</td><td>4.5</td><td>5.0</td><td>-2.9</td><td>1321</td></tr><tr><td>30002021005B</td><td>3.7</td><td>-5.3</td><td>0.5</td><td>1321</td></tr><tr><td>30002021007A</td><td>4.2</td><td>3.1</td><td>-3.6</td><td>1321</td></tr><tr><td>30002021007B</td><td>3.7</td><td>-5.4</td><td>-0.8</td><td>1321</td></tr><tr><td>30002021009A</td><td>1.7</td><td>-0.3</td><td>-1.7</td><td>1321</td></tr><tr><td>30002021009B</td><td>4.4</td><td>-6.1</td><td>1.2</td><td>1321</td></tr><tr><td>30002021011A</td><td>4.3</td><td>-4.4</td><td>3.1</td><td>1321</td></tr><tr><td>30002021011B</td><td>4.7</td><td>-6.9</td><td>0.3</td><td>1321</td></tr><tr><td>30002021013A</td><td>7.9</td><td>8.2</td><td>5.5</td><td>1321</td></tr><tr><td>30002021013B</td><td>6.1</td><td>1.8</td><td>6.0</td><td>1321</td></tr><tr><td>30002021015A</td><td>4.5</td><td>3.7</td><td>3.7</td><td>1321</td></tr><tr><td>30002021015B</td><td>6.0</td><td>0.6</td><td>5.9</td><td>1321</td></tr><tr><td>30002021017A</td><td>2.2</td><td>2.1</td><td>1.6</td><td>1321</td></tr><tr><td>30002021017B</td><td>2.9</td><td>4.1</td><td>0.6</td><td>1321</td></tr><tr><td>30002021019A</td><td>7.1</td><td>10.0</td><td>-1.6</td><td>1321</td></tr><tr><td>30002021019B</td><td>10.0</td><td>11.2</td><td>6.4</td><td>1321</td></tr><tr><td>30002021021A</td><td>1.8</td><td>-0.6</td><td>-1.8</td><td>1321</td></tr><tr><td>30002021021B</td><td>7.2</td><td>9.4</td><td>3.2</td><td>1321</td></tr><tr><td>30002021023A</td><td>1.2</td><td>1.8</td><td>0.1</td><td>1321</td></tr><tr><td>30002021023B</td><td>7.9</td><td>-6.2</td><td>-6.6</td><td>1321</td></tr><tr><td>30002021025A</td><td>7.7</td><td>11.2</td><td>0.0</td><td>1321</td></tr><tr><td>30002021025B</td><td>9.3</td><td>13.3</td><td>1.0</td><td>1321</td></tr><tr><td>30002021027A</td><td>0.6</td><td>0.9</td><td>0.1</td><td>1321</td></tr><tr><td>30002021027B</td><td>8.7</td><td>11.7</td><td>-3.2</td><td>1321</td></tr><tr><td>30002021029A</td><td>10.2</td><td>12.1</td><td>-5.9</td><td>1321</td></tr><tr><td>30002021029B</td><td>5.9</td><td>5.9</td><td>-4.3</td><td>1321</td></tr><tr><td>30002021031A</td><td>9.3</td><td>10.9</td><td>5.5</td><td>1321</td></tr><tr><td>30002021031B</td><td>7.2</td><td>0.8</td><td>7.2</td><td>1321</td></tr><tr><td>30002021033A</td><td>7.2</td><td>1.3</td><td>-7.1</td><td>1321</td></tr><tr><td>30002021033B</td><td>14.4</td><td>-20.6</td><td>-2.6</td><td>1321</td></tr><tr><td>30002021034A</td><td>10.5</td><td>-8.7</td><td>-8.7</td><td>1321</td></tr><tr><td>30002021034B</td><td>9.8</td><td>10.8</td><td>6.3</td><td>1321</td></tr><tr><td>30002021036A</td><td>5.7</td><td>-8.4</td><td>-0.1</td><td>1321</td></tr><tr><td>40001022002A</td><td>4.9</td><td>-5.9</td><td>-2.9</td><td>786</td></tr><tr><td>40001022002B</td><td>6.4</td><td>-9.5</td><td>0.3</td><td>786</td></tr><tr><td>40014017001A</td><td>9.0</td><td>6.9</td><td>-7.7</td><td>1321</td></tr><tr><td>40014017001B</td><td>7.7</td><td>6.6</td><td>6.2</td><td>1321</td></tr></tbody></table>

**Notes.** The 90% confidence statistical uncertainties of the astrometric corrections are estimated to be <2" for NARCS 999 and 5–6" for all other NARCS sources. (2) Angular distance between original pointing and boresight-corrected pointing. (5) NARCS ID of source used to determine astrometric correction.

NuSTAR's point-spread function (PSF) being larger and its background being higher and more complex compared to other focusing X-ray telescopes, such as Chandra and XMM-Newton, the utility of typical detection algorithms, such as `wavdetect` (Freeman et al. 2002), is limited when applied to NuSTAR data. One way of dealing with this problem is to add an additional level of screening to the results of conventional algorithms, calculating the significance of detections by independent means and setting a significance detection threshold. The trial map

technique is more direct, skipping over the initial step of using a detection algorithm such as `wavdetect`.

To make a trial map, for each sky pixel, we calculate the probability of acquiring more than the total observed counts within a source region due to a random background fluctuation. For each pixel, the source and background regions are defined as a circle and an annulus, respectively, centered on that pixel. The mean background counts expected within the source region are estimated from the counts in the background region scaled by the ratio of the areas and exposure values of the source and background regions. Using background regions that are symmetric around the central pixel helps to account for spatial variations of the background. In making trial maps, we plot the inverse of the random chance probability, which is the number of random trials required to produce the observed counts simply by random background fluctuations, such that brighter sources with higher significance have higher values in the maps.

We generated trial maps using three different source region sizes with radii of 9"/5, 12", and 17" (corresponding to 15%, 22%, and 30% enclosures of the PSF, respectively) and six different energy bands (3–78, 3–10, 10–40, 40–78, 10–20, and 20–40 keV). The source region sizes we used are slightly larger than those used in the analysis of the NuSTAR GC survey, since the smaller sizes are especially suited for picking out relatively bright sources in areas of diffuse emission, but in the Norma region there is no evident diffuse emission apart from stray light and ghost rays. The inner and outer radii of the background regions are 51" (corresponding to 70% of the PSF) and 85" (equal to 5/3 of the inner radius), respectively, in all cases. Figure 2 shows trial maps made using the 22% PSF enclosure and the 3–10, 10–20, and 20–40 keV bands; the three energy bands are combined into a three-color image so that spectral differences between sources can be seen.

## 4.2. Detection Thresholds and Source Selection

When considering how to set detection thresholds for our trial maps, we excluded the observations from the first mini-survey and observation 30001008002, since they have significantly higher levels of stray light and ghost ray contamination than the rest of the survey; in the remainder of this paper, we will refer to this subset of observations as the “clean” sample. Figure 3 shows the fractional distributions of the values from the clean trial maps using source region sizes of 22% PSF enclosures. As can be seen, the distribution for the 40–78 keV band is very close to that expected for a Poissonian distribution of random background fluctuations, and, in fact, no sources are clearly visible in the 40–78 keV clean trial maps.

Following the procedure described in Hong et al. (2016) to establish detection thresholds, we began by cross-correlating each trial map with the NARCS source catalog. Figure 4 shows the maximum trial map value within 10" of the locations of the NARCS sources detected at $>3\sigma$ in the 2–10 keV band as a function of Chandra photon flux. Above Chandra fluxes of $6 \times 10^{-6} \text{ cm}^{-2} \text{ s}^{-1}$, more than 1/3 of the NARCS sources have trial map values that are significantly higher than the bulk of the NARCS sources clustered between trial map values of $10^{0.3}$ and $10^3$. For Chandra fluxes lower than $2 \times 10^{-6} \text{ cm}^{-2} \text{ s}^{-1}$, the distribution of trial map values is uncorrelated with source flux, having a linear Pearson correlation coefficient $|p| < 0.04$ for all trial maps.
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Composite trial map showing the 3–10 keV band in red, 10–20 keV band in green, and 20–40 keV band in blue. The colors are scaled by the logarithmic trial map values. Tier 1 sources are labeled in green, if they were observed by NARCS or were previously well-studied, or cyan, if they were discovered by the NuSTAR Norma survey. Tier 2 sources are labeled in yellow. The streaks in the vicinity of NNR 2 are due to stray light that has not been removed because NNR 2 is partially embedded in it. The small streaks seen in the area covered by the first mini-survey are due to ghost rays from NNR 1.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>**Figure 3.** Distribution of trial map values in different energy bands for 22% PSF enclosures. The x-axis is shown in a double logarithmic scale. The 40–78 keV distribution closely matches the random distribution expected due to Poissonian fluctuations of the background; this is consistent with the fact that, among the clean observations included in creating this plot, only one source is detected in the 40–78 keV band. The vertical dashed line shows the detection threshold set for the 3–10 keV band trial map. The excess of high trial map values relative to the 40–78 keV band distribution is due to the presence of sources, stray light, and ghost rays; the excess of low trial map values results from the vicinity of bright sources, which effectively increase the local background.</figcaption></figure>

For a source to be considered for the final catalog, we require that it exceed the detection threshold in at least two trial maps. If all 18 trial maps were independent of each other, the expected number of false sources ($N_F$) would be equal to $N_{\text{can}} C (18, 2) p^{16} (1-p)^2$, where $N_{\text{can}}$ is the number of NARCS

<figure><img src="image_6.png" /><figcaption>**Figure 4.** Trial map value in the 3–10 keV band using 22% PSF enclosures vs. Chandra 2–10 keV photon flux for NARCS sources in the surveyed NuSTAR area. Fluxes of sources in the gray region are uncorrelated with the trial map values and used to set the detection threshold, which is shown by the red horizontal line. Sources above the horizontal line in at least two trial maps are tier 1 sources, while bright sources below that line but to the right of the vertical dashed line are tier 2 candidates.</figcaption></figure>

sources included in a NuSTAR counterpart search, $C(i, j)$ is a binomial coefficient, and $p$ is the fraction of false sources to be rejected in each map (Hong et al. 2016). However, the trial maps are not completely independent, given that their energy ranges overlap. Thus, to at least partly account for the fact that some of the trial maps are correlated, we set a stringent limit on
the expected number of false sources, setting $N_F = 0.5$. Since the long-term variability of the NARCS sources is unknown, we search for *NuSTAR* detections among all NARCS sources. Thus, in the clean map regions, $N_{can} = 579$; limiting $N_F$ to 0.5 requires a rejection percentage $p = 99.76\%$. Making a cumulative distribution function of the trial map values of uncorrelated NARCS sources lying in the gray area of Figure 4, we determine the corresponding trial value threshold for each trial map; the detection thresholds range from $10^{5.2}$ in the 20–40 keV band with 15% PSF enclosures to $10^{10.3}$ in the 3–10 keV band with 30% PSF enclosures.

Having established detection thresholds for each trial map, we first search for any *Chandra* sources detected by *NuSTAR*. We cross-correlate all NARCS sources detected at $>3\sigma$ in the 2–10 keV *Chandra* band with the trial maps of the full set of observations, including those with significant background contamination. We consider all NARCS sources that exceed the detection threshold in at least two trial maps as tier 1 candidate sources. All sources with 2–10 keV *Chandra* flux $>6 \times 10^{-6} \text{ cm}^{-2} \text{ s}^{-1}$ that are not tier 1 sources are considered tier 2 candidate sources, regardless of their trial map values. Although, for tier 2 sources, we do not expect to be able to retrieve significant spectral information, we can at least check for significant variability between the *Chandra* and *NuSTAR* observations and place upper limits on the flux above 10 keV. We also perform a blind search for *NuSTAR* sources that were not detected in NARCS; we consider any clusters of pixels that exceed the detection threshold in at least three trial maps as additional tier 1 candidate sources.

We then inspect all the candidate sources. First, we check whether *NuSTAR* sources matched to *Chandra* counterparts are unique matches. We find 13 cases in which multiple NARCS sources were associated with a single *NuSTAR* detection due to *NuSTAR*'s much larger PSF; however, in all these cases, one NARCS source was more clearly centered on the *NuSTAR* position and was also significantly brighter, demonstrating the more likely association. We then visually inspect all tier 1 candidate sources without NARCS associations to ensure they are not associated with artifacts due to stray light, ghost rays, or the edges of the FOVs. Based on this visual inspection, we exclude three candidate sources located at the edges of the FOVs, the stray light region near NNR 2, and 21 candidates without a clear point-like morphology that are located in the first mini-survey area contaminated by ghost rays. In addition, since tier 2 candidate sources do not exceed the trial map detection thresholds, in order for them to be included in our final catalog, we require that their aperture photometry have a signal-to-noise ratio (S/N) $> 3\sigma$ in at least one of the 3–10, 3–40, or 10–20 keV energy bands (see Section 5.2 for details). In total, after these different screenings, 28 tier 1 candidates and 10 tier 2 candidates are included in our final source list, shown in Table 5.

To determine the best position of the tier 1 *NuSTAR* sources, we applied the DAOPHOT “FIND” algorithm in the proximity of each source in the 3–10 keV trial map with 22% PSF enclosure; we found that using the centroid algorithm on the trial maps rather than the mosaic images yielded better results, allowing the algorithm to converge for all tier 1 sources with lower statistical errors. When applying the centroid algorithm, we used the 3–10 keV, 22% PSF trial map, since all the tier 1 sources are clearly discernible in it.

The tier 2 sources are not bright enough for the centroid algorithm to yield reliable results, so we simply adopted the *Chandra* positions for these sources. The offsets between the tier 1 sources and their *Chandra* counterparts vary from 0.9 to 14.0, excluding two extended sources (NNR 8 and 21) whose *Chandra* positions were determined subjectively by eye. The offsets of four *NuSTAR* point sources from their *Chandra* counterparts are larger than the 90% *NuSTAR* positional uncertainties. We estimated the *NuSTAR* positional uncertainty for each tier 1 source as the quadrature sum of the statistical and systematic uncertainties. We calculated the statistical error by performing Gaussian fits to histograms of the spatial count distributions in the x- and y-directions in a 25 × 25 pixel image cutout centered on the source position. These statistical errors are approximate, since the *NuSTAR* PSF has non-Gaussian wings, but comparison of the errors derived using the Gaussian approximation to those derived from the accurate PSF simulations performed for some of the brighter sources (see Section 3) indicates that this approximation is accurate to 10%. For the systematic uncertainty, we assumed the nominal 8" astrometric accuracy (Harrison et al. 2013) for sources located in observations that were not astrometrically corrected and the uncertainties calculated in Section 3 for sources in astrometrically corrected observations. Looking carefully at the four sources with the largest offsets, the similarity between their fluxes and/or spectral properties in the 2–10 keV band between *Chandra* and *NuSTAR* suggests that they are true counterparts despite the large positional offsets. The fact that 17% of the *NuSTAR* offsets exceed the 90% positional uncertainties suggests that the *NuSTAR* positional uncertainty is slightly underestimated. Large offsets between *NuSTAR* positions and soft X-ray counterparts are also seen in the *NuSTAR* serendipitous survey, where Lansbury et al. (2017) find that the 90% positional accuracy of *NuSTAR* varies from 12" for the most significant detections to 20" for the least significant detections. The large *NuSTAR* offsets in the serendipitous survey suggest that the 90% *NuSTAR* systematic uncertainty is larger than 8", which would help to explain some of the large offsets seen for sources in the Norma survey.

Table 5 provides information about the detection, position, and *Chandra* counterparts of all NNR sources. The tier 1 sources include five sources not detected in NARCS; one of them is the well-known LMXB 4U 1630–472 (Kuulkers et al. 1997), while the others are new transient sources discussed in Section 5.5.

# 5. Aperture Photometry

## 5.1. Defining Source and Background Regions

For photometry and spectral extraction, we used circular source regions and, whenever possible, annular background regions centered on the source positions provided in Table 5. At energies below 20 keV, the *NuSTAR* background is not uniform; it is dominated by nonfocused emission, which exhibits spatial variations due to shadowing of the focal plane (Harrison et al. 2013). Using aperture regions that are symmetric about the source position helps to compensate for this nonuniformity. We performed our photometric analysis with two different source extraction regions with 30" and 40" radii (corresponding to roughly 50% and 60% PSF enclosures, respectively) to assess possible systematic errors associated
<table><caption>Table 5
Source List</caption><thead><tr><th>Src<br>No.<br>(1)</th><th>R.A.<br>(J2000 °)<br>(2)</th><th>Decl.<br>(J2000 °)<br>(3)</th><th>ℓ<br>(J2000 °)<br>(4)</th><th>b<br>(J2000 °)<br>(5)</th><th>Unc.<br>(′′)<br>(6)</th><th>Source Name<br>(7)</th><th>NARCS<br>ID<br>(8)</th><th>Offset<br>(′′)<br>(9)</th><th>Exp.<br>(ks)<br>(10)</th><th>No. Trials<br>(10<sup>x</sup>)<br>(11)</th><th>Band<br>(keV)<br>(12)</th><th>EEF<br>(%)<br>(13)</th><th>No.<br>Det.<br>(14)</th></tr></thead><tbody><tr><td colspan="14"><strong>Tier 1</strong></td></tr><tr><td>1</td><td>248.5070</td><td>-47.3923</td><td>336.9119</td><td>0.2506</td><td>8</td><td>4U 1630-472</td><td>...</td><td>...</td><td>63</td><td>1596934.6</td><td>3-78</td><td>30</td><td>18</td></tr><tr><td>2</td><td>249.7733</td><td>-46.7041</td><td>338.0014</td><td>0.0746</td><td>3<sup>a</sup></td><td>IGR J16393-4643</td><td>999</td><td>2</td><td>101</td><td>15406.9</td><td>3-78</td><td>30</td><td>18</td></tr><tr><td>3</td><td>250.1813</td><td>-46.5272</td><td>338.3198</td><td>-0.0173</td><td>6<sup>a</sup></td><td>CXOU J164043.5-463135</td><td>1321<sup>b</sup></td><td>2</td><td>1039</td><td>1180.9</td><td>3-78</td><td>30</td><td>17</td></tr><tr><td>4</td><td>249.4627</td><td>-46.9299</td><td>337.6914</td><td>0.0821</td><td>6<sup>a</sup></td><td>CXOU J163750.8-465545</td><td>750</td><td>3</td><td>96</td><td>141.2</td><td>3-10</td><td>30</td><td>10</td></tr><tr><td>5</td><td>249.5112</td><td>-47.2327</td><td>337.4885</td><td>-0.1451</td><td>6<sup>a</sup></td><td>CXOU J163802.6-471358</td><td>786</td><td>1</td><td>200</td><td>132.6</td><td>3-10</td><td>30</td><td>14</td></tr><tr><td>6</td><td>248.4812</td><td>-47.6342</td><td>336.7224</td><td>0.0993</td><td>8</td><td>CXOU J163355.1-473804</td><td>78</td><td>4</td><td>43</td><td>92.3</td><td>3-78</td><td>30</td><td>13</td></tr><tr><td>7</td><td>250.1214</td><td>-46.3929</td><td>338.3930</td><td>0.1026</td><td>8</td><td>CXOU J164029.5-462329</td><td>1278/9<sup>c</sup></td><td>7</td><td>215</td><td>77.6</td><td>3-10</td><td>30</td><td>6</td></tr><tr><td>8</td><td>248.9483</td><td>-47.6217</td><td>336.9445</td><td>-0.1241</td><td>9</td><td>CXOU J163547.0-473739</td><td>365<sup>d</sup></td><td>22<sup>a</sup></td><td>94</td><td>64.9</td><td>3-78</td><td>30</td><td>12</td></tr><tr><td>9</td><td>249.8060</td><td>-46.4027</td><td>338.2412</td><td>0.2586</td><td>8</td><td>CXOU J163912.9-462357</td><td>1024</td><td>13</td><td>87</td><td>45.4</td><td>3-10</td><td>30</td><td>12</td></tr><tr><td>10</td><td>248.6407</td><td>-47.6439</td><td>336.7881</td><td>0.0138</td><td>8</td><td>NuSTAR J163433-4738.7</td><td>...</td><td>...</td><td>45<sup>e</sup></td><td>40.6</td><td>3-10</td><td>30</td><td>6</td></tr><tr><td>11</td><td>250.1467</td><td>-46.4991</td><td>338.3251</td><td>0.0191</td><td>9</td><td>CXOU J164035.5-462951</td><td>1301</td><td>7</td><td>1123</td><td>34.9</td><td>3-10</td><td>30</td><td>10</td></tr><tr><td>12</td><td>250.1143</td><td>-46.4226</td><td>338.3676</td><td>0.0865</td><td>9</td><td>CXOU J164027.8-462513</td><td>1276</td><td>9</td><td>654</td><td>31.1</td><td>3-10</td><td>30</td><td>10</td></tr><tr><td>13</td><td>249.9911</td><td>-46.4329</td><td>338.3035</td><td>0.1432</td><td>9</td><td>CXOU J163957.8-462549</td><td>1181</td><td>8</td><td>208</td><td>28.4</td><td>3-78</td><td>30</td><td>12</td></tr><tr><td>14</td><td>249.9943</td><td>-46.8584</td><td>337.9869</td><td>-0.1410</td><td>9</td><td>CXOU J163957.2-465126</td><td>1180</td><td>14</td><td>69</td><td>28.0</td><td>3-10</td><td>30</td><td>6</td></tr><tr><td>15</td><td>250.3823</td><td>-46.5145</td><td>338.4208</td><td>-0.1127</td><td>9</td><td>CXOU J164130.8-463048</td><td>1379</td><td>10</td><td>39</td><td>27.7</td><td>3-10</td><td>30</td><td>6</td></tr><tr><td>16</td><td>248.4639</td><td>-47.7762</td><td>336.6102</td><td>0.0115</td><td>10</td><td>CXOU J163350.9-474638</td><td>72</td><td>6</td><td>37</td><td>21.8</td><td>3-78</td><td>30</td><td>11</td></tr><tr><td>17</td><td>249.9421</td><td>-46.4023</td><td>338.3039</td><td>0.1888</td><td>9</td><td>CXOU J163946.1-462359</td><td>1137</td><td>8</td><td>161</td><td>19.4</td><td>3-10</td><td>30</td><td>6</td></tr><tr><td>18</td><td>248.3743</td><td>-47.5569</td><td>336.7301</td><td>0.2048</td><td>9</td><td>CXOU J163329.5-473332</td><td>38</td><td>9</td><td>37</td><td>18.4</td><td>3-78</td><td>30</td><td>6</td></tr><tr><td>19</td><td>250.3176</td><td>-46.5373</td><td>338.3743</td><td>-0.0943</td><td>9</td><td>NuSTAR J164116-4632.2</td><td>...</td><td>13</td><td>424<sup>c</sup></td><td>15.7</td><td>3-10</td><td>30</td><td>5</td></tr><tr><td>20</td><td>250.5927</td><td>-46.7153</td><td>338.3652</td><td>-0.3538</td><td>9</td><td>NuSTAR J164222-4642.9</td><td>...</td><td>4</td><td>123</td><td>14.9</td><td>3-10</td><td>30</td><td>8</td></tr><tr><td>21</td><td>248.9882</td><td>-47.3188</td><td>337.1864</td><td>0.0601</td><td>12</td><td>CXOU J163555.4-471907</td><td>402/4<sup>f</sup></td><td>18<sup>a</sup></td><td>47</td><td>14.8</td><td>3-10</td><td>30</td><td>3</td></tr><tr><td>22</td><td>250.1156</td><td>-46.8060</td><td>338.0812</td><td>-0.1684</td><td>10</td><td>CXOU J164027.6-464814</td><td>1273</td><td>7</td><td>66</td><td>13.4</td><td>3-10</td><td>20</td><td>4</td></tr><tr><td>23</td><td>249.0619</td><td>-46.8736</td><td>337.5493</td><td>0.3228</td><td>11</td><td>CXOU J163614.2-465222</td><td>454</td><td>7</td><td>86</td><td>12.8</td><td>3-10</td><td>30</td><td>6</td></tr><tr><td>24</td><td>248.9650</td><td>-47.5894</td><td>336.9760</td><td>-0.1106</td><td>11</td><td>CXOU J163551.8-473523</td><td>391</td><td>3</td><td>187</td><td>12.8</td><td>3-78</td><td>30</td><td>4</td></tr><tr><td>25</td><td>249.0020</td><td>-47.8078</td><td>336.8313</td><td>-0.2763</td><td>10</td><td>NuSTAR J163600-4748.4</td><td>...</td><td>6</td><td>77</td><td>11.8</td><td>3-78</td><td>30</td><td>5</td></tr><tr><td>26</td><td>249.8911</td><td>-46.9254</td><td>337.8899</td><td>-0.1330</td><td>9</td><td>CXOU J163933.2-465530</td><td>1090</td><td>7</td><td>58</td><td>10.8</td><td>3-78</td><td>30</td><td>9</td></tr><tr><td>27</td><td>250.1304</td><td>-46.8142</td><td>338.0817</td><td>-0.1814</td><td>13</td><td>CXOU J164031.0-464845</td><td>1291</td><td>6</td><td>121</td><td>9.9</td><td>10-20</td><td>30</td><td>3</td></tr><tr><td>28</td><td>249.2382</td><td>-46.8161</td><td>337.6730</td><td>0.2722</td><td>11</td><td>CXOU J163657.1-464903</td><td>585</td><td>6</td><td>26</td><td>8.5</td><td>3-10</td><td>20</td><td>2</td></tr><tr><td colspan="14"><strong>Tier 2</strong></td></tr><tr><td>29</td><td>250.1010</td><td>-46.5335</td><td>338.23700</td><td>0.0666</td><td>...</td><td>CXOU J164002.4-463200</td><td>1203</td><td>...</td><td>212</td><td>8.7</td><td>3-10</td><td>30</td><td>0</td></tr><tr><td>30</td><td>250.5191</td><td>-46.7281</td><td>338.32231</td><td>-0.3243</td><td>...</td><td>CXOU J164204.5-464341</td><td>1408</td><td>...</td><td>177</td><td>7.2</td><td>3-10</td><td>30</td><td>0</td></tr><tr><td>31</td><td>248.3784</td><td>-47.4266</td><td>336.82764</td><td>0.2912</td><td>...</td><td>CXOU J163330.8-472535</td><td>40</td><td>...</td><td>11</td><td>6.0</td><td>3-78</td><td>30</td><td>0</td></tr><tr><td>32</td><td>248.6447</td><td>-47.2967</td><td>337.04544</td><td>0.2468</td><td>...</td><td>CXOU J163434.7-471748</td><td>139</td><td>...</td><td>20</td><td>5.6</td><td>10-20</td><td>30</td><td>0</td></tr><tr><td>33</td><td>250.0287</td><td>-46.4872</td><td>338.28012</td><td>0.0878</td><td>...</td><td>CXOU J164006.8-462913</td><td>1216</td><td>...</td><td>434</td><td>5.3</td><td>3-10</td><td>30</td><td>0</td></tr><tr><td>34</td><td>249.8351</td><td>-46.8352</td><td>337.93184</td><td>-0.0443</td><td>...</td><td>CXOU J163920.4-465006</td><td>1039</td><td>...</td><td>29</td><td>5.1</td><td>3-10</td><td>30</td><td>0</td></tr><tr><td>35</td><td>248.9010</td><td>-47.0967</td><td>337.31056</td><td>0.2536</td><td>...</td><td>CXOU J163536.2-470548</td><td>325</td><td>...</td><td>115</td><td>4.6</td><td>10-20</td><td>20</td><td>0</td></tr><tr><td>36</td><td>250.3453</td><td>-46.7582</td><td>338.22112</td><td>-0.2546</td><td>...</td><td>CXOU J164122.8-464529</td><td>1374</td><td>...</td><td>178</td><td>4.4</td><td>3-10</td><td>30</td><td>0</td></tr></tbody></table>
<table><caption>**Table 5**
(Continued)</caption><thead><tr><th>Src<br>No.<br>(1)</th><th>R.A.<br>(J2000 °)<br>(2)</th><th>Decl.<br>(J2000 °)<br>(3)</th><th>ℓ<br>(J2000 °)<br>(4)</th><th>b<br>(J2000 °)<br>(5)</th><th>Unc.<br>(")<br>(6)</th><th>Source Name<br>(7)</th><th>NARCS<br>ID<br>(8)</th><th>Offset<br>(")<br>(9)</th><th>Exp.<br>(ks)<br>(10)</th><th>No. Trials<br>(10<sup>x</sup>)<br>(11)</th><th>Band<br>(keV)<br>(12)</th><th>EEF<br>(%)<br>(13)</th><th>No.<br>Det.<br>(14)</th></tr></thead><tbody><tr><td>37</td><td>248.9518</td><td>-47.3590</td><td>337.14012</td><td>0.0512</td><td>...</td><td>CXOU J163548.4-472132</td><td>373</td><td>...</td><td>89</td><td>3.6</td><td>40-78</td><td>15</td><td>0</td></tr><tr><td>38</td><td>248.4062</td><td>-47.4119</td><td>336.85116</td><td>0.2874</td><td>...</td><td>CXOU J163337.4-472442</td><td>52</td><td>...</td><td>21</td><td>2.3</td><td>3-10</td><td>30</td><td>0</td></tr></tbody></table>

**Notes.** (1) NNR source ID. (2)–(5) R.A., decl., Galactic longitude, and Galactic latitude of source determined from centroid algorithm for tier 1 sources and adopting *Chandra* positions from Fornasini et al. (2014) for tier 2 sources. (6) 90% confidence positional uncertainty, including statistical and systematic uncertainties summed in quadrature. In most cases, the 90% confidence systematic uncertainty is 8"; however, for sources that were used to derive astrometric corrections (*), the 90% systematic uncertainty is estimated based on simulations (2" for NARCS 999 and 6" for all sources marked with *). Uncertainties for tier 2 sources are not provided, since the positions of these sources are simply set to the *Chandra* positions. (7) NARCS source name or other commonly used name for the source. For *NuSTAR* discoveries, a *NuSTAR* name is provided. (8) NARCS catalog ID number. (9) Angular distance between the source positions in *NuSTAR* and *Chandra* observations. For tier 2 sources, no offset is shown, since the *Chandra*-determined position is adopted for the *NuSTAR* analysis. (10) Total *NuSTAR* exposure, including both modules (FPMA and FPMB) and all observations used in measuring the photometric properties of the source (see Section 5 for details). (11) Maximum value from the trial maps at the location of the source. This value is the number of random trials required to produce the observed counts from a random background fluctuation. For extended sources, this is the maximum trial map value within 30" of the listed source location. (12) Energy band of the trial map in which the maximum trial value for the source is measured. (13) PSF enclosed energy fraction of the trial map in which the maximum trial value for the source is measured. (14) Total number of trial maps in which the source exceeds the detection threshold. There are 18 trial maps using six different energy bands and three different PSF enclosure fractions. (15) Tier 1 sources are those detected in at least two trial maps. Tier 2 sources are NARCS sources with 2–10 keV fluxes >6 × 10<sup>-6</sup> ph cm<sup>-2</sup> s<sup>-1</sup> that do not meet the *NuSTAR* detection threshold requirements but have S/N > 3 in the 3–10, 10–20, or 3–40 keV bands (S/N values can be found in Table 6).

a These large offsets are due to the fact that the positions for these extended sources were determined by eye in NARCS.

b Point source embedded in extended emission. We treat it as a point source and leave the detailed analysis of the extended emission to Gotthelf et al. (2014).

c Blend of two *Chandra* sources that are also blended in NARCS but resolved in *Chandra* ObsID 11008 (Rahoui et al. 2014).

d Extended source.

e For these transient sources, the exposure times listed only include observations in which the source was detected at a >2σ level.

f In *Chandra*, point source 402 is resolved within extended emission (404), but, in *NuSTAR*, the two are not distinguishable, so we treat it as an extended source.
with aperture selection. The default background regions are annuli with 60" inner radii and 90" outer radii. For NNR 8 and 21, which appear extended and are not fully contained within the default source regions, we adopted radii of 45" and 60" for the small and large circular source regions, respectively, and annular background regions with 80" inner radii and 110" outer radii. We adjusted the centers of the aperture regions for NNR 8 and 21 by 8" and 5", respectively, so that they were more centered with respect to the full extended emission rather than the peak of the emission.<fnref n="23" />

For about 1/3 of sources, it was necessary to modify the background aperture regions. In order to prevent contamination to the background from other sources, it is preferable for background regions not to extend within 60" of any tier 1 source. In addition, above 20 keV, as the relative contribution of the internal background becomes more significant, the background is fairly uniform across any given detector but differs between detectors (Harrison et al. 2013; Wik et al. 2014), so it is advantageous for the background region to be located on the same detector as the source region. Furthermore, when a source is located close to the edge of the FOV, using an annular background region may not sample a statistically large enough number of background counts. Finally, although we removed the most significant patches of stray light and ghost ray contamination from the NuSTAR observations, nonuniform low-level contamination remained. Thus, we modified the background region in situations where the default background region comes within 60" of any tier 1 source, the low-level contamination from stray light or ghost rays appears to differ significantly between the source and default background regions, or >50% of the annular background region falls outside the observation area or on a detector different from the one where the source is located. In these cases, we adopted a circle with a 70" radius for the background region and placed it in as ideal a location as possible following these criteria:

i. Keeping the region as close to the source as possible to minimize variations due to background inhomogeneities but at least 60" away from the source and any tier 1 sources.

ii. Maximizing the fraction of the background region area that falls on the same detector as the source region.

iii. Placing the background region at a location that exhibits a level of low-level stray light or ghost ray contamination similar to that of the source region.

For a given source, background aperture regions were defined for each observation and FPM individually, since stray light and ghost ray contamination and the fraction of the default annular background that lies on a given detector vary depending on the observation and the module. Furthermore, if a source fell close to the edge of an observation, such that >50% of the area of a 40" radius source region was outside the observation area, that observation was not used to extract photometric or spectral information for the source. Thus, the exposure value at the location of a source in the mosaicked exposure map may be higher than the effective exposure for the source based only on observations used for photometric analysis; the latter effective exposure is the value reported in Table 5. Table 6 provides the results of our aperture

photometry and includes flags that indicate which sources required modified background regions.

The only exceptions to this method of defining background regions are NNR 22 and 27. These sources are only separated by 47" and thus contaminate each other's default background regions, although they do not suffer from any additional background problems. Therefore, since annular background regions are preferable for minimizing the vignetting effect, we simply redefined their background regions as an annulus with an 80" inner radius and 110" outer radius centered between the two sources. Due to their proximity, the photometric and spectral properties of these sources as derived from the 40" radius circular apertures are less reliable than those from the 30" radius apertures.

## 5.2. Net Counts and Source Significance

Having defined aperture regions, we extracted the source and background counts for each source in each observation. We then calculated the expected number of background counts ($\langle c_{bkg} \rangle$) in each source region by multiplying the counts in the background region by the ratio $(A_{src}E_{src})/(A_{bkg}E_{bkg})$, where $A_{src}$ and $A_{bkg}$ are the areas in units of pixels and $E_{src}$ and $E_{bkg}$ are the exposures (without vignetting corrections) of the source and background regions, respectively. Then, for each source, we summed the source counts ($C_{src}$), total background counts ($C_{bkg}$), background counts expected in the source region ($\langle C_{bkg} \rangle$), and exposures across all observations and modules in seven different energy bands: 3–78, 3–40, 40–78, 3–10, 10–40, 10–20, and 20–40 keV. The $1\sigma$ errors in the total counts were calculated using the recommended approximations for upper and lower limits in Gehrels (1986). Then, the net source counts ($C_{net}$) were calculated by subtracting the total expected background counts in the source region from the total source counts.

In each energy band, we then calculated the S/N of the photometric measurements from the probability that the source could be generated by a noise fluctuation of the local background using the following equation from Weisskopf et al. (2007):

$$ P(\ge C_{src}|C_{bkg}; C_{net} = 0) = \sum_{c=C_{src}}^{C_{bkg}+C_{src}} \frac{(C_{bkg} + C_{src})!}{c!(C_{bkg} + C_{src} - c)!} \left(\frac{f}{1+f}\right)^c \times \left(1 - \frac{f}{1+f}\right)^{C_{bkg}+C_{src}-c}, \quad (1) $$

where $f = \langle C_{bkg} \rangle / C_{bkg}$. Using this probability, we defined the S/N as the equivalent Gaussian significance in units of the standard deviation (e.g., $P = 0.0013$ corresponds to $S/N = 3\sigma$). These S/N measurements were used to select which tier 2 sources to include in our catalog but not to set detection thresholds for tier 1 sources, which are determined by the trial maps. Only five sources have photometric measurements with $S/N \ge 3\sigma$ above 20 keV. Therefore, we focused the remainder of our analysis on the 3–40, 3–10, and 10–20 keV energy bands. Of the tier 2 source candidates, we only included those with $S/N \ge 3\sigma$ in at least one of these three energy bands, using either of the two source aperture regions, in our final source list. Table 6 provides the significance of each source in our final catalog in these three

<footnote n="23">²³ The adjusted locations of the aperture regions for NNR 8 and 21 are $(\alpha, \delta) = (248.9468, -47.6238)$ and $(248.9875, -47.3200)$, respectively.</footnote>
<table><caption>Table 6
Photometry</caption><thead><tr><th rowspan="2">Source No.<br>(1)</th><th rowspan="2">S/N<br>3–40 keV<br>(2)</th><th rowspan="2">S/N<br>3–10 keV<br>(3)</th><th rowspan="2">S/N<br>10–20 keV<br>(4)</th><th rowspan="2">Net Counts<br>3–40 keV<br>(5)</th><th colspan="2">Ph. Flux (10<sup>-6</sup> cm<sup>-2</sup> s<sup>-1</sup>)</th><th colspan="2">En. Flux (10<sup>-14</sup> erg cm<sup>-2</sup> s<sup>-1</sup>)</th><th rowspan="2">Hardness Ratio<br>(10)</th><th rowspan="2">E<sub>50</sub><br>(keV)<br>(11)</th><th rowspan="2">QR<br>(12)</th><th colspan="2">Var. Flag</th><th rowspan="2">Aperture Flag<br>(15)</th></tr><tr><th>3–10 keV<br>(6)</th><th>10–20 keV<br>(7)</th><th>3–10 keV<br>(8)</th><th>10–20 keV<br>(9)</th><th>NuST.<br>(13)</th><th>Chan.<br>(14)</th></tr></thead><tbody><tr><td colspan="14" style="text-align: left; font-weight: bold;">Tier 1</td></tr><tr><td>1</td><td>134534.5</td><td>142889.4</td><td>15742.0</td><td>3214900 ± 18000</td><td>598180 ± 340</td><td>19112 ± 55</td><td>473890 ± 270</td><td>38210 ± 110</td><td>-0.9246</td><td>5.3245</td><td>1.0334</td><td>1</td><td>—</td><td>pcm</td></tr><tr><td></td><td>130019.1</td><td>138317.1</td><td>15422.4</td><td>4079200 ± 2000</td><td>603320 ± 300</td><td>19060 ± 49</td><td>477130 ± 240</td><td>38119 ± 100</td><td>± 0.0008</td><td>± 0.0006</td><td>± 0.0004</td><td></td><td></td><td>pcm</td></tr><tr><td>2</td><td>616.4</td><td>350.0</td><td>648.6</td><td>37360 ± 200</td><td>1634 ± 14</td><td>1710 ± 14</td><td>1748 ± 14</td><td>3954 ± 32</td><td>-0.112</td><td>9.83</td><td>1.077</td><td>sp<sup>a</sup></td><td>slp</td><td>pcm</td></tr><tr><td></td><td>581.4</td><td>331.0</td><td>614.8</td><td>46720 ± 240</td><td>1623 ± 13</td><td>1694 ± 13</td><td>1737 ± 13</td><td>3914 ± 29</td><td>± 0.006</td><td>± 0.03</td><td>± +0.006</td><td></td><td></td><td>pcm</td></tr><tr><td>3</td><td>144.2</td><td>128.5</td><td>84.4</td><td>9590 ± 120</td><td>50.1 ± 0.8</td><td>21.5 ± 0.5</td><td>46.4 ± 0.7</td><td>48.7 ± 1.2</td><td>-0.41 ± 0.01</td><td>8.0 ± 0.1</td><td>0.90 ± +0.02</td><td>sp<sup>a</sup></td><td>slp</td><td>pcm</td></tr><tr><td></td><td>153.3</td><td>140.4</td><td>87.4</td><td>13550 ± 150</td><td>57.9 ± 0.8</td><td>23.3 ± 0.5</td><td>53.3 ± 0.7</td><td>52.7 ± 1.2</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>4</td><td>21.3</td><td>24.6</td><td>6.5</td><td>556 ± 32</td><td>49.1 ± 2.7</td><td>7.6 ± 1.3</td><td>40.3 ± 2.2</td><td>18.8 ± 3.2</td><td>-0.72 ± 0.06</td><td>6.4 ± 0.1</td><td>0.92 ± +0.10</td><td>1</td><td>slp</td><td>pcm</td></tr><tr><td></td><td>20.3</td><td>23.3</td><td>5.9</td><td>723 ± 40</td><td>50.8 ± 2.7</td><td>7.8 ± 1.4</td><td>41.8 ± 2.2</td><td>19.7 ± 3.4</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>5</td><td>23.0</td><td>22.9</td><td>9.6</td><td>842 ± 42</td><td>23.6 ± 1.3</td><td>6.4 ± 0.8</td><td>21.6 ± 1.2</td><td>13.7 ± 2.0</td><td>-0.55 ± 0.06</td><td>7.8 ± 0.3</td><td>0.93 ± +0.06</td><td></td><td></td><td>p</td></tr><tr><td></td><td>21.4</td><td>21.3</td><td>9.1</td><td>1087 ± 55</td><td>24.0 ± 1.4</td><td>6.9 ± 0.9</td><td>22.0 ± 1.2</td><td>14.7 ± 2.2</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>6</td><td>14.3</td><td>12.8</td><td>6.9</td><td>359 ± 29</td><td>77.8 ± 7.7</td><td>18.0 ± 3.2</td><td>67.7 ± 6.2</td><td>40.0 ± 7.1</td><td>-0.63 ± 0.09</td><td>6.5 ± 0.2</td><td>0.91 ± +0.14</td><td></td><td></td><td>pc</td></tr><tr><td></td><td>13.4</td><td>11.7</td><td>7.3</td><td>464 ± 38</td><td>76.0 ± 8.0</td><td>21.2 ± 3.3</td><td>67.6 ± 6.5</td><td>46.4 ± 7.5</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>7</td><td>17.5</td><td>20.6</td><td>1.7</td><td>621 ± 40</td><td>37.4 ± 2.2</td><td>1.1 ± 0.8</td><td>29.6 ± 1.7</td><td>1.7<sup>+2.0</sup><sub>-1.7</sub></td><td>-0.92 ± 0.08</td><td>5.5 ± 0.2</td><td>0.90 ± +0.06</td><td></td><td></td><td>pc</td></tr><tr><td></td><td>17.0</td><td>19.6</td><td>3.0</td><td>835 ± 53</td><td>38.1 ± 2.2</td><td>2.3 ± 1.0</td><td>30.6 ± 1.8</td><td>4.4 ± 2.2</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>8</td><td>24.9</td><td>22.3</td><td>14.4</td><td>884 ± 41</td><td>40.5 ± 2.3</td><td>17.0 ± 1.4</td><td>37.6 ± 2.0</td><td>37.4 ± 3.3</td><td>-0.41 ± 0.05</td><td>8.0 ± 0.2</td><td>0.90 ± +0.06</td><td></td><td></td><td>e</td></tr><tr><td></td><td>21.9</td><td>20.0</td><td>12.5</td><td>1083 ± 52</td><td>44.9 ± 2.5</td><td>17.6 ± 1.6</td><td>40.7 ± 2.2</td><td>38.3 ± 3.7</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>9</td><td>13.4</td><td>13.3</td><td>7.1</td><td>303 ± 26</td><td>33.9 ± 3.2</td><td>14.1 ± 2.5</td><td>32.1 ± 3.0</td><td>30.8 ± 5.6</td><td>-0.47 ± 0.09</td><td>7.5 ± 0.4</td><td>1.02 ± +0.11</td><td></td><td></td><td>p</td></tr><tr><td></td><td>11.9</td><td>12.5</td><td>5.8</td><td>371 ± 34</td><td>37.1 ± 3.4</td><td>12.9 ± 2.5</td><td>33.4 ± 3.0</td><td>28.5 ± 5.9</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>10</td><td>9.7</td><td>10.6</td><td>1.7</td><td>240 ± 27</td><td>84.0 ± 9.1</td><td>4.0<sup>+3.1</sup><sub>-2.8</sub></td><td>67.3 ± 7.1</td><td>8.0<sup>+7.0</sup><sub>-6.3</sub></td><td>-0.89<sup>+0.14</sup><sub>-0.11</sub></td><td>5.6 ± 0.3</td><td>0.83 ± +0.11</td><td>1</td><td>—</td><td>p</td></tr><tr><td></td><td>6.5</td><td>6.9</td><td>1.6</td><td>220 ± 35</td><td>56.2 ± 9.1</td><td>3.8 ± 3.1</td><td>46.5 ± 7.1</td><td>6.9 ± 6.9</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>11</td><td>17.1</td><td>18.8</td><td>6.8</td><td>1310 ± 81</td><td>9.8 ± 0.6</td><td>1.9 ± 0.3</td><td>8.3 ± 0.5</td><td>4.1 ± 0.8</td><td>-0.64 ± 0.07</td><td>6.4 ± 0.1</td><td>0.92 ± +0.08</td><td>1</td><td>slp</td><td>pcm</td></tr><tr><td></td><td>17.1</td><td>19.0</td><td>6.6</td><td>1830 ± 110</td><td>10.9 ± 0.6</td><td>2.1 ± 0.4</td><td>9.4 ± 0.5</td><td>4.6 ± 0.8</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>12</td><td>12.6</td><td>13.9</td><td>5.0</td><td>687 ± 58</td><td>11.1 ± 0.9</td><td>2.2 ± 0.5</td><td>9.7 ± 0.8</td><td>4.5 ± 1.2</td><td>-0.65 ± 0.09</td><td>6.6 ± 0.2</td><td>1.06 ± +0.15</td><td></td><td></td><td>pcm</td></tr><tr><td></td><td>12.2</td><td>13.7</td><td>5.0</td><td>929 ± 79</td><td>12.1 ± 1.0</td><td>2.5 ± 0.6</td><td>10.5 ± 0.8</td><td>5.3 ± 1.4</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>13</td><td>10.5</td><td>8.6</td><td>6.6</td><td>339 ± 35</td><td>10.1 ± 1.5</td><td>5.8 ± 1.0</td><td>9.7 ± 1.3</td><td>13.4 ± 2.4</td><td>-0.34 ± 0.11</td><td>8.9 ± 0.7</td><td>0.95 ± +0.11</td><td></td><td></td><td>p</td></tr><tr><td></td><td>9.3</td><td>7.4</td><td>6.1</td><td>418 ± 47</td><td>9.5 ± 1.6</td><td>5.9 ± 1.0</td><td>9.2 ± 1.3</td><td>13.5 ± 2.5</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>14</td><td>7.7</td><td>9.9</td><td>0.9</td><td>159 ± 23</td><td>20.9 ± 2.5</td><td>&lt;3.6</td><td>17.2 ± 2.1</td><td>&lt;8.8</td><td>&gt; -1</td><td>5.7 ± 0.4</td><td>1.11 ± +0.19</td><td></td><td></td><td>p</td></tr><tr><td></td><td>6.5</td><td>9.3</td><td>0.3</td><td>187 ± 30</td><td>21.6 ± 2.5</td><td>&lt;3.2</td><td>17.7 ± 2.1</td><td>&lt;7.8</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>15</td><td>6.0</td><td>7.8</td><td>0.6</td><td>89 ± 16</td><td>28.6 ± 4.4</td><td>&lt;3.9</td><td>23.5 ± 3.6</td><td>&lt;8.5</td><td>&gt; -1</td><td>5.6 ± 0.6</td><td>0.87 ± +0.13</td><td>s</td><td>slp</td><td>p</td></tr><tr><td></td><td>6.0</td><td>7.5</td><td>0.3</td><td>125 ± 22</td><td>31.2 ± 4.6</td><td>&lt;3.9</td><td>24.7 ± 3.7</td><td>&lt;9.0</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>16</td><td>9.6</td><td>8.5</td><td>4.2</td><td>287 ± 32</td><td>60.8 ± 8.9</td><td>10.2<sup>+3.0</sup><sub>-2.8</sub></td><td>54.9 ± 7.1</td><td>22.5<sup>+6.7</sup><sub>-6.1</sub></td><td>-0.71 ± 0.14</td><td>6.4 ± 0.3</td><td>0.85 ± +0.10</td><td></td><td></td><td>p</td></tr><tr><td></td><td>9.4</td><td>8.5</td><td>3.4</td><td>393 ± 44</td><td>65.7 ± 9.6</td><td>8.6 ± 2.8</td><td>60.2 ± 7.7</td><td>18.7 ± 6.3</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>17</td><td>7.8</td><td>8.1</td><td>3.0</td><td>215 ± 30</td><td>9.0 ± 1.5</td><td>2.5 ± 1.0</td><td>9.2 ± 1.3</td><td>5.0 ± 2.3</td><td>-0.62 ± 0.15</td><td>7.3 ± 0.5</td><td>1.13 ± +0.22</td><td></td><td></td><td>p</td></tr><tr><td></td><td>7.5</td><td>7.3</td><td>3.9</td><td>292 ± 40</td><td>9.2 ± 1.7</td><td>3.8 ± 1.1</td><td>9.3 ± 1.4</td><td>8.3 ± 2.6</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>18</td><td>6.3</td><td>6.6</td><td>1.9</td><td>134 ± 23</td><td>47.7 ± 8.0</td><td>3.7<sup>+2.4</sup><sub>-2.7</sub></td><td>38.1 ± 6.2</td><td>5.8<sup>+5.6</sup><sub>-5.8</sub></td><td>-0.78<sup>+0.21</sup><sub>-0.20</sub></td><td>5.9 ± 0.7</td><td>0.54 ± +0.09</td><td></td><td>s</td><td>pc</td></tr><tr><td></td><td>5.4</td><td>5.8</td><td>1.5</td><td>159 ± 30</td><td>47.8 ± 8.5</td><td>3.3<sup>+3.3</sup><sub>-3.1</sub></td><td>37.5 ± 6.6</td><td>5.6<sup>+7.4</sup><sub>-5.6</sub></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>19</td><td>10.3</td><td>11.7</td><td>2.7</td><td>399 ± 41</td><td>11.0 ± 1.1</td><td>1.6 ± 0.6</td><td>9.5 ± 0.9</td><td>3.7 ± 1.5</td><td>-0.77 ± 0.12</td><td>6.6 ± 0.2</td><td>1.09 ± +0.18</td><td>1</td><td>—</td><td>p</td></tr><tr><td></td><td>9.0</td><td>10.5</td><td>2.0</td><td>487 ± 56</td><td>11.1 ± 1.2</td><td>1.3 ± 0.7</td><td>9.4 ± 1.0</td><td>3.1 ± 1.6</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>20</td><td>5.9</td><td>6.7</td><td>3.3</td><td>126 ± 23</td><td>10.0 ± 1.9</td><td>4.5<sup>+1.5</sup><sub>-1.3</sub></td><td>9.4 ± 1.6</td><td>11.7<sup>+3.6</sup><sub>-3.3</sub></td><td>-0.53 ± 0.17</td><td>6.8 ± 0.6</td><td>1.27 ± +0.39</td><td>1</td><td>—</td><td>p</td></tr><tr><td></td><td>6.4</td><td>7.1</td><td>3.5</td><td>191 ± 31</td><td>11.8 ± 2.0</td><td>5.0 ± 1.4</td><td>11.1 ± 1.7</td><td>12.4 ± 3.3</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>21</td><td>13.4</td><td>13.5</td><td>6.1</td><td>312 ± 26</td><td>46.4 ± 4.1</td><td>12.1 ± 2.4</td><td>39.9 ± 3.5</td><td>26.0 ± 5.4</td><td>-0.58 ± 0.09</td><td>6.7 ± 0.3</td><td>0.79 ± +0.11</td><td></td><td></td><td>e</td></tr><tr><td></td><td>12.3</td><td>12.7</td><td>5.5</td><td>408 ± 35</td><td>52.3 ± 4.5</td><td>13.4 ± 2.7</td><td>45.4 ± 3.8</td><td>29.9 ± 6.3</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>22</td><td>6.0</td><td>7.4</td><td>1.4</td><td>96 ± 18</td><td>17.5 ± 2.7</td><td>2.1<sup>+2.0</sup><sub>-2.6</sub></td><td>15.0 ± 2.3</td><td>4.3<sup>+3.2</sup><sub>-3.3</sub></td><td>-0.75<sup>+0.22</sup><sub>-0.21</sub></td><td>6.9 ± 0.5</td><td>1.30 ± +0.43</td><td></td><td></td><td>p</td></tr><tr><td></td><td>5.9</td><td>7.6</td><td>1.4</td><td>132 ± 23</td><td>20.0 ± 2.9</td><td>2.3 ± 2.2</td><td>17.3 ± 2.4</td><td>4.3<sup>+5.1</sup><sub>-4.3</sub></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>23</td><td>6.0</td><td>5.9</td><td>2.7</td><td>108 ± 20</td><td>8.4 ± 1.8</td><td>2.6<sup>+1.1</sup><sub>-1.1</sub></td><td>7.6 ± 1.5</td><td>6.0<sup>+2.8</sup><sub>-2.8</sub></td><td>-0.55 ± 0.20</td><td>7.4 ± 0.9</td><td>0.91 ± +0.30</td><td></td><td></td><td>p</td></tr><tr><td></td><td>4.8</td><td>4.9</td><td>2.2</td><td>122 ± 26</td><td>7.5 ± 1.9</td><td>2.1 ± 1.1</td><td>7.1 ± 1.6</td><td>4.4 ± 2.6</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>24</td><td>6.7</td><td>5.0</td><td>4.0</td><td>198 ± 31</td><td>6.8 ± 1.2</td><td>2.3 ± 0.6</td><td>5.3 ± 1.0</td><td>4.7 ± 1.5</td><td>-0.37 ± 0.18</td><td>9.0 ± 2.1</td><td>0.35 ± +0.10</td><td></td><td></td><td>pcm</td></tr><tr><td></td><td>5.4</td><td>3.5</td><td>3.0</td><td>222 ± 42</td><td>5.9 ± 1.3</td><td>1.9 ± 0.7</td><td>4.6 ± 1.0</td><td>3.7 ± 1.6</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>25</td><td>6.0</td><td>5.8</td><td>1.4</td><td>98 ± 18</td><td>6.4 ± 1.5</td><td>1.4<sup>+0.9</sup><sub>-0.8</sub></td><td>6.4 ± 1.3</td><td>3.8<sup>+2.4</sup><sub>-2.1</sub></td><td>-0.74<sup>+0.25</sup><sub>-0.24</sub></td><td>8.1 ± 0.9</td><td>0.60 ± +0.25</td><td>1</td><td>—</td><td>p</td></tr><tr><td></td><td>6.2</td><td>5.5</td><td>2.0</td><td>144 ± 24</td><td>6.8 ± 1.6</td><td>1.8<sup>+1.0</sup><sub>-0.9</sub></td><td>6.7 ± 1.3</td><td>4.6<sup>+2.4</sup><sub>-2.2</sub></td><td></td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>
<table><caption>Table 6
(Continued)</caption><thead><tr><th rowspan="2">Source<br>No.<br>(1)</th><th rowspan="2">S/N<br>3–40 keV<br>(2)</th><th rowspan="2">S/N<br>3–10 keV<br>(3)</th><th rowspan="2">S/N<br>10–20 keV<br>(4)</th><th rowspan="2">Net Counts<br>3–40 keV<br>(5)</th><th colspan="2">Ph. Flux (10<sup>-6</sup> cm<sup>-2</sup> s<sup>-1</sup>)</th><th colspan="2">En. Flux (10<sup>-14</sup> erg cm<sup>-2</sup> s<sup>-1</sup>)</th><th rowspan="2">Hardness<br>Ratio<br>(10)</th><th rowspan="2">E<sub>50</sub><br>(keV)<br>(11)</th><th rowspan="2">QR<br>(12)</th><th colspan="2">Var. Flag</th><th rowspan="2">Aperture<br>Flag<br>(15)</th></tr><tr><th>3–10 keV<br>(6)</th><th>10–20 keV<br>(7)</th><th>3–10 keV<br>(8)</th><th>10–20 keV<br>(9)</th><th>NuST.<br>(13)</th><th>Chan.<br>(14)</th></tr></thead><tbody><tr><td rowspan="2">26</td><td>6.0</td><td>5.7</td><td>4.3</td><td>107 ± 19</td><td>11.5 ± 2.2</td><td>6.6 ± 1.7</td><td>9.5 ± 1.8</td><td>16.6 ± 4.1</td><td rowspan="2">-0.25 ± 0.16</td><td rowspan="2">8.3 ± 0.9</td><td rowspan="2">0.55 ± +0.16</td><td rowspan="2">1</td><td rowspan="2"></td><td rowspan="2">pcm</td></tr><tr><td>6.1</td><td>6.1</td><td>4.1</td><td>152 ± 26</td><td>13.2 ± 2.3</td><td>6.4 ± 1.7</td><td>11.5 ± 2.0</td><td>14.8 ± 4.1</td><td>-0.20 ± 0.14</td><td>8.3 ± 1.1</td><td>0.75 ± +0.18</td><td>p</td></tr><tr><td rowspan="2">27</td><td>7.3</td><td>6.0</td><td>5.3</td><td>179 ± 26</td><td>11.0 ± 2.1</td><td>8.5 ± 1.8</td><td>10.0 ± 1.8</td><td>20.3 ± 4.3</td><td rowspan="2">-0.17<sup>+0.36</sup><sub>-0.30</sub></td><td rowspan="2">10.3 ± 4.1</td><td rowspan="2">0.42 ± +0.26</td><td rowspan="2">1</td><td rowspan="2"></td><td rowspan="2">p</td></tr><tr><td>7.4</td><td>6.5</td><td>5.0</td><td>252 ± 35</td><td>13.9 ± 2.3</td><td>9.1 ± 1.9</td><td>12.4 ± 2.0</td><td>22.5 ± 4.6</td><td>-0.17<sup>+0.36</sup><sub>-0.30</sub></td><td>8.3 ± 1.1</td><td>0.75 ± +0.18</td><td>p</td></tr><tr><td rowspan="2">28</td><td>3.5</td><td>2.7</td><td>2.7</td><td>32<sup>+11</sup><sub>-10</sub></td><td>15.3<sup>+7.0</sup><sub>-5.9</sub></td><td>11.6<sup>+6.1</sup><sub>-4.9</sub></td><td>11.9<sup>+5.7</sup><sub>-4.9</sub></td><td>28.7<sup>+15.3</sup><sub>-12.2</sub></td><td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td></tr><tr><td>3.0</td><td>2.1</td><td>2.2</td><td>37 ± 13</td><td>11.5<sup>+6.6</sup><sub>-5.8</sub></td><td>10.4<sup>+4.9</sup><sub>-4.9</sub></td><td>9.4<sup>+5.6</sup><sub>-4.9</sub></td><td>25.7<sup>+14.8</sup><sub>-12.2</sub></td><td></td><td></td><td></td><td></td></tr><tr><td colspan="16" style="text-align: left; font-weight: bold;">Tier 2</td></tr><tr><td rowspan="2">29</td><td>5.5</td><td>6.0</td><td>2.3</td><td>169 ± 32</td><td>9.1 ± 1.7</td><td>2.3 ± 1.1</td><td>8.1 ± 1.4</td><td>5.0 ± 2.7</td><td rowspan="2">-0.60 ± 0.20</td><td rowspan="2">7.5 ± 0.8</td><td rowspan="2">0.94 ± +0.30</td><td rowspan="2">1</td><td rowspan="2"></td><td rowspan="2">pcm</td></tr><tr><td>4.7</td><td>5.5</td><td>3.0</td><td>201 ± 43</td><td>9.0 ± 1.8</td><td>3.5 ± 1.3</td><td>8.2 ± 1.5</td><td>7.5 ± 3.0</td><td>-0.61<sup>+0.42</sup><sub>-0.34</sub></td><td>6.3 ± 0.6</td><td>0.93 ± +0.41</td><td>pcm</td></tr><tr><td rowspan="2">30</td><td>4.7</td><td>6.1</td><td>0.9</td><td>128 ± 28</td><td>6.6 ± 1.3</td><td>&lt;2.4</td><td>6.1 ± 1.1</td><td>&lt;15.9</td><td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td></tr><tr><td>3.9</td><td>5.8</td><td>0.8</td><td>147 ± 39</td><td>7.9 ± 1.5</td><td>&lt;2.5</td><td>6.6 ± 1.2</td><td>&lt;6.2</td><td>-0.11<sup>+0.49</sup><sub>-0.39</sub></td><td>10.1 ± 3.8</td><td>0.68<sup>+0.56</sup><sub>-0.68</sub></td><td>pc</td></tr><tr><td rowspan="2">31</td><td>4.6</td><td>3.9</td><td>1.5</td><td>29<sup>+9</sup><sub>-8</sub></td><td>37.7<sup>+15.8</sup><sub>-13.3</sub></td><td>10.4<sup>+10.2</sup><sub>-7.5</sub></td><td>35.3<sup>+13.7</sup><sub>-11.6</sub></td><td>23.7<sup>+23.5</sup><sub>-17.2</sub></td><td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td></tr><tr><td>4.5</td><td>3.7</td><td>1.7</td><td>38<sup>+11</sup><sub>-10</sub></td><td>29.9<sup>+13.9</sup><sub>-12.2</sub></td><td>12.3<sup>+9.7</sup><sub>-7.7</sub></td><td>33.1<sup>+12.6</sup><sub>-11.0</sub></td><td>28.1<sup>+22.4</sup><sub>-17.6</sub></td><td>-0.49<sup>+0.34</sup><sub>-0.33</sub></td><td>6.2 ± 1.4</td><td>0.83 ± +0.38</td><td>p</td></tr><tr><td rowspan="2">32</td><td>2.5</td><td>1.9</td><td>1.9</td><td>18<sup>+9</sup><sub>-8</sub></td><td>9.0<sup>+6.4</sup><sub>-5.3</sub></td><td>8.5<sup>+6.2</sup><sub>-4.9</sub></td><td>8.4<sup>+5.6</sup><sub>-4.6</sub></td><td>19.1<sup>+14.6</sup><sub>-11.5</sub></td><td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td></tr><tr><td>3.0</td><td>1.6</td><td>2.3</td><td>32<sup>+12</sup><sub>-11</sub></td><td>10.0<sup>+6.8</sup><sub>-5.8</sub></td><td>11.7<sup>+6.5</sup><sub>-5.4</sub></td><td>8.1<sup>+5.7</sup><sub>-4.8</sub></td><td>26.4<sup>+15.3</sup><sub>-12.6</sub></td><td>-0.25 ± 0.23</td><td>7.4 ± 1.0</td><td>0.82 ± +0.37</td><td>p</td></tr><tr><td rowspan="2">33</td><td>2.5</td><td>3.4</td><td>0.5</td><td>113 ± 45</td><td>3.2 ± 1.3</td><td>&lt;1.6</td><td>3.2 ± 1.0</td><td>&lt;17.4</td><td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td></tr><tr><td>2.0</td><td>2.8</td><td>0.0</td><td>125 ± 63</td><td>2.9 ± 1.4</td><td>&lt;1.3</td><td>3.0 ± 1.1</td><td>&lt;3.1</td><td>-0.56<sup>+0.51</sup><sub>-0.44</sub></td><td>6.5 ± 2.0</td><td>0.32<sup>+0.50</sup><sub>-0.32</sub></td><td>p</td></tr><tr><td rowspan="2">34</td><td>3.1</td><td>3.3</td><td>1.5</td><td>46 ± 15</td><td>14.6 ± 4.7</td><td>5.1<sup>+4.2</sup><sub>-3.7</sub></td><td>12.6 ± 4.1</td><td>11.1<sup>+10.0</sup><sub>-8.9</sub></td><td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td></tr><tr><td>2.4</td><td>3.2</td><td>0.5</td><td>49 ± 21</td><td>17.4 ± 5.2</td><td>&lt;7.9</td><td>13.9 ± 4.5</td><td>&lt;18.7</td><td>-0.11<sup>+0.49</sup><sub>-0.39</sub></td><td>10.1 ± 3.8</td><td>0.68<sup>+0.56</sup><sub>-0.68</sub></td><td>p</td></tr><tr><td rowspan="2">35</td><td>4.1</td><td>3.6</td><td>3.3</td><td>85 ± 22</td><td>5.4 ± 1.6</td><td>3.4<sup>+1.2</sup><sub>-1.1</sub></td><td>4.4 ± 1.3</td><td>8.3<sup>+2.8</sup><sub>-2.5</sub></td><td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td></tr><tr><td>3.6</td><td>4.2</td><td>1.5</td><td>104 ± 30</td><td>6.6 ± 1.8</td><td>1.6 ± 1.1</td><td>5.7 ± 1.5</td><td>3.8 ± 2.5</td><td>-0.25 ± 0.23</td><td>7.4 ± 1.0</td><td>0.82 ± +0.37</td><td>p</td></tr><tr><td rowspan="2">36</td><td>3.5</td><td>1.7</td><td>3.2</td><td>101 ± 29</td><td>3.3 ± 1.5</td><td>3.5 ± 1.2</td><td>2.1 ± 1.2</td><td>8.4 ± 2.9</td><td rowspan="2">0.17 ± 0.31</td><td rowspan="2">11.8 ± 3.3</td><td rowspan="2">0.48<sup>+0.42</sup><sub>-0.48</sub></td><td rowspan="2">1</td><td rowspan="2">sl</td><td rowspan="2">p</td></tr><tr><td>4.2</td><td>2.4</td><td>3.5</td><td>168 ± 41</td><td>4.7 ± 1.7</td><td>4.4 ± 1.3</td><td>3.1 ± 1.4</td><td>10.6 ± 3.2</td><td>-0.56<sup>+0.51</sup><sub>-0.44</sub></td><td>6.5 ± 2.0</td><td>0.32<sup>+0.50</sup><sub>-0.32</sub></td><td>p</td></tr><tr><td rowspan="2">37</td><td>2.2</td><td>2.2</td><td>1.0</td><td>40 ± 18</td><td>3.4 ± 1.6</td><td>1.2<sup>+1.1</sup><sub>-1.0</sub></td><td>2.7 ± 1.3</td><td>3.2<sup>+2.7</sup><sub>-2.4</sub></td><td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td></tr><tr><td>3.4</td><td>3.2</td><td>1.4</td><td>84 ± 25</td><td>6.7 ± 1.9</td><td>1.8<sup>+1.2</sup><sub>-1.1</sub></td><td>5.0 ± 1.6</td><td>4.8<sup>+2.9</sup><sub>-2.7</sub></td><td>-0.56<sup>+0.51</sup><sub>-0.44</sub></td><td>6.5 ± 2.0</td><td>0.32<sup>+0.50</sup><sub>-0.32</sub></td><td>p</td></tr><tr><td rowspan="2">38</td><td>2.8</td><td>3.7</td><td>0.5</td><td>23<sup>+10</sup><sub>-9</sub></td><td>11.4<sup>+4.3</sup><sub>-3.8</sub></td><td>&lt;4.4</td><td>10.7<sup>+3.7</sup><sub>-3.2</sub></td><td>&lt;11.5</td><td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td></tr><tr><td>3.2</td><td>3.3</td><td>1.6</td><td>37 ± 12</td><td>11.4<sup>+4.3</sup><sub>-3.9</sub></td><td>3.6<sup>+2.6</sup><sub>-2.2</sub></td><td>10.0<sup>+3.6</sup><sub>-3.2</sub></td><td>8.9<sup>+6.2</sup><sub>-5.1</sub></td><td>-0.11<sup>+0.49</sup><sub>-0.39</sub></td><td>7.1 ± 1.6</td><td>0.61<sup>+0.60</sup><sub>-0.61</sub></td><td>p</td></tr></tbody></table>

Notes. (2)–(9) S/Ns, net counts, photon flux, and energy flux of the source in the specified energy bands. Values in the top (bottom) row for each entry are based on using source aperture regions with small (large) radii. All other table column values are based on using small aperture regions. (10) Hardness ratio is defined as $(H-S)/(H+S)$, where H represents the net counts in the 10–20 keV band and S represents the net counts in the 3–10 keV band. (11)–(12) Median energy in the 3–40 keV band and the y-value of the quantile plot, defined as $3(E_{25}-3 \text{ keV})/(E_{75}-3 \text{ keV})$. (13) Flags indicating source variability: “s” = short timescale ($<$ a few hours) variability, “l” = long timescale (weeks–years) variability, “p” = periodic modulations detected. See Section 5.4 for details. (14) Variability flags from Fornasini et al. (2014): “s” = short timescale ($<$ a few hours) variability (within a single observation, the Kolmogorov–Smirnov test probability that the source is constant is $<$0.3%), “l” = long timescale (days–weeks) variability (the 0.5–10, 0.5–2, or 2–10 keV photon flux varies by $>$3σ between NARCS observations), “p” = periodic modulations detected by the $Z_n^2$ test, “–” = source not detected in NARCS. (15) “p” = point-source region aperture (circle with 30′′/40′′ radius), “e” = extended source aperture (circle with 45′′/60′′ radius), “c” = background region is a circle with 70′′ radius offset from the source rather than an annulus centered on the source, “m” = stray light and background spatial variations require background regions to be modified for each observation.

<footnote>a Periodic variability for NNR 2 detected by Bodaghee et al. (2016) and for NNR 3 by Gotthelf et al. (2014).</footnote>
energy bands, the net counts in the 3–40 keV band, and additional photometric properties described in the following sections. We estimate that local spatial variations of the background could affect the S/N values reported in this table by ±0.4σ and change the measured net counts and fluxes by ±5%, variations that are smaller than the statistical uncertainties of the photometric measurements.

### 5.3. Photon and Energy Fluxes

In Section 5.7, we describe how we derived fluxes from spectral modeling. However, for all sources, we also derived fluxes in a model-independent way, since the spectral fitting of faint sources is prone to significant uncertainty. For each source and background region in each observation and module, we used *nuproducts* to extract a list of photon counts as a function of energy and generate both an ARF and a response matrix file (RMF); the ARFs are scaled by the PSF energy fraction enclosed by the aperture region. We first calculated the source photon flux within each observation and module in the 3–10 and 10–20 keV bands by dividing the counts in each channel by the corresponding ARF, summing all these values within the given energy band, and dividing by the source region exposure. The estimated background contribution, scaled from the photon flux measured in the background region, was subtracted. These photon flux measurements assume a quantum efficiency of 1; this is a decent approximation for the *NuSTAR* CdZnTe detectors, which have a quantum efficiency of 0.98 over the vast majority of the *NuSTAR* energy range (Bhalerao 2012). If the significance of a source in a particular observation was <1σ, then we calculated a 90% confidence upper limit to its photon flux by converting the probability distribution of true source counts (from Equation (A21) in Weisskopf et al. 2007) to a photon flux distribution using the source region effective area.

For the five transient sources that were detected by *NuSTAR* but not by NARCS, we looked at the light curves of the 3–10 keV photon fluxes to check whether they are detected at >2σ confidence in individual *NuSTAR* observations. We found that NNR 1 is only detected in ObsIDs 40014008002 and 40014009001, NNR 10 is only detected in ObsID 40014007001 (which is consistent with T14), and NNR 19 is only detected in ObsIDs 30002021005, 30002021007, 30002021009, 30002021011, and 30002021013. Excluding the observations in which the transient sources are not detected, we reevaluated their 3–40 keV net counts and source significance as described in Section 5.2 and continued to exclude these observations for these sources when determining their other average photometric and spectral properties. Thus, the photometric and spectral properties derived for NNR 1, 10, 19, and 25 should be considered as their average properties during high flux states.

For each source, we then computed average 3–10 and 10–20 keV photon fluxes by combining the count lists and ARFs from different observations and modules. These measurements are presented in Table 6. We also calculated the average 3–10 and 10–20 keV energy flux for each source using the same model-independent method but with the additional step of multiplying the source counts in each channel by the channel energy. Fluxes derived using the two different source region sizes are in 1σ agreement with one another, except for three sources that are located in regions of diffuse emission or ghost rays and thus do not appear as exactly

point-like. Comparing the model-independent fluxes with those we derived from spectral modeling (see Section 5.7) for tier 1 sources, we find that they are in good agreement when using the smaller aperture regions but show a significant number of discrepancies at >2σ confidence when using the larger aperture regions. In the larger aperture regions, while the net number of source counts is higher, so is the background/source count ratio, which is why in most cases the source significance derived from the larger aperture regions is slightly lower. As a result, accurate background subtraction is more important when using the larger aperture regions, and it is not surprising that our crude subtraction method, which assumes a spectrally flat background, for the model-independent fluxes leads to discrepancies with the spectral fluxes.

### 5.4. X-Ray Variability

*NuSTAR*'s high time resolution allows us to characterize the timing properties of detected sources over a range of timescales. *NuSTAR*'s time resolution is good to ~2 ms rms after being corrected for thermal drift of the onboard clock, and the absolute accuracy is known to be better than <3 ms (Mori et al. 2014; Madsen et al. 2015). For our timing studies, all photon arrival times were converted to barycentric dynamical time (TDB) using the *NuSTAR* coordinates of each point source.

To characterize the source variability on ~hourly timescales, we used the Kolmogorov–Smirnov (KS) statistic to compare the temporal distributions of X-ray events extracted from source and background apertures in the 3–20 keV energy band. The background light curve acts as a model for the count rate variations expected in the source region due to the background. The maximal difference between the two cumulative normalized light curves gives the probability that they are drawn from the same distribution, i.e., that the light curve in the source region is consistent with that expected from the background plus a source with constant flux. Any source with a KS statistic lower than 0.05% in any observation is flagged as short-term variable by an “s” in Table 6. For each source, we ran the KS test independently for each of the observations in which it was covered. Since the KS test was applied 160 times in total, the adopted threshold corresponds to ≲1 spurious detection. We identified two sources as variable using the KS test. An examination of the light curves of these sources, NNR 2 (presented in B16) and NNR 15 (Figure 5), shows clear variability on ~hourly timescales.

We checked for variability of the NNR sources on week-to-year timescales by comparing the flux detected between repeated *NuSTAR* observations. Sources were flagged as long-term variable with an “l” in Table 6 if their 3–10 keV photon flux differed by >3σ based on their flux measured uncertainties; given the number of flux comparisons performed, this 3σ threshold should result in ≲1 spurious detection. NNR 1, 10, 11, 19, and 29 were found to be variable using this criterion. In addition, we compared *Chandra* and *NuSTAR* fluxes to check for variability on year timescales. For all sources with sufficient photon statistics, we compared the joint spectral fits to *Chandra* and *NuSTAR* data (see Section 5.7 for details) and identified sources with normalizations that differed at the >90% confidence level. Since we performed these joint fits for 24 sources, we would expect as many as 2 spurious detections of variability. But we made the criterion more stringent by requiring that, for a source to be considered
<figure><img src="image_7.png" /><figcaption>**Figure 5.** Light curve of NNR 15 in the *NuSTAR* 3–20 keV band from ObsID 40014016001, FPMA and FPMB combined, as measured from an aperture region with a 30′′ radius (top) and a 40′′ radius (middle). The light curve exhibits evident short-term variability. The bottom panel displays the light curve extracted from the background aperture region. The blue dashed lines in the top two panels show the mean background count rate scaled by the source region area. The light curves display the average count rate in each 1 ks time interval; note that during some of these time intervals, the effective exposure time is less than 1 ks due to Earth occultations or periods of poor data quality.</figcaption></figure>

variable between the *Chandra* and *NuSTAR* observations, its *Chandra* and *NuSTAR* normalizations must be inconsistent regardless of which of three different spectral models is adopted. This more selective criterion is only met by NNR 4, 11, and 27. For fainter sources (NNR 29–38), we considered a range of spectral models that would be consistent with their quantile values and assessed whether their 2–10 keV *Chandra* flux was incompatible with their average 3–10 keV *NuSTAR* flux at >90% confidence, regardless of the spectral model assumed. NNR 28, 35, and 36 are found to be variable by this criterion. In Table 7, we provide maximum photon fluxes and the ratio of maximum and minimum fluxes for all *NuSTAR* sources that demonstrate X-ray variability; the transient sources, NNR 1, 10, 19, 20, and 25—which are detected by *NuSTAR* but not detected in NARCS—are flagged as long-term variable and included in this table as well.

We searched for a periodic signal from the *NuSTAR* sources with sufficient counts to detect a coherent timing signal, determined as follows. The ability to detect pulsations depends strongly on the source and background counts and the number of search trials. For a sinusoidal signal, the aperture counts (source plus background) necessary to detect a signal of pulsed fraction $f_p$ is $N = 2S/f_p^2$, where S is the power associated with the single trial false detection probability of a test signal $\rho = e^{-S/2}$; S is distributed as $\chi^2$ with two degrees of freedom (van der Klis 1989). In practice, for a blind search, we need to take into account the number of frequencies tested, $N_{trials} = T_{span}/f_{Nyq}$, when $T_{span}$ is the data span and $f_{Nyq} = 250$ Hz, the effective *NuSTAR* Nyquist frequency. In computing N, we must allow for the reduced sensitivity of the search due to background contamination in the source aperture ($N_b$); the minimum detectable pulse fraction $f_p$ (min) is then increased by $(N_s + N_b)/N_b$.

We computed the detectability in individual observations for each source in our sample and considered those suitable for a pulsar search, with $f_p$ (min) > 50% at the $3\sigma$ level. For the three brightest sources in the Norma survey, the timing properties are already presented elsewhere: (i) the

<table><caption>**Table 7**
X-Ray Variability</caption><thead><tr><th>Source No. (1)</th><th>NuSTAR Var. Flag (2)</th><th>Maximum 3–10 keV Flux (10<sup>-6</sup> ph cm<sup>-2</sup> s<sup>-1</sup>) (3)</th><th>Variability Amplitude 3–10 keV (4)</th><th>Criteria for Long-Term Var. Detection (5)</th></tr></thead><tbody><tr><td colspan="5"><strong>Tier 1</strong></td></tr><tr><td>1</td><td>1</td><td>641200 ± 700</td><td>>427500</td><td>T, N</td></tr><tr><td>2</td><td>sp</td><td>10100 ± 700</td><td>>34</td><td>...</td></tr><tr><td>4</td><td>1</td><td>71<sup>+2</sup><sub>-5</sub></td><td>1.5<sup>+0.1</sup><sub>-0.2</sub></td><td>CS</td></tr><tr><td>10</td><td>1</td><td>84 ± 9</td><td>>56</td><td>T, N</td></tr><tr><td>11</td><td>1</td><td>26 ± 4</td><td>>18</td><td>N, CS</td></tr><tr><td>15</td><td>s</td><td>220 ± 40</td><td>>6</td><td>...</td></tr><tr><td>19</td><td>1</td><td>11 ± 1</td><td>>7</td><td>T, N</td></tr><tr><td>20</td><td>1</td><td>10 ± 2</td><td>>2</td><td>CS</td></tr><tr><td>25</td><td>1</td><td>6 ± 1</td><td>>4</td><td>T</td></tr><tr><td>27</td><td>1</td><td>11 ± 2</td><td>2.2<sup>+1.6</sup><sub>-0.9</sub></td><td>CS</td></tr><tr><td>28</td><td>1</td><td>15<sup>+7</sup><sub>-6</sub></td><td>11<sup>+8</sup><sub>-6</sub></td><td>CQ</td></tr><tr><td colspan="5"><strong>Tier 2</strong></td></tr><tr><td>29</td><td>1</td><td>40 ± 7</td><td>6.5 ± 1.3</td><td>N</td></tr><tr><td>35</td><td>1</td><td>13 ± 2</td><td>2.5 ± 0.6</td><td>CQ</td></tr><tr><td>36</td><td>1</td><td>9<sup>+2</sup><sub>-1</sub></td><td>2.8 ± 1.4</td><td>CQ</td></tr></tbody></table>

**Notes.** (2) *NuSTAR* variability flag: “s” = short timescale (< a few hours) variability, “l” = long timescale (weeks–years) variability, “p” = periodic modulations detected. See Section 5.4 for details. (3) Maximum 3–10 keV photon flux from either *Chandra* or *NuSTAR* photometry (based on 30′′ radius aperture regions). (4) Ratio of maximum to minimum 3–10 keV photon fluxes. (5) Criteria by which long-term variability was determined for sources flagged with “l”: T = transient source is detected by *NuSTAR* but falls below the survey sensitivity of NARCS, N = photon flux varies by >3σ between different *NuSTAR* observations, CS = cross-normalization between *Chandra* and *NuSTAR* spectra is inconsistent at >90% confidence, CQ = *Chandra* 2–10 keV and *NuSTAR* 3–10 keV photon fluxes are inconsistent at >90% confidence when adopting a range of spectral models consistent with the quantile values of the source.

quasi-periodic oscillations of the BH binary 4U 1630–472 (NNR 1), extensively studied using the *Rossi X-Ray Timing Explorer* (Dieters et al. 2000; Tomsick & Kaaret 2000; Seifina et al. 2014); (ii) the HMXB pulsar IGR J16393–4643 (NNR 2), with a period of 904 s, whose spin-up rate was determined from recent *NuSTAR* observations (B16); and (iii) the *NuSTAR*-discovered 206 ms pulsar PSR J1640–4631 (NNR 3), associated with the TeV source HESS J1640–465 (G14; Archibald et al. 2016).

For NNR 4, 5, 8, and 21, we extracted event lists in the 3–20 keV band from $r = 40''$ radius apertures and searched for periodic signals between 4 ms and 100 s. For each source, we evaluated the power at each frequency (oversampling by a factor of 2) using the unbinned $Z_n^2$ test statistic (Buccheri et al. 1983) summed over $n = 1, 2, 3, 5$ harmonics, to be sensitive to both broad and narrow pulse profiles. We repeated our search for an additional combination of energy ranges $3 < E < 25$ keV, $3 < E < 10$ keV, $10 < E < 25$ keV, and $10 < E < 40$ keV and aperture size $r < 20''$ and $r < 30''$. For all these searches, no significant signals were detected. For NNR 5 and 8, we can constrain the pulsed fraction of X-ray emission to be <45% and <48%, respectively, at the $3\sigma$ confidence. We also performed periodic searches for longer periods, with special attention to NNR 4, for which *Chandra* detected a 7150 s period, but we were unable to pick out any
<table><caption>**Table 8**

Properties of *Chandra* Counterparts to *NuSTAR* Discoveries</caption><thead><tr><th>Source<br>No.<br>(1)</th><th>R.A.<br>J2000 (°)<br>(2)</th><th>Decl.<br>(3)</th><th>Position<br>Uncertainty (′′)<br>(4)</th><th>Significance<br>0.5–10 keV<br>(5)</th><th>Net Counts<br>0.5–10 keV<br>(6)</th><th>E<sub>50</sub><br>(keV)<br>(7)</th><th>QR<br>(8)</th></tr></thead><tbody><tr><td>19</td><td>250.315033</td><td>−46.540543</td><td>0.68</td><td>15</td><td>245<sup>+17</sup><sub>−16</sub></td><td>2.9 ± 0.2</td><td>0.92 ± 0.06</td></tr><tr><td>20</td><td>250.591644</td><td>−46.716049</td><td>0.87</td><td>2.9</td><td>3<sup>+3</sup><sub>−2</sub></td><td>...</td><td>...</td></tr><tr><td>25</td><td>248.999542</td><td>−47.807671</td><td>0.71</td><td>6</td><td>33<sup>+7</sup><sub>−6</sub></td><td>2.3 ± 0.4</td><td>0.9 ± 0.3</td></tr></tbody></table>

Note. (4) 90% statistical and systematic positional uncertainties summed in quadrature.

ª The *Chandra* counterpart of NNR 20 has too few counts to perform quantile analysis. The energies of the three photons attributed to this source are 4.2, 5.7, and 7.0 keV. Since the *Chandra* effective area is higher at softer energies, the fact that no photons are detected with energies <4 keV suggests that this source is subject to high levels of absorption.

signals that could clearly be attributed to the *NuSTAR* sources due to the artifacts introduced by *NuSTAR*'s orbital occultations to the Fourier power spectrum.

## 5.5. Chandra Follow-Up of NuSTAR Discoveries

As discussed in Section 2, we triggered *Chandra* follow-up observations for the four sources discovered by *NuSTAR*: NNR 10, 19, 20, and 25. NNR 10, 19, and 25 were not detected by NARCS despite its much higher sensitivity compared to the *NuSTAR* Norma survey, indicating that these are transient sources. NNR 20 falls outside the area surveyed by *Chandra*, but our follow-up *Chandra* observations show that its flux is also highly variable.

The analysis of the *Chandra* follow-up of NNR 10 is presented in T14, while the analysis of the other three observations, which are listed in Table 3, is described here. The archival *Chandra* observation 7591 (see Table 2, which provides additional coverage of NNR 19) was also subjected to the same analysis. The *Chandra* observations were processed using CIAO version 4.7 adopting standard procedures. Then, we used *wavdetect* to determine the positions of the *Chandra* sources in the vicinity of the *NuSTAR* sources. The statistical uncertainties of the *Chandra* positions were calculated using the parameterization in Equation (5) of Hong et al. (2005); the 90% statistical uncertainty was then combined with *Chandra*'s 0.64 systematic uncertainty<fnref n="24" /> in quadrature. Since NNR 19 was also detected in an archival *Chandra* observation, we averaged the positions determined from ObsIDs 7591 and 16170. The *Chandra* positions and uncertainties are reported in Table 8. The *Chandra* follow-up observations of NNR 19, 20, and 25 are shown in Figure 6, where green circles indicate the *NuSTAR* source positions and magenta circles show the locations of the nearest *Chandra* sources.

The closest *Chandra* source to NNR 19 is located at a distance of 13.2, which is outside of the 90% confidence *NuSTAR* error circle. However, as noted in Table 5, a few of the NARCS counterparts have similarly large offsets, suggesting that in some cases the systematic *NuSTAR* positional uncertainties may be underestimated. The fact that only 3 days elapsed between the *NuSTAR* and *Chandra* observations of NNR 19 strengthens the case that these sources are indeed associated. Furthermore, this *Chandra* source was detected in 2007 in *Chandra* ObsID 7591 but undetected in 2011 in ObsID 12508; the fact that this *Chandra* source is a transient boosts the probability that it is the counterpart of NNR 19.

The only *Chandra* source in the vicinity of NNR 20 lies within the *NuSTAR* error circle but is only detected at 2.9σ confidence. NNR 20 was not covered by previous *Chandra* observations, including NARCS; thus, before our follow-up observation (ObsID 16171), we did not know whether this source was a transient. Based on its *NuSTAR* 3–10 keV flux, we would have expected to detect at least 10 counts from its *Chandra* counterpart if it was persistent. Thus, even if it is not definite that the weak *Chandra* detection is truly the counterpart of NNR 20, the lack of any brighter *Chandra* sources proves that NNR 20 is a variable source.

Follow-up observations of NNR 25 were performed 34 days after the *NuSTAR* observations, and a *Chandra* source is clearly detected within the *NuSTAR* error circle. This *Chandra* source was not detected during the 2011 NARCS observations; its transient nature boosts the probability that it is the true counterpart of the transient NNR 25. As was done by F14 for all of the NARCS sources, we searched for infrared counterparts to the *NuSTAR*-discovered sources in the VVV survey. We did not find any infrared counterparts to NNR 19, 20, or 25 within the 95% uncertainty of the *Chandra*-derived positions.

In order to extract photometric and spectral information for each *Chandra* counterpart, we defined source aperture regions as circles with 2.5 radii and background regions as annuli with 15" inner radii and 44" outer radii. As the counterpart of NNR 19 was at a larger angular offset from the *Chandra* aimpoint in ObsID 7591, and the *Chandra* PSF increases in size with angular offset, the circular source region used for this observation had a 5" radius. For each source in each *Chandra* observation, we calculated the net 0.5–10 keV counts, detection significance, and quantile values (see Section 5.6), which are provided in Table 8.

## 5.6. Hardness Ratio and Quantile Analysis

Since spectral fitting can be unreliable or impractical for faint sources, we used hardness ratios and quantile values (Hong et al. 2004) to probe and compare the spectral properties of the *NuSTAR* sources. In order to reduce the level of background contamination and prevent the hardness ratios and quantile values from being skewed toward the values of the *NuSTAR* background, we opted to use the aperture regions with smaller radii to derive these spectral parameters. The hardness ratio for each source is calculated as $(H - S)/(H + S)$, where H is the counts in the hard (10–20 keV) band and S is the counts in the soft (3–10 keV) band. The *NuSTAR* hardness ratios are listed in Table 6.

<footnote n="24">²⁴ See http://cxc.harvard.edu/cal/ASPECT/celmon.</footnote>
<figure><img src="image_8.png" /><figcaption>**Figure 6.** *Chandra* follow-up observations of NuSTAR transients in the 0.5–10 keV band (see Table 3). NuSTAR source positions are shown with 90% confidence error circles in green, and the locations of the nearest Chandra sources are indicated with 90% confidence error circles in magenta. The NuSTAR and Chandra positional uncertainties are provided in Tables 5 and 8 and are approximately 10'' and 0.''7, respectively, for all three sources.</figcaption></figure>

While hardness ratios are the most widely used proxy for spectral hardness of faint X-ray sources, they are subject to selection effects associated with having to choose two particular energy bands, and they do not yield meaningful information for sources that have zero net counts in one of the two energy bands. Therefore, we also calculated quantile values for each source in the 3–40 keV band; these values are the median energy ($E_{50}$) and the energies below which 25% and 75% of the source counts reside ($E_{25}$ and $E_{75}$, respectively). The latter energies were combined into a single quantile ratio (QR), which is a measure of how broad or peaked the spectrum is and is defined as $QR = 3(E_{25} - E_{min})/(E_{75} - E_{min})$, where $E_{min}$ is the lower bound of the energy band: 3 keV for NuSTAR and 0.5 keV for Chandra. The NuSTAR median energy and QR value of each source are provided in Table 6 and shown in Figure 7(a). The gridlines in the figure indicate where a source with a particular blackbody, bremsstrahlung, or power-law spectrum would fall in the NuSTAR quantile space. Gridlines that are roughly vertical represent different temperatures ($kT$) or photon indices ($\Gamma$), while roughly horizontal gridlines represent different values of the absorbing column density along the line of sight to the source ($N_H$).

Figure 7(b) shows the quantile values of the *Chandra* counterparts of the NuSTAR sources in the 0.5–10 keV band. Most of these values are taken from the NARCS catalog (F14). The quantile values for the *Chandra* counterparts of NNR 19 and 25 were derived using the aperture regions described in Section 5.5; the values for NNR 19 derived from ObsIDs 7591 and 16170 were combined in a weighted average. The *Chandra* counterpart of NNR 20 only has 3 counts, which are too few for quantile analysis; however, all three photons have energies >4 keV, indicating that this source is subject to significant absorption because *Chandra*'s effective area peaks below 2 keV. Finally, we did not adopt the NARCS catalog quantile values for the extended sources because they were derived using aperture regions whose position and extent were determined by eye and that removed embedded point sources not distinguishable with NuSTAR. Therefore, we recalculated the quantile values for the extended sources using circular aperture regions with 45'' radii centered on the NuSTAR-determined positions of NNR 8 and 21. These *Chandra* quantiles are weighted averages of the values derived from

ObsIDs 12528 and 12529<sup>25</sup> for the counterpart of NNR 8 and ObsIDs 12523 and 12526 for the counterpart of NNR 21.

As can be seen in Figure 7, the *Chandra* quantiles can easily differentiate between foreground sources and those subject to high levels of absorption due to gas along the line of sight. The integrated column density of neutral and molecular hydrogen due to the interstellar medium along the line of sight in the Norma region varies from 4 to $9 \times 10^{22} \text{cm}^{-2}$, as derived from the sum of $N_{HI}$ measured by the Leiden/Argentine/Bonn survey (Kalberla et al. 2005) and $N_{H_2}$ estimated from the Millimeter-Wave Telescope CO survey (Bronfman et al. 1989) using the $N_{H_2}/I_{CO}$ factor from Dame et al. (2001). Since these surveys have 0.5 resolution, the interstellar $N_{HI+H_2}$ values we derive are averages over 0.25 deg<sup>2</sup> regions, so it is possible that the interstellar absorption is actually higher or lower along particular lines of sight due to the clumpy nature of molecular clouds. Thus, the sources whose X-ray spectra show column densities in excess of these values may be located behind dense molecular clouds or suffer from additional absorption due to gas or dust local to the X-ray source. The NuSTAR quantiles are not particularly sensitive to $N_H$ but instead are able to separate sources with intrinsically soft and hard spectra, regardless of their level of absorption. Thus, the combination of quantile values in the *Chandra* and NuSTAR bands allows us to learn a fair amount about the spectral properties of sources that are too faint for spectral fitting and provide a check on spectral fitting results that can depend on the choice of binning for low photon statistics.

## 5.7. Spectral Analysis

For all tier 1 sources with >100 net counts in the 40'' radius aperture in the 3–40 keV band, we performed spectral analysis using XSPEC version 12.8.2 (Arnaud 1996), jointly fitting the NuSTAR and Chandra data when it was available. All spectral parameters were tied together for these joint fits, except for a

<footnote n="25"><sup>25</sup> The *Chandra* counterpart of NNR 8 is also observed in ObsID 12525. However, in this observation, a nearby transient point source that falls within the aperture region is visible. Comparing the 3–10 keV photon fluxes of NNR 8 in *Chandra* and *NuSTAR*, it does not appear that this nearby transient was present during the *NuSTAR* observation, and therefore we decided not to include ObsID 12525 in our *Chandra* analysis.</footnote>
<figure><img src="image_9.png" /><figcaption>**Figure 7.** Quantile diagrams showing the quantile ratio on the y-axis and the median energy on the x-axis (or median energy "normalized" by the *Chandra* 0.5–10 keV band for panel (a)). Quantile values of tier 1 sources are shown with black circles, and those of tier 2 sources are shown with gray triangles. Comparing the positions of sources in the quantile diagrams to the spectral model gridlines provides a rough measurement of their spectral parameters. The *Chandra* quantiles are very sensitive to the amount of absorption suffered by a source, while the *NuSTAR* quantiles are more useful for separating sources with different spectral slopes. To improve the legibility of the plots, $1\sigma$ error bars have been scaled down by 50%. As a visual aide, the corner boxes in each plot show the mean $1\sigma$ uncertainty for the tier 1 sources and the same mean error scaled by 50%. (a) The *NuSTAR* 3–40 keV background has $E_{50} = 10–15$ keV and $QR = 0.4–0.6$, which is why several tier 2 sources, which are most affected by the background, are found near that position in the diagram. Grids representing absorbed bremsstrahlung, blackbody, and power-law models are shown in blue, green, and orange, respectively. Roughly vertical grid lines represent different values of the temperature ($kT$) or photon index ($\Gamma$). Primarily horizontal grid lines represent $N_H = 10^{22}$, $10^{23}$, and $5 \times 10^{23}$ cm$^{-2}$ from bottom to top. (b) A grid of a power-law spectral model attenuated by interstellar absorption is overlaid. Red (primarily vertical) lines represent values of the photon index $\Gamma = 0, 1, 2, 3$, and 4 from right to left. Blue (primarily horizontal) lines represent values of the hydrogen column density $N_H = 0.01, 0.1, 0.4, 1, 4, 10$, and 40 in units of $10^{22}$ cm$^{-2}$ from bottom to top.</figcaption></figure>
cross-normalization factor between the *Chandra* and *NuSTAR* observations that was left as a free parameter to account for source variability and differences in instrumental calibrations (measured to be consistent to 10% precision; Madsen et al. 2015). We also included a cross-normalization constant between NuSTAR FPMA and FPMB in our models; for most sources, due to limited photon statistics, the errors on this normalization constant are large, and the constant is consistent with 1.0 to better than 90% confidence. Thus, for the *NuSTAR* sources detected with lowest significance (i.e., with trial map values <10¹⁵), we fixed the FPMA/FPMB normalization constant to 1. To maximize the number of counts per spectral bin, we used the larger aperture source regions to extract information for spectral fitting; however, for NNR 22 and 27, which are only separated by 47"), we extracted spectral information from 30" source regions to limit the blending of the two sources. The spectra of the *Chandra* counterparts were extracted as described in F14 for the NARCS sources and Section 5.5 for the counterparts of the *NuSTAR* discoveries; however, for the extended counterparts of NNR 8 and 21, we defined the aperture regions as 60" radius circles centered on the *NuSTAR*-derived position in order to match the *NuSTAR* extraction region.

The *Chandra* and *NuSTAR* spectra were grouped into bins of >2–10σ confidence, depending on the net counts of each source. For the three brightest sources, which have been carefully analyzed in other papers, we adopted simplified versions of the best-fitting models found in King et al. (2014), B16, and G14 in order to easily measure their observed and unabsorbed fluxes in the 3–10 and 10–20 keV bands, which we used to calculate the logN–logS distribution of our survey (Section 6.3). For the other tier 1 sources, we fitted absorbed power-law, bremsstrahlung, and collisionally ionized models; we employed the tbabs absorption model with solar abundances from Wilms et al. (2000) and photoionization cross-sections from Verner et al. (1996). When Fe line emission was clearly visible between 6.4 and 7.1 keV, we also included a Gaussian line in the spectral models. Due to NuSTAR’s 0.4 keV resolution at 6–7 keV energies, multiple Fe lines would appear blended in our spectra, especially given the low photon statistics. Thus, measurements of the Fe line parameters should be interpreted as the average energy of the Fe line complex and the combined equivalent width of the Fe lines. If Fe line emission was not evident, the source spectrum was first fit without a Gaussian component. Then, having determined which of the three spectral models best fit the spectrum, a Gaussian component was added in order to place constraints on the strength of Fe line emission that may not be visible due to poor photon statistics. The central energy of this Gaussian component was constrained to be between 6.3 and 7.1 keV, and its width was fixed to zero; we tested the effect of fixing the width to values as high as 0.1 keV, but the impact on the results was negligible. Then, the 90% upper limit on the line normalization was used to calculate the 90% upper limit on the Fe line equivalent width. In addition, when significant residuals remained at soft energies, we introduced a partial covering model (pcfabs) to test whether it provided a significant improvement of the χ² statistic. Including this component substantially improved χ² for NNR 4 and 6;

however, for NNR 6, the $N_H$ of the partial absorber could not be well constrained, and the covering fraction was found to be consistent with 1.0 to 90% confidence. Thus, since the spectral quality of NNR 6 was not good enough to constrain the additional pcfabs component, we did not include it in our final model fit for NNR 6.

The results of our spectral analysis can be found in Table 9, and the spectra and fit residuals are shown in Figure 8 and the Appendix. As can be seen, spectra with <300 *NuSTAR* counts cannot place strong constraints on the spectral parameters. However, we nonetheless include these results in order to be able to compare nonparametric fluxes with spectrally derived fluxes and as a reference to aid the design of future *NuSTAR* surveys.

We used the model fit with the best reduced χ² statistic to determine observed energy fluxes for each source in the 2–10, 3–10, and 10–20 keV bands and conversion factors from photon fluxes to unabsorbed energy fluxes, which are listed in Table 10. These conversion factors are used to calculate the logN–logS distribution for unabsorbed fluxes (see Section 6.3). The faintest tier 1 source, NNR 28, does not have enough counts to permit spectral fitting; based on its quantile values, it has $N_H \approx 10^{23}$ cm⁻² and $\Gamma \approx 1.8$. Fixing the parameters of an absorbed power-law model to these values while allowing the *Chandra* and *NuSTAR* normalizations to vary independently, we fit the unbinned spectra of NNR 28 using the C-statistic (Cash 1979) and find a goodness of fit lower than 28%. The observed and unabsorbed fluxes of NNR 28 measured from these fits are included in Table 10.

To ensure that these results were not significantly dependent on the binning that was chosen, we compared the best-fitting parameters with those derived by fitting unbinned spectra using the C-statistic and the locations of sources in the quantile diagrams. No significant discrepancies were found except for sources with strong Fe lines, which is to be expected, since the quantile grids do not account for the presence of Fe lines. However, for NNR 17, our analysis yielded a harder spectrum than that found by B14. This source lies in the ghost ray pattern of 4U 1630–472, making background subtraction particularly challenging. The background region we selected contains higher ghost ray contamination than the background chosen by B14; we consider our selection more appropriate, given that this source resides in a region of high ghost ray contamination. Since the spectrum of 4U 1630–472 is dominated by a blackbody component with $kT \approx 1.4$ keV, the fact that B14 measured a softer spectrum for NNR 17 than we did, with $\Gamma = 3.7 \pm 0.5$ rather than $2.0_{-0.8}^{+1.0}$, suggests that the background contribution from ghost rays may have been underestimated by B14. The photon index we measured is also more consistent with the hard photon index indicated by the *Chandra* quantiles (see Figure 7(b)).

# 6. Discussion

## 6.1. Classification of NuSTAR Sources

The X-ray spectral and timing properties of the *NuSTAR* sources, as well as information about their optical and infrared counterparts, can help identify their physical nature. The three brightest sources in the *NuSTAR* Norma survey are well
<table><caption>Table 9
Spectral Fits</caption><thead><tr><th>Src No.<br>(1)</th><th>Model<br>tbabs*X<br>(2)</th><th>N/C<br>norm<br>(3)</th><th>FPMA/B<br>norm<br>(4)</th><th>N<sub>H</sub><br>(10<sup>22</sup> cm<sup>-2</sup>)<br>(5)</th><th>Γ<br>(6)</th><th>E<sub>cut</sub><br>(keV)<br>(7)</th><th>Power-law<br>norm<br>(8)</th><th>kT<sub>BB</sub><br>(keV)<br>(9)</th><th>Bbody<br>norm<br>(10)</th><th>χ<sup>2</sup>/dof<br>(11)</th><th>Bin<br>(σ)<br>(12)</th><th>(13)</th><th>Comments</th></tr></thead><tbody><tr><td>1</td><td>PL +diskbb</td><td>...</td><td>0.978<sup>+0.01</sup><sub>-0.02</sub></td><td>12.47 ± 0.08</td><td>2.15 ± 0.03</td><td>...</td><td>0.22 ± 0.02</td><td>1.425<sup>+0.002</sup><sub>-0.003</sub></td><td>192 ± 2</td><td>2.68/806</td><td>10</td><td>See King et al. (2014) for fit including disk reflection and wind absorption.</td><td></td></tr><tr><td>2</td><td>cutoffpl +bbodyrad</td><td>0.67<sup>+0.02</sup><sub>-0.01</sub></td><td>1.02<sup>+0.03</sup><sub>-0.02</sub></td><td>46.0 ± 1.5</td><td>-2.5<sup>+0.4</sup><sub>-0.5</sub></td><td>4.05<sup>+0.33</sup><sub>-0.06</sub></td><td>1.3<sup>+1.5</sup><sub>-0.1</sub> × 10<sup>-5</sup></td><td>1.56<sup>+0.06</sup><sub>-0.08</sub></td><td>0.75<sup>+0.12</sup><sub>-0.08</sub></td><td>1.14/1096</td><td>5, 5</td><td>See Bodaghee et al. (2016) for fit including cyclotron absorption line.</td><td></td></tr><tr><td>3</td><td>PL</td><td>3.4<sup>+1.0</sup><sub>-0.7</sub></td><td>1.04<sup>+0.04</sup><sub>-0.03</sub></td><td>12 ± 2</td><td>1.71 ± 0.06</td><td></td><td>6.7<sup>+1.0</sup><sub>-0.6</sub></td><td>...</td><td>...</td><td>...</td><td>1.02/263</td><td>3, 5</td><td>Chandra only includes the point source, while NuSTAR includes extended emission. See Gotthelf et al. (2014) for detailed analysis.</td></tr><tr><td>4</td><td>PCA* (PL+G)</td><td>0.59<sup>+0.10</sup><sub>-0.09</sub></td><td>0.92<sup>+0.15</sup><sub>-0.13</sub></td><td>0.35<sup>+0.11</sup><sub>-0.10</sub></td><td>2.34 ± 0.22</td><td></td><td>56<sup>+26</sup><sub>-17</sub></td><td>6.65<sup>+0.10</sup><sub>-0.06</sub></td><td>0.9<sup>+0.2</sup><sub>-0.1</sub></td><td>6.2<sup>+2.4</sup><sub>-1.9</sub></td><td>1.19/154</td><td>3, 3</td><td>pcfabs reduces χ<sup>2</sup> by ≈ 0.2. For PL, N<sub>H,cvr</sub> = 6<sup>+2</sup><sub>-1</sub> × 10<sup>22</sup> cm<sup>-2</sup>, cvrf = 0.77<sup>+0.06</sup><sub>-0.08</sub>. For Brand AP, N<sub>H,cvr</sub> = 5 ± 2 × 10<sup>22</sup> cm<sup>-2</sup>, cvrf = 0.5 ± 0.1.</td></tr><tr><td></td><td>PCA* (BR+G)</td><td>0.57<sup>+0.09</sup><sub>-0.08</sub></td><td>0.90<sup>+0.15</sup><sub>-0.12</sub></td><td>0.14 ± 0.08</td><td>7.9<sup>+2.4</sup><sub>-1.7</sub></td><td>24<sup>+4</sup><sub>-3</sub></td><td>6.65<sup>+0.09</sup><sub>-0.06</sub></td><td>0.8 ± 0.2</td><td>5.6<sup>+2.1</sup><sub>-1.8</sub></td><td>1.20/154</td><td>3, 3</td><td>...</td><td></td></tr><tr><td></td><td>PCA* (AP+G)</td><td>0.56<sup>+0.04</sup><sub>-0.08</sub></td><td>0.90<sup>+0.16</sup><sub>-0.12</sub></td><td>0.13<sup>+0.09</sup><sub>-0.08</sub></td><td>7.4<sup>+2.1</sup><sub>-1.5</sub></td><td>68<sup>+9</sup><sub>-9</sub></td><td>6.56<sup>+0.12</sup><sub>-0.17</sub></td><td>0.2 ± 0.1</td><td>2.1<sup>+2.2</sup><sub>-1.6</sub></td><td>1.19/154</td><td>3, 3</td><td>...</td><td></td></tr><tr><td>5</td><td>PL</td><td>1.3<sup>+0.5</sup><sub>-0.3</sub></td><td>0.9<sup>+0.3</sup><sub>-0.2</sub></td><td>27<sup>+10</sup><sub>-8</sub></td><td>2.3 ± 0.3</td><td></td><td>28<sup>+40</sup><sub>-16</sub></td><td>6.3–7.1</td><td>&lt;0.36</td><td>&lt;1.3</td><td>1.07/47</td><td>3, 3</td><td>...</td></tr><tr><td></td><td>BR</td><td>1.3<sup>+0.5</sup><sub>-0.3</sub></td><td>0.9<sup>+0.3</sup><sub>-0.2</sub></td><td>21<sup>+8</sup><sub>-6</sub></td><td>10<sup>+5</sup><sub>-3</sub></td><td>9<sup>+5</sup><sub>-3</sub></td><td>...</td><td>...</td><td>...</td><td>...</td><td>1.07/47</td><td>3, 3</td><td>...</td></tr><tr><td></td><td>AP</td><td>1.2<sup>+0.5</sup><sub>-0.3</sub></td><td>0.9<sup>+0.3</sup><sub>-0.2</sub></td><td>17<sup>+5</sup><sub>-5</sub></td><td>13<sup>+5</sup><sub>-3</sub></td><td>21<sup>+8</sup><sub>-7</sub></td><td>...</td><td>...</td><td>...</td><td>...</td><td>1.15/47</td><td>3, 3</td><td>...</td></tr><tr><td>6</td><td>PL+G</td><td>1.0 ± 0.2</td><td>0.9<sup>+0.3</sup><sub>-0.2</sub></td><td>5 ± 1</td><td>1.5 ± 0.3</td><td>13<sup>+7</sup><sub>-4</sub></td><td>6.5<sup>+0.3</sup><sub>-1.7</sub></td><td>1.5 ± 0.5</td><td>11<sup>+62</sup><sub>-5</sub></td><td>1.79/27</td><td>5, 3</td><td>...</td><td></td></tr><tr><td></td><td>BR+G</td><td>1.0<sup>+0.3</sup><sub>-0.2</sub></td><td>0.98 ± 0.25</td><td>4.3<sup>+0.9</sup><sub>-1.5</sub></td><td>&gt;15</td><td>18<sup>+3</sup><sub>-2</sub></td><td>6.5<sup>+0.4</sup><sub>-0.3</sub></td><td>1.3 ± 0.4</td><td>10<sup>+7</sup><sub>-5</sub></td><td>1.72/27</td><td>5, 3</td><td>...</td><td></td></tr><tr><td></td><td>AP+G</td><td>1.0 ± 0.2</td><td>1.0<sup>+0.3</sup><sub>-0.2</sub></td><td>4.3<sup>+0.9</sup><sub>-0.7</sub></td><td>&gt;15</td><td>51<sup>+9</sup><sub>-6</sub></td><td>6.4 ± 0.4</td><td>1.2 ± 0.5</td><td>9<sup>+6</sup><sub>-5</sub></td><td>1.69/27</td><td>5, 3</td><td>...</td><td></td></tr><tr><td>7</td><td>PL+G</td><td>1.0 ± 0.2</td><td>0.8<sup>+0.2</sup><sub>-0.1</sub></td><td>15<sup>+3</sup><sub>-2</sub></td><td>3.4<sup>+0.4</sup><sub>-0.3</sub></td><td>220<sup>+80</sup><sub>-90</sub></td><td>6.76 ± 0.12</td><td>0.65 ± 0.20</td><td>2.1<sup>+1.1</sup><sub>-0.9</sub></td><td>0.92/75</td><td>2.5, 2.5</td><td>apec abundance = 0.5 ± 0.3.<br>NARCS 1278 flux is 30% of total<br>(Rahoui et al. 2014).</td><td></td></tr><tr><td></td><td>BR+G</td><td>1.0 ± 0.2</td><td>0.9<sup>+0.2</sup><sub>-0.1</sub></td><td>11<sup>+2</sup><sub>-1</sub></td><td>3.4<sup>+0.7</sup><sub>-0.6</sub></td><td>32<sup>+6</sup><sub>-7</sub></td><td>6.76 ± 0.12</td><td>0.5 ± 0.2</td><td>1.8<sup>+1.1</sup><sub>-0.9</sub></td><td>0.93/75</td><td>2.5, 2.5</td><td>...</td><td></td></tr><tr><td></td><td>AP</td><td>1.0 ± 0.2</td><td>0.9<sup>+0.2</sup><sub>-0.1</sub></td><td>11 ± 2</td><td>3.2<sup>+0.8</sup><sub>-0.5</sub></td><td>100<sup>+30</sup><sub>-25</sub></td><td>...</td><td>...</td><td>...</td><td>0.89/77</td><td>2.5, 2.5</td><td>...</td><td></td></tr><tr><td>8</td><td>PL</td><td>1.0 ± 0.2</td><td>...</td><td>14<sup>+7</sup><sub>-5</sub></td><td>1.8 ± 0.2</td><td>18<sup>+15</sup><sub>-8</sub></td><td>6.3–7.1</td><td>&lt;0.26</td><td>&lt;1.9</td><td>1.01/27</td><td>3, 5</td><td>Only FPMA used.</td><td></td></tr><tr><td></td><td>BR</td><td>1.0 ± 0.2</td><td>...</td><td>12<sup>+4</sup><sub>-4</sub></td><td>25<sup>+22</sup><sub>-9</sub></td><td>15<sup>+5</sup><sub>-4</sub></td><td>...</td><td>...</td><td>...</td><td>1.03/27</td><td>3, 5</td><td>...</td><td></td></tr><tr><td></td><td>AP</td><td>1.0 ± 0.2</td><td>...</td><td>10<sup>+4</sup><sub>-3</sub></td><td>&gt;21</td><td>44<sup>+11</sup><sub>-10</sub></td><td>...</td><td>...</td><td>...</td><td>1.14/27</td><td>3, 5</td><td>...</td><td></td></tr><tr><td>9</td><td>PL+G</td><td>0.9<sup>+0.3</sup><sub>-0.2</sub></td><td>0.9<sup>+0.3</sup><sub>-0.2</sub></td><td>7<sup>+3</sup><sub>-2</sub></td><td>1.5 ± 0.3</td><td>9<sup>+7</sup><sub>-4</sub></td><td>6.5 ± 1.2</td><td>0.6 ± 0.4</td><td>3<sup>+25</sup><sub>-2</sub></td><td>0.84/29</td><td>3, 2.5</td><td>...</td><td></td></tr><tr><td></td><td>BR+G</td><td>0.9<sup>+0.3</sup><sub>-0.2</sub></td><td>0.9<sup>+0.3</sup><sub>-0.2</sub></td><td>7<sup>+2</sup><sub>-1</sub></td><td>&gt;15</td><td>11<sup>+6</sup><sub>-5</sub></td><td>6.4<sup>+0.7</sup><sub>-0.3</sub></td><td>0.5 ± 0.3</td><td>3<sup>+8</sup><sub>-2</sub></td><td>0.83/29</td><td>3, 2.5</td><td>...</td><td></td></tr><tr><td></td><td>AP+G</td><td>0.8<sup>+0.3</sup><sub>-0.2</sub></td><td>0.9 ± 0.2</td><td>7<sup>+2</sup><sub>-1</sub></td><td>&gt;15</td><td>33<sup>+8</sup><sub>-6</sub></td><td>6.4<sup>+0.5</sup><sub>-0.4</sub></td><td>0.4 ± 0.2</td><td>2.2<sup>+2.7</sup><sub>-1.7</sub></td><td>0.82/29</td><td>3, 2.5</td><td>...</td><td></td></tr><tr><td>10</td><td>PL</td><td>...</td><td>0.9<sup>+0.6</sup><sub>-0.3</sub></td><td>28</td><td>4.1<sup>+0.9</sup><sub>-0.8</sub></td><td>1800<sup>+6400</sup><sub>-1400</sub></td><td>6.3–7.1</td><td>&lt;0.61</td><td>&lt;4.2</td><td>1.09/10</td><td>2</td><td>N<sub>H</sub> set to values from Jakobsen et al. (2014).</td><td></td></tr><tr><td></td><td>BR</td><td>...</td><td>0.9<sup>+0.6</sup><sub>-0.3</sub></td><td>17</td><td>3<sup>+2</sup><sub>-1</sub></td><td>70<sup>+100</sup><sub>-40</sub></td><td>...</td><td>...</td><td>...</td><td>1.14/10</td><td>2</td><td>...</td><td></td></tr><tr><td></td><td>AP</td><td>...</td><td>0.9<sup>+0.6</sup><sub>-0.4</sub></td><td>17</td><td>1.9<sup>+5.0</sup><sub>-0.7</sub></td><td>330<sup>+800</sup><sub>-290</sub></td><td>...</td><td>...</td><td>...</td><td>1.77/10</td><td>2</td><td>...</td><td></td></tr><tr><td>11</td><td>PL</td><td>5<sup>+7</sup><sub>-2</sub></td><td>1.3<sup>+0.9</sup><sub>-0.8</sub></td><td>11<sup>+11</sup><sub>-9</sub></td><td>2.3 ± 0.4</td><td>2.4<sup>+6.1</sup><sub>-2.0</sub></td><td>...</td><td>...</td><td>...</td><td>1.32/42</td><td>3, 3</td><td>...</td><td></td></tr><tr><td></td><td>BR</td><td>6<sup>+21</sup><sub>-3</sub></td><td>1.3<sup>+1.1</sup><sub>-1.0</sub></td><td>&lt;14</td><td>10<sup>+6</sup><sub>-3</sub></td><td>0.7<sup>+0.9</sup><sub>-0.6</sub></td><td>6.3–7.1</td><td>&lt;0.35</td><td>&lt;0.1</td><td>1.27/42</td><td>3, 3</td><td>...</td><td></td></tr><tr><td></td><td>AP</td><td>17<sup>+15</sup><sub>-12</sub></td><td>1.4<sup>+3.4</sup><sub>-0.6</sub></td><td>&lt;5</td><td>14<sup>+5</sup><sub>-3</sub></td><td>0.6<sup>+1.6</sup><sub>-0.3</sub></td><td>...</td><td>...</td><td>...</td><td>1.33/42</td><td>3, 3</td><td>...</td><td></td></tr></tbody></table>
<table><caption>Table 9
(Continued)</caption><thead><tr><th>Src No.</th><th>Model tbabs*X</th><th>N/C norm</th><th>FPMA/B norm</th><th>N<sub>H</sub> (10<sup>22</sup> cm<sup>-2</sup>)</th><th>Γ or kT(keV)</th><th>Norm (10<sup>-5</sup>) (14)</th><th>Line En. (keV) (16)</th><th>Line Equation (keV) (17)</th><th>Line norm (10<sup>-6</sup>) (18)</th><th>χ<sup>2</sup>/dof</th><th>Bin (σ)</th><th>Comments</th></tr></thead><tbody><tr><td rowspan="3">12</td><td>PL+G</td><td>1.1<sup>+0.5</sup><sub>-0.3</sub></td><td>1.0<sup>+0.4</sup><sub>-0.3</sub></td><td>20<sup>+9</sup><sub>-6</sub></td><td>2.4 ± 0.5</td><td>14<sup>+26</sup><sub>-9</sub></td><td>6.78<sup>+0.14</sup><sub>-0.12</sub></td><td>1.2 ± 0.4</td><td>1.7<sup>+1.2</sup><sub>-0.8</sub></td><td>1.14/33</td><td>2.5, 2.5</td><td>...</td></tr><tr><td>BR+G</td><td>1.0<sup>+0.5</sup><sub>-0.3</sub></td><td>1.0<sup>+0.4</sup><sub>-0.3</sub></td><td>16<sup>+7</sup><sub>-5</sub></td><td>9<sup>+8</sup><sub>-3</sub></td><td>5 ± 2</td><td>6.77<sup>+0.13</sup><sub>-0.12</sub></td><td>1.2<sup>+0.5</sup><sub>-0.3</sub></td><td>1.6<sup>+1.2</sup><sub>-0.8</sub></td><td>1.17/33</td><td>2.5, 2.5</td><td>...</td></tr><tr><td>AP</td><td>1.1<sup>+0.5</sup><sub>-0.3</sub></td><td>1.0<sup>+0.4</sup><sub>-0.3</sub></td><td>19<sup>+7</sup><sub>-5</sub></td><td>6<sup>+3</sup><sub>-1</sub></td><td>17<sup>+8</sup><sub>-6</sub></td><td>...</td><td>...</td><td>...</td><td>1.21/36</td><td>2.5, 2.5</td><td>...</td></tr><tr><td rowspan="3">13</td><td>PL</td><td>1.5<sup>+1.1</sup><sub>-0.6</sub></td><td>1.0<sup>+0.7</sup><sub>-0.4</sub></td><td>9<sup>+17</sup><sub>-6</sub></td><td>1.0 ± 0.5</td><td>0.7<sup>+1.8</sup><sub>-0.5</sub></td><td>6.3–7.1</td><td>&lt;2.7</td><td>&lt;3.4</td><td>1.22/23</td><td>2, 2</td><td>...</td></tr><tr><td>BR</td><td>1.6<sup>+1.1</sup><sub>-0.6</sub></td><td>1.0<sup>+0.7</sup><sub>-0.4</sub></td><td>11<sup>+11</sup><sub>-6</sub></td><td>&gt;31</td><td>2.9<sup>+2.2</sup><sub>-0.4</sub></td><td>...</td><td>...</td><td>...</td><td>1.25/23</td><td>2, 2</td><td>...</td></tr><tr><td>AP</td><td>1.6 ± 0.7</td><td>1.0<sup>+0.7</sup><sub>-0.4</sub></td><td>13<sup>+24</sup><sub>-7</sub></td><td>&gt;21</td><td>7<sup>+1</sup><sub>-3</sub></td><td>...</td><td>...</td><td>...</td><td>1.28/23</td><td>2, 2</td><td>...</td></tr><tr><td rowspan="3">14</td><td>PL+G</td><td>0.7<sup>+0.2</sup><sub>-0.1</sub></td><td>...</td><td>29<sup>+9</sup><sub>-7</sub></td><td>4.1<sup>+1.2</sup><sub>-0.9</sub></td><td>760<sup>+5700</sup><sub>-610</sub></td><td>6.59<sup>+0.08</sup><sub>-0.06</sub></td><td>1.8 ± 0.5</td><td>6<sup>+3</sup><sub>-2</sub></td><td>1.07/28</td><td>3, 2.5</td><td>Only FPMB used.</td></tr><tr><td>BR+G</td><td>0.7<sup>+0.2</sup><sub>-0.1</sub></td><td>...</td><td>22<sup>+7</sup><sub>-5</sub></td><td>2.4<sup>+1.4</sup><sub>-0.9</sub></td><td>60<sup>+130</sup><sub>-30</sub></td><td>6.59<sup>+0.10</sup><sub>-0.06</sub></td><td>1.7<sup>+0.6</sup><sub>-0.4</sub></td><td>5<sup>+3</sup><sub>-2</sub></td><td>1.08/28</td><td>3, 2.5</td><td>...</td></tr><tr><td>AP</td><td>0.8 ± 0.2</td><td>...</td><td>25<sup>+7</sup><sub>-5</sub></td><td>2.1<sup>+0.9</sup><sub>-0.5</sub></td><td>190<sup>+240</sup><sub>-90</sub></td><td>...</td><td>...</td><td>...</td><td>1.11/31</td><td>3, 2.5</td><td>...</td></tr><tr><td rowspan="3">15</td><td>PL</td><td>1.9<sup>+1.4</sup><sub>-0.8</sub></td><td>0.7<sup>+0.4</sup><sub>-0.3</sub></td><td>&lt;0.4</td><td>2.6 ± 0.4</td><td>13<sup>+4</sup><sub>-4</sub></td><td>6.3–7.1</td><td>&lt;1.7</td><td>&lt;2.1</td><td>0.75/20</td><td>3, 2</td><td>...</td></tr><tr><td>BR</td><td>1.8<sup>+1.1</sup><sub>-0.8</sub></td><td>0.6<sup>+0.3</sup><sub>-0.2</sub></td><td>&lt;0.08</td><td>2.9<sup>+1.0</sup><sub>-0.7</sub></td><td>11 ± 2</td><td>...</td><td>...</td><td>...</td><td>0.90/20</td><td>3, 2</td><td>...</td></tr><tr><td>AP</td><td>1.8<sup>+1.1</sup><sub>-0.8</sub></td><td>0.6<sup>+0.3</sup><sub>-0.2</sub></td><td>&lt;0.10</td><td>2.9<sup>+0.8</sup><sub>-0.7</sub></td><td>28<sup>+9</sup><sub>-7</sub></td><td>...</td><td>...</td><td>...</td><td>0.82/19</td><td>3, 2</td><td>...</td></tr><tr><td rowspan="3">16</td><td>PL</td><td>1.6<sup>+0.7</sup><sub>-0.5</sub></td><td>0.7<sup>+0.3</sup><sub>-0.2</sub></td><td>19<sup>+6</sup><sub>-5</sub></td><td>2.9<sup>+0.6</sup><sub>-0.5</sub></td><td>130<sup>+240</sup><sub>-80</sub></td><td>...</td><td>...</td><td>...</td><td>1.05/24</td><td>3, 2.5</td><td>Harder spectrum than that found by Bodaghee et al. (2014) due to different background regions.</td></tr><tr><td>BR</td><td>1.6<sup>+0.6</sup><sub>-0.5</sub></td><td>0.7<sup>+0.3</sup><sub>-0.2</sub></td><td>14<sup>+4</sup><sub>-3</sub></td><td>5<sup>+3</sup><sub>-1</sub></td><td>25<sup>+14</sup><sub>-8</sub></td><td>6.3–7.1</td><td>&lt;0.8</td><td>&lt;4.8</td><td>0.95/24</td><td>3, 2.5</td><td>...</td></tr><tr><td>AP</td><td>1.2<sup>+0.5</sup><sub>-0.4</sub></td><td>0.7 ± 0.2</td><td>13 ± 3</td><td>6<sup>+4</sup><sub>-2</sub></td><td>58<sup>+27</sup><sub>-15</sub></td><td>...</td><td>...</td><td>...</td><td>1.06/24</td><td>3, 2.5</td><td>...</td></tr><tr><td rowspan="3">17</td><td>PL</td><td>1.1<sup>+1.2</sup><sub>-0.5</sub></td><td>0.8<sup>+0.6</sup><sub>-0.4</sub></td><td>21<sup>+32</sup><sub>-16</sub></td><td>2.0<sup>+1.0</sup><sub>-0.8</sub></td><td>6<sup>+65</sup><sub>-5</sub></td><td>6.3–7.1</td><td>&lt;1.0</td><td>&lt;1.3</td><td>0.94/13</td><td>2, 2</td><td>...</td></tr><tr><td>BR</td><td>1.1<sup>+1.3</sup><sub>-0.5</sub></td><td>0.8<sup>+0.6</sup><sub>-0.4</sub></td><td>16<sup>+24</sup><sub>-12</sub></td><td>&gt;6</td><td>3<sup>+6</sup><sub>-2</sub></td><td>...</td><td>...</td><td>...</td><td>0.95/13</td><td>2, 2</td><td>...</td></tr><tr><td>AP</td><td>1.1<sup>+1.3</sup><sub>-0.5</sub></td><td>0.8<sup>+0.6</sup><sub>-0.4</sub></td><td>14<sup>+17</sup><sub>-10</sub></td><td>&gt;8</td><td>9<sup>+9</sup><sub>-5</sub></td><td>...</td><td>...</td><td>...</td><td>0.94/13</td><td>2, 2</td><td>...</td></tr><tr><td rowspan="3">18</td><td>PL</td><td>1.1<sup>+0.6</sup><sub>-0.5</sub></td><td>0.8<sup>+0.7</sup><sub>-0.4</sub></td><td>19<sup>+9</sup><sub>-6</sub></td><td>2.6<sup>+1.0</sup><sub>-0.8</sub></td><td>50<sup>+260</sup><sub>-40</sub></td><td>...</td><td>...</td><td>...</td><td>1.87/13</td><td>3, 2</td><td>...</td></tr><tr><td>BR</td><td>1.1<sup>+0.6</sup><sub>-0.4</sub></td><td>0.8<sup>+0.7</sup><sub>-0.4</sub></td><td>16<sup>+7</sup><sub>-4</sub></td><td>6<sup>+11</sup><sub>-3</sub></td><td>16<sup>+19</sup><sub>-6</sub></td><td>6.3–7.1</td><td>&lt;0.9</td><td>&lt;3</td><td>1.81/13</td><td>3, 2</td><td>...</td></tr><tr><td>AP</td><td>1.0<sup>+0.5</sup><sub>-0.4</sub></td><td>0.8<sup>+0.8</sup><sub>-0.4</sub></td><td>13<sup>+7</sup><sub>-3</sub></td><td>9<sup>+24</sup><sub>-6</sub></td><td>33<sup>+51</sup><sub>-8</sub></td><td>...</td><td>...</td><td>...</td><td>1.97/13</td><td>3, 2</td><td>...</td></tr><tr><td rowspan="3">19</td><td>PL</td><td>1.0 ± 0.3</td><td>1.2<sup>+0.6</sup><sub>-0.4</sub></td><td>1.7<sup>+0.8</sup><sub>-0.6</sub></td><td>1.7<sup>+0.3</sup><sub>-0.4</sub></td><td>4 ± 2</td><td>...</td><td>...</td><td>...</td><td>1.66/25</td><td>3, 3</td><td>N /C = 0.8<sup>+0.3</sup><sub>-0.2</sub> for Chandra ObsID 7591.</td></tr><tr><td>BR</td><td>1.0<sup>+0.4</sup><sub>-0.2</sub></td><td>1.1<sup>+0.6</sup><sub>-0.3</sub></td><td>1.4<sup>+0.5</sup><sub>-0.4</sub></td><td>13<sup>+18</sup><sub>-5</sub></td><td>4.0<sup>+0.8</sup><sub>-0.6</sub></td><td>...</td><td>...</td><td>...</td><td>1.44/25</td><td>3, 3</td><td>...</td></tr><tr><td>AP</td><td>1.0 ± 0.3</td><td>1.1<sup>+0.6</sup><sub>-0.3</sub></td><td>1.4<sup>+0.5</sup><sub>-0.4</sub></td><td>11<sup>+18</sup><sub>-4</sub></td><td>12 ± 2</td><td>6.3–7.1</td><td>&lt;1.3</td><td>&lt;2.4</td><td>1.44/25</td><td>3, 3</td><td>...</td></tr><tr><td rowspan="3">20</td><td>PL</td><td>1</td><td>1</td><td>70<sup>+130</sup><sub>-50</sub></td><td>2.6<sup>+2.1</sup><sub>-1.4</sub></td><td>52<sup>+15000</sup><sub>-50</sub></td><td>6.3–7.1</td><td>&lt;0.6</td><td>&lt;6.4</td><td>1.26/11</td><td>2, 2</td><td>If the cross-normalization constant between Chandra and NuSTAR is left as a free parameter, N/C > 2 at 90% confidence.</td></tr><tr><td>BR</td><td>1</td><td>1</td><td>60<sup>+90</sup><sub>-40</sub></td><td>&gt;3</td><td>8<sup>+80</sup><sub>-5</sub></td><td>...</td><td>...</td><td>...</td><td>1.31/11</td><td>2, 2</td><td>...</td></tr><tr><td>AP</td><td>1</td><td>1</td><td>50<sup>+50</sup><sub>-30</sub></td><td>&gt;6</td><td>18<sup>+27</sup><sub>-8</sub></td><td>...</td><td>...</td><td>...</td><td>1.30/11</td><td>2, 2</td><td>...</td></tr><tr><td rowspan="3">21</td><td>PL</td><td>0.9 ± 0.2</td><td>...</td><td>26<sup>+9</sup><sub>-7</sub></td><td>2.6<sup>+0.5</sup><sub>-0.4</sub></td><td>120<sup>+200</sup><sub>-70</sub></td><td>6.3–7.1</td><td>&lt;0.5</td><td>&lt;4.6</td><td>1.01/47</td><td>3, 3</td><td>Only FPMB used. Point-source (NARCS 402) flux is 20% ± 5% of total.</td></tr><tr><td>BR</td><td>0.9 ± 0.2</td><td>...</td><td>20<sup>+7</sup><sub>-5</sub></td><td>8<sup>+5</sup><sub>-2</sub></td><td>31<sup>+15</sup><sub>-9</sub></td><td>...</td><td>...</td><td>...</td><td>1.04/47</td><td>3, 3</td><td>...</td></tr><tr><td>AP</td><td>0.8 ± 0.2</td><td>...</td><td>17<sup>+5</sup><sub>-4</sub></td><td>10<sup>+7</sup><sub>-3</sub></td><td>71<sup>+22</sup><sub>-16</sub></td><td>...</td><td>...</td><td>...</td><td>1.13/47</td><td>3, 3</td><td>...</td></tr><tr><td rowspan="3">22</td><td>PL</td><td>1.7<sup>+1.5</sup><sub>-0.8</sub></td><td>1</td><td>13<sup>+12</sup><sub>-7</sub></td><td>2.0<sup>+1.3</sup><sub>-1.2</sub></td><td>5.1<sup>+41</sup><sub>-44</sub></td><td>...</td><td>...</td><td>...</td><td>1.92/10</td><td>2, 2</td><td>...</td></tr><tr><td>BR</td><td>1.6<sup>+1.4</sup><sub>-0.8</sub></td><td>1</td><td>11<sup>+10</sup><sub>-5</sub></td><td>&gt;4</td><td>3<sup>+6</sup><sub>-1</sub></td><td>...</td><td>...</td><td>...</td><td>1.91/10</td><td>2, 2</td><td>...</td></tr><tr><td>AP</td><td>1.8<sup>+1.3</sup><sub>-0.7</sub></td><td>1</td><td>13<sup>+13</sup><sub>-5</sub></td><td>5<sup>+23</sup><sub>-3</sub></td><td>12<sup>+24</sup><sub>-6</sub></td><td>6.3–7.1</td><td>&lt;3.8</td><td>&lt;4.4</td><td>1.67/10</td><td>2, 2</td><td>...</td></tr><tr><td rowspan="3">23</td><td>PL</td><td>1.7<sup>+2.1</sup><sub>-0.9</sub></td><td>1</td><td>7<sup>+65</sup><sub>-5</sub></td><td>1.8<sup>+2.0</sup><sub>-0.8</sub></td><td>1.6<sup>+340</sup><sub>-13</sub></td><td>...</td><td>...</td><td>...</td><td>0.83/6</td><td>2, 2</td><td>...</td></tr></tbody></table>
<table><caption>**Table 9**
(Continued)</caption><thead><tr><th>Src<br>No.</th><th>Model<br>tbabs*X</th><th>N/C<br>norm</th><th>FPMA/B<br>norm</th><th>N<sub>H</sub><br>(10<sup>22</sup> cm<sup>-2</sup>)</th><th>Γ or<br>kT(keV)<br>(14)</th><th>Norm<br>(10<sup>-5</sup>)<br>(15)</th><th>Line En.<br>(keV)<br>(16)</th><th>Line Equation<br>(keV)<br>(17)</th><th>Line norm<br>(10<sup>-6</sup>)<br>(18)</th><th>χ<sup>2</sup>/dof</th><th>Bin<br>(σ)</th><th>Comments</th></tr></thead><tbody><tr><td rowspan="4">24</td><td>BR</td><td>1.7<sup>+2.0</sup><sub>-0.9</sub></td><td>1</td><td>6<sup>+49</sup><sub>-4</sub></td><td>&gt;4</td><td>1.5<sup>+14.5</sup><sub>-0.9</sub></td><td>...</td><td>...</td><td>...</td><td>0.75/6</td><td>2, 2</td><td>...</td></tr><tr><td>AP</td><td>1.7<sup>+2.0</sup><sub>-0.9</sub></td><td>1</td><td>7<sup>+29</sup><sub>-5</sub></td><td>&gt;5</td><td>4.6<sup>+13</sup><sub>-2.6</sub></td><td>6.3-7.1</td><td>&lt;3.9</td><td>&lt;2.9</td><td>0.64/6</td><td>2, 2</td><td>...</td></tr><tr><td>PL</td><td>0.7 ± 0.3</td><td>1</td><td>28<sup>+12</sup><sub>-8</sub></td><td>5.0<sup>+2.2</sup><sub>-1.4</sub></td><td>1180<sup>+41220</sup><sub>-1070</sub></td><td>6.3-7.1</td><td>&lt;16</td><td>&lt;6.2</td><td>1.25/23</td><td>2, 2</td><td>...</td></tr><tr><td>BR</td><td>0.7 ± 0.3</td><td>1</td><td>20<sup>+8</sup><sub>-6</sub></td><td>1.7<sup>+1.3</sup><sub>-0.7</sub></td><td>50<sup>+290</sup><sub>-30</sub></td><td>...</td><td>...</td><td>...</td><td>1.33/23</td><td>2, 2</td><td>...</td></tr><tr><td rowspan="4">25</td><td>AP</td><td>0.7 ± 0.3</td><td>1</td><td>24<sup>+7</sup><sub>-6</sub></td><td>1.4<sup>+0.8</sup><sub>-0.4</sub></td><td>160<sup>+500</sup><sub>-110</sub></td><td>...</td><td>...</td><td>...</td><td>1.32/23</td><td>2, 2</td><td>...</td></tr><tr><td>PL</td><td>1.3<sup>+1.3</sup><sub>-0.6</sub></td><td>...</td><td>3.1<sup>+3.8</sup><sub>-2.8</sub></td><td>1.8 ± 0.7</td><td>1.9<sup>+4.2</sup><sub>-1.5</sub></td><td>...</td><td>...</td><td>...</td><td>1.14/8</td><td>2, 2</td><td>Only FPMA used.</td></tr><tr><td>BR</td><td>1.3<sup>+1.7</sup><sub>-0.6</sub></td><td>...</td><td>2.3<sup>+3.0</sup><sub>-2.2</sub></td><td>&gt;6</td><td>1.7<sup>+1.1</sup><sub>-1.0</sub></td><td>...</td><td>...</td><td>...</td><td>1.05/8</td><td>2, 2</td><td>...</td></tr><tr><td>AP</td><td>1.3<sup>+1.7</sup><sub>-0.6</sub></td><td>...</td><td>2.3<sup>+2.9</sup><sub>-2.1</sub></td><td>&gt;6</td><td>5 ± 3</td><td>6.3-7.1</td><td>&lt;2.1</td><td>&lt;1.8</td><td>1.02/8</td><td>2, 2</td><td>...</td></tr><tr><td rowspan="4">26</td><td>PL</td><td>1.3<sup>+2.8</sup><sub>-0.7</sub></td><td>1</td><td>30<sup>+35</sup><sub>-23</sub></td><td>1.5<sup>+1.0</sup><sub>-0.9</sub></td><td>2.8<sup>+30</sup><sub>-2.5</sub></td><td>6.3-7.1</td><td>&lt;1.2</td><td>&lt;1.9</td><td>1.57/10</td><td>2, 2</td><td>...</td></tr><tr><td>BR</td><td>1.3<sup>+2.6</sup><sub>-0.6</sub></td><td>1</td><td>28<sup>+31</sup><sub>-21</sub></td><td>&gt;9</td><td>3.5<sup>+5.5</sup><sub>-2.5</sub></td><td>...</td><td>...</td><td>...</td><td>1.58/10</td><td>2, 2</td><td>...</td></tr><tr><td>AP</td><td>1.3<sup>+2.5</sup><sub>-0.6</sub></td><td>1</td><td>28<sup>+28</sup><sub>-19</sub></td><td>&gt;13</td><td>11<sup>+12</sup><sub>-8</sub></td><td>...</td><td>...</td><td>...</td><td>1.59/10</td><td>2, 2</td><td>...</td></tr><tr><td>PL</td><td>4.4<sup>+5.9</sup><sub>-3.5</sub></td><td>1</td><td>&lt;23</td><td>0.9<sup>+0.8</sup><sub>-0.4</sub></td><td>0.18<sup>+2.68</sup><sub>-0.09</sub></td><td>6.3-7.1</td><td>&lt;1.1</td><td>&lt;0.5</td><td>0.85/8</td><td>2, 2</td><td>...</td></tr><tr><td rowspan="3">27</td><td>BR</td><td>6.3<sup>+6.2</sup><sub>-5.2</sub></td><td>1</td><td>&lt;27</td><td>&gt;23</td><td>0.7<sup>+6.0</sup><sub>-0.3</sub></td><td>...</td><td>...</td><td>...</td><td>0.95/8</td><td>2, 2</td><td>...</td></tr><tr><td>AP</td><td>7.4<sup>+10.2</sup><sub>-2.7</sub></td><td>1</td><td>&lt;34</td><td>&gt;20</td><td>1.4<sup>+13.8</sup><sub>-0.7</sub></td><td>...</td><td>...</td><td>...</td><td>1.08/8</td><td>2, 2</td><td>...</td></tr></tbody></table>

**Notes.** Errors provided are 90% confidence intervals, except for errors on the line equivalent widths, which are $1\sigma$ confidence intervals. (2) For sources NNR 1–3, which have been analyzed in more detail in other papers, we present the results of simplified models used to derive the fluxes and conversion factors in Table 10. For all other sources, we present fits using power-law (PL), bremsstrahlung (BR), and collisionally ionized apec models (AP). Some models include a Gaussian line (G) or partial covering absorption (PCA). The best-fitting model for each source is written in italics. (3) Multiplicative constant included in all spectral models. The constant is set to 1.0 for Chandra, and, if enough spectral bins are available, it is allowed to vary independently for NuSTAR FPMA and FPMB. Here N/C provides the ratio of the NuSTAR FPMA constant relative to Chandra. (4) Ratio of the FPMA to FPMB fitting constants, providing the cross-calibration of the two NuSTAR modules. For sources with insufficient photon statistics, this ratio is set to 1.0. (11) Reduced $\chi^2$ statistic and degrees of freedom for the best-fitting model. (12) Minimum significance of bins for Chandra and NuSTAR spectra. (16)–(18) Central energy, equivalent width, and normalization of a Gaussian model accounting for Fe line emission. In cases where a Fe line is clearly visible in the spectrum, a Gaussian line (G) is included in the model; otherwise, we provide the results of the best-fit models without a Gaussian line and an upper limit to the Fe line equivalent width derived by adding a Gaussian component as described in Section 5.7.
<figure><img src="image_10.png" /><figcaption>**Figure 8.** Example *Chandra* and *NuSTAR* spectra with residuals of the best-fitting models. *Chandra* data is shown in black, *NuSTAR* FPMA data are shown in red, and FPMB data are shown in blue. Additional spectra are shown in the Appendix. Spectral analysis results can be found in Table 9.</figcaption></figure>

studied and classified: 4U 1630–472 (NNR 1) is a BH LMXB (e.g., Barret et al. 1996; Klein-Wolt et al. 2004), IGR J16393–4643 (NNR 2) is an NS HMXB (Bodaghee et al. 2006; B16), and HESS J1640–465 (NNR 3) is a

pulsar and associated pulsar wind nebula (PWN; G14; Archibald et al. 2016). Here we present the most likely classifications of the fainter *NuSTAR* sources and their hard X-ray properties.
<table><caption>**Table 10**
Spectrally Derived Fluxes</caption><thead><tr><th rowspan="3">Src<br>No.<br>(1)</th><th colspan="3">Ph. Flux (10<sup>-6</sup> cm<sup>-2</sup> s<sup>-1</sup>)</th><th colspan="3">Abs. Flux (10<sup>-14</sup> erg cm<sup>-2</sup> s<sup>-1</sup>)</th><th colspan="3">Ph. flux to unabs. flux (10<sup>-9</sup> erg ph<sup>-1</sup>)</th></tr><tr><th>Chandra<br>2-10 keV<br>(2)</th><th>NuSTAR<br>3-10 keV<br>(3)</th><th>NuSTAR<br>10-20 keV<br>(4)</th><th>Chandra<br>2-10 keV<br>(5)</th><th>NuSTAR<br>3-10 keV<br>(6)</th><th>NuSTAR<br>10-20 keV<br>(7)</th><th>Chandra<br>2-10 keV<br>(8)</th><th>NuSTAR<br>3-10 keV<br>(9)</th><th>NuSTAR<br>10-20 keV<br>(10)</th></tr></thead><tbody><tr><td>1</td><td>...</td><td>614000 ± 300</td><td>19950<sup>+30</sup><sub>-50</sub></td><td>...</td><td>490300<sup>+200</sup><sub>-300</sub></td><td>39590<sup>+60</sup><sub>-100</sub></td><td>...</td><td>12.2</td><td>20.9</td></tr><tr><td>2</td><td>2200<sup>+200</sup><sub>-500</sub></td><td>1500<sup>+100</sup><sub>-300</sub></td><td>1700<sup>+500</sup><sub>-700</sub></td><td>2500<sup>+300</sup><sub>-600</sub></td><td>1700<sup>+200</sup><sub>-300</sub></td><td>4000<sup>+1000</sup><sub>-2000</sub></td><td>27.0</td><td>24.7</td><td>26.3</td></tr><tr><td>3</td><td>19<sup>+0.2</sup><sub>-0.3</sub></td><td>56.0<sup>+0.3</sup><sub>-1.9</sub></td><td>22.8<sup>+0.1</sup><sub>-0.8</sub></td><td>16 ± 2</td><td>52.1<sup>+0.2</sup><sub>-1.8</sub></td><td>51.3<sup>+0.3</sup><sub>-1.9</sub></td><td>14.4</td><td>12.6</td><td>23.4</td></tr><tr><td>4</td><td>101<sup>+3</sup><sub>-7</sub></td><td>43<sup>+1</sup><sub>-1</sub></td><td>7.0<sup>+0.4</sup><sub>-0.7</sub></td><td>71<sup>+2</sup><sub>-6</sub></td><td>36<sup>+1</sup><sub>-3</sub></td><td>15.3<sup>+0.8</sup><sub>-1.6</sub></td><td>9.3</td><td>9.7</td><td>22.3</td></tr><tr><td>5</td><td>16<sup>+2</sup><sub>-4</sub></td><td>21.8<sup>+0.4</sup><sub>-3.6</sub></td><td>7.1<sup>+0.2</sup><sub>-1.4</sub></td><td>15<sup>+4</sup><sub>-4</sub></td><td>20.6<sup>+0.3</sup><sub>-3.4</sub></td><td>15.7<sup>+0.3</sup><sub>-3.0</sub></td><td>26.5</td><td>19.0</td><td>24.1</td></tr><tr><td>6</td><td>82<sup>+3</sup><sub>-7</sub></td><td>68<sup>+4</sup><sub>-9</sub></td><td>21<sup>+2</sup><sub>-5</sub></td><td>68<sup>+3</sup><sub>-7</sub></td><td>63<sup>+3</sup><sub>-8</sub></td><td>47<sup>+4</sup><sub>-11</sub></td><td>9.9</td><td>10.2</td><td>22.7</td></tr><tr><td>7</td><td>42<sup>+1</sup><sub>-3</sub></td><td>36<sup>+1</sup><sub>-1</sub></td><td>1.7<sup>+0.2</sup><sub>-0.4</sub></td><td>32<sup>+1</sup><sub>-3</sub></td><td>29.1<sup>+0.9</sup><sub>-2.5</sub></td><td>3.3<sup>+0.4</sup><sub>-0.8</sub></td><td>14.3</td><td>11.7</td><td>20.7</td></tr><tr><td>8</td><td>43<sup>+3</sup><sub>-7</sub></td><td>40.3<sup>+0.6</sup><sub>-6.4</sub></td><td>16.6<sup>+0.3</sup><sub>-2.6</sub></td><td>39<sup>+3</sup><sub>-6</sub></td><td>37.8<sup>+0.5</sup><sub>-5.8</sub></td><td>37.5<sup>+0.8</sup><sub>-6.0</sub></td><td>15.7</td><td>13.4</td><td>23.5</td></tr><tr><td>9</td><td>43 ± 3</td><td>33<sup>+2</sup><sub>-3</sub></td><td>12.5<sup>+0.9</sup><sub>-1.1</sub></td><td>37 ± 3</td><td>31 ± 2</td><td>28 ± 2</td><td>11.2</td><td>10.9</td><td>22.9</td></tr><tr><td>10</td><td>...</td><td>55<sup>+3</sup><sub>-24</sub></td><td>3.6<sup>+0.3</sup><sub>-2.1</sub></td><td>...</td><td>44<sup>+2</sup><sub>-19</sub></td><td>7.5<sup>+0.7</sup><sub>-4.4</sub></td><td>...</td><td>23.2</td><td>23.0</td></tr><tr><td>11</td><td>2.6<sup>+0.6</sup><sub>-1.1</sub></td><td>10.3<sup>+0.2</sup><sub>-4.1</sub></td><td>2.15<sup>+0.03</sup><sub>-0.98</sub></td><td>2.0<sup>+0.6</sup><sub>-0.9</sub></td><td>9.0<sup>+0.2</sup><sub>-3.5</sub></td><td>4.65<sup>+0.06</sup><sub>-2.13</sub></td><td>9.8</td><td>10.0</td><td>22.0</td></tr><tr><td>12</td><td>10.4<sup>+0.8</sup><sub>-3.9</sub></td><td>10.4<sup>+0.2</sup><sub>-3.1</sub></td><td>2.35<sup>+0.03</sup><sub>-0.8</sub></td><td>9<sup>+2</sup><sub>-3</sub></td><td>9.7<sup>+0.1</sup><sub>-2.8</sub></td><td>5.18<sup>+0.06</sup><sub>-1.70</sub></td><td>20.7</td><td>15.6</td><td>23.5</td></tr><tr><td>13</td><td>6.4<sup>+0.9</sup><sub>-2.8</sub></td><td>8.4<sup>+0.2</sup><sub>-3.4</sub></td><td>5.8<sup>+0.1</sup><sub>-2.4</sub></td><td>6<sup>+1</sup><sub>-3</sub></td><td>8.27<sup>+0.09</sup><sub>-3.38</sub></td><td>13.5<sup>+0.2</sup><sub>-5.5</sub></td><td>12.5</td><td>11.9</td><td>23.7</td></tr><tr><td>14</td><td>28.3<sup>+0.9</sup><sub>-4.1</sub></td><td>20<sup>+1</sup><sub>-4</sub></td><td>1.1<sup>+0.2</sup><sub>-0.4</sub></td><td>23.5<sup>+0.9</sup><sub>-3.7</sub></td><td>16.9<sup>+0.9</sup><sub>-3.4</sub></td><td>2.4<sup>+0.4</sup><sub>-0.8</sub></td><td>48.5</td><td>22.5</td><td>23.1</td></tr><tr><td>15</td><td>25 ± 4</td><td>30<sup>+3</sup><sub>-5</sub></td><td>3.6<sup>+0.5</sup><sub>-0.9</sub></td><td>14<sup>+3</sup><sub>-2</sub></td><td>23<sup>+3</sup><sub>-4</sub></td><td>8<sup>+1</sup><sub>-2</sub></td><td>5.9</td><td>7.8</td><td>21.7</td></tr><tr><td>16</td><td>41<sup>+7</sup><sub>-7</sub></td><td>69<sup>+4</sup><sub>-13</sub></td><td>9.1<sup>+0.9</sup><sub>-2.2</sub></td><td>34<sup>+2</sup><sub>-6</sub></td><td>60<sup>+3</sup><sub>-11</sub></td><td>19<sup>+2</sup><sub>-5</sub></td><td>16.0</td><td>13.1</td><td>21.8</td></tr><tr><td>17</td><td>8<sup>+2</sup><sub>-3</sub></td><td>31<sup>+3</sup><sub>-12</sub></td><td>5.1<sup>+0.6</sup><sub>-2.5</sub></td><td>7<sup>+2</sup><sub>-3</sub></td><td>9.0<sup>+0.4</sup><sub>-2.6</sub></td><td>8.2<sup>+0.5</sup><sub>-2.6</sub></td><td>20.3</td><td>15.9</td><td>23.8</td></tr><tr><td>18</td><td>26<sup>+1</sup><sub>-11</sub></td><td>11<sup>+1</sup><sub>-1</sub></td><td>1.9<sup>+0.3</sup><sub>-0.9</sub></td><td>22<sup>+1</sup><sub>-10</sub></td><td>28<sup>+2</sup><sub>-11</sub></td><td>11<sup>+1</sup><sub>-5</sub></td><td>17.0</td><td>13.9</td><td>22.2</td></tr><tr><td>19</td><td>15<sup>+3</sup><sub>-4</sub></td><td>10.1<sup>+0.4</sup><sub>-1.8</sub></td><td>1.7<sup>+0.4</sup><sub>-1.7</sub></td><td>11<sup>+2</sup><sub>-3</sub></td><td>8.8<sup>+0.4</sup><sub>-1.7</sub></td><td>4.2<sup>+0.7</sup><sub>-2.0</sub></td><td>7.8</td><td>9.0</td><td>21.8</td></tr><tr><td>20</td><td>7.3<sup>+0.2</sup><sub>-5.5</sub></td><td>7.3<sup>+0.1</sup><sub>-5.2</sub></td><td>4.4<sup>+0.2</sup><sub>-2.5</sub></td><td>7.8<sup>+0.2</sup><sub>-5.8</sub></td><td>7.79<sup>+0.09</sup><sub>-5.52</sub></td><td>9.7<sup>+0.6</sup><sub>-5.6</sub></td><td>80.5</td><td>52.4</td><td>28.3</td></tr><tr><td>21</td><td>49<sup>+1</sup><sub>-15</sub></td><td>43.4<sup>+0.9</sup><sub>-12.2</sub></td><td>11.5<sup>+0.4</sup><sub>-4.2</sub></td><td>43.9<sup>+0.8</sup><sub>-14.1</sub></td><td>40.0<sup>+0.6</sup><sub>-11.3</sub></td><td>25<sup>+1</sup><sub>-9</sub></td><td>27.9</td><td>19.1</td><td>23.9</td></tr><tr><td>22</td><td>8.4<sup>+0.5</sup><sub>-4.9</sub></td><td>13.4<sup>+0.1</sup><sub>-9.2</sub></td><td>1.7<sup>+0.3</sup><sub>-1.6</sub></td><td>7.1<sup>+0.4</sup><sub>-4.6</sub></td><td>12.0<sup>+0.1</sup><sub>-2.3</sub></td><td>3.6<sup>+0.8</sup><sub>-3.3</sub></td><td>14.9</td><td>12.7</td><td>21.8</td></tr><tr><td>23</td><td>6<sup>+1</sup><sub>-4</sub></td><td>7.9<sup>+0.1</sup><sub>-4.9</sub></td><td>1.9<sup>+0.2</sup><sub>-1.8</sub></td><td>5<sup>+1</sup><sub>-3</sub></td><td>7.1<sup>+0.1</sup><sub>-4.7</sub></td><td>4.3<sup>+0.6</sup><sub>-4.0</sub></td><td>11.1</td><td>10.7</td><td>22.3</td></tr><tr><td>24</td><td>9.8<sup>+0.2</sup><sub>-3.0</sub></td><td>6.3<sup>+0.7</sup><sub>-2.4</sub></td><td>0.16<sup>+0.03</sup><sub>-0.09</sub></td><td>6.9<sup>+0.2</sup><sub>-2.3</sub></td><td>4.7<sup>+0.5</sup><sub>-1.8</sub></td><td>0.32<sup>+0.06</sup><sub>-0.18</sub></td><td>78.9</td><td>25.8</td><td>22.5</td></tr><tr><td>25</td><td>8<sup>+1</sup><sub>-5</sub></td><td>7.6<sup>+0.1</sup><sub>-3.7</sub></td><td>1.8<sup>+0.2</sup><sub>-1.6</sub></td><td>6<sup>+1</sup><sub>-4</sub></td><td>6.7<sup>+0.2</sup><sub>-3.3</sub></td><td>4.0<sup>+0.6</sup><sub>-3.5</sub></td><td>8.4</td><td>9.4</td><td>22.1</td></tr><tr><td>26</td><td>6.1<sup>+0.7</sup><sub>-3.0</sub></td><td>7.8<sup>+0.2</sup><sub>-4.2</sub></td><td>5.4<sup>+0.1</sup><sub>-3.1</sub></td><td>6.3<sup>+0.8</sup><sub>-3.1</sub></td><td>8.0<sup>+0.1</sup><sub>-4.2</sub></td><td>12.4<sup>+0.2</sup><sub>-7.1</sub></td><td>23.9</td><td>19.7</td><td>25.0</td></tr><tr><td>27</td><td>3.0<sup>+0.2</sup><sub>-2.0</sub></td><td>10.1<sup>+0.6</sup><sub>-5.7</sub></td><td>6.2<sup>+0.3</sup><sub>-4.2</sub></td><td>2.4<sup>+0.2</sup><sub>-1.6</sub></td><td>9.5<sup>+0.4</sup><sub>-5.2</sub></td><td>14.3<sup>+0.7</sup><sub>-9.6</sub></td><td>8.1</td><td>9.4</td><td>23.2</td></tr><tr><td>28<sup>a</sup></td><td>2.1 ± 0.7</td><td>24.9<sup>+0.1</sup><sub>-4.6</sub></td><td>8.79<sup>+0.06</sup><sub>-2.07</sub></td><td>1.8 ± 0.6</td><td>22.6<sup>+0.1</sup><sub>-4.1</sub></td><td>19.8<sup>+0.1</sup><sub>-4.7</sub></td><td>13.1</td><td>11.8</td><td>23.2</td></tr></tbody></table>

Note. These fluxes and conversion factors are determined from spectral fitting. The NuSTAR fluxes represent the average of the FPMA and FPMB fluxes. Errors provided are 1σ confidence intervals.

<sup>a</sup> Due to the poor photon statistics of NNR 28, it was not possible to perform spectral fitting in the same way as for the other sources. Adopting an absorbed power-law model for this source with $\Gamma = 1.8$ and $N_H = 10^{23} \text{ cm}^{-2}$, we determined the Chandra and NuSTAR fluxes using the C-statistic (see Section 5.7 for more details).

### 6.1.1. Colliding Wind Binaries

Two of the NuSTAR sources in the Norma region are likely CWBs: NNR 7 and 14.

NNR 7 actually consists of two Chandra sources blended together due to NuSTAR's PSF. In Chandra ObsID 11008, where these two sources are resolved, they exhibit very similar spectral properties ($N_H$ and $kT$ values are consistent at the <1σ level), but the 0.5–10 keV flux of NARCS 1279 is 2 times higher than the flux of NARCS 1278. These sources are blended in Chandra ObsIDs 12508 and 12509 because they are far off-axis, and the combined flux of the two sources is a factor of 3 higher in these later observations. Spectroscopic follow-up of the near-IR counterparts of both of these Chandra sources revealed that they are Wolf–Rayet stars of spectral type WN8 (Rahoui et al. 2014). These stars belong to the young massive cluster Mercer 81 (Mercer et al. 2005), located at a distance of $11 \pm 2$ kpc (Davies et al. 2012). The Chandra spectra of these sources were better fit by thermal plasma models than power-law models, suggesting that these sources were more likely to be CWBs than HMXBs with

compact objects accreting from the powerful Wolf–Rayet stellar winds.

The NuSTAR data provide even stronger support for the CWB hypothesis for NNR 7. Joint fitting of the Chandra (from NARCS) and NuSTAR spectra of these blended sources reveals that they fall off steeply above 1 keV and show prominent Fe line emission, primarily due to Fe XXV based on its $6.76 \pm 0.1$ keV line energy (House 1969). The spectra are best fit by an apec thermal model with $kT = 3.2^{+0.8}_{-0.5}$ keV and a metal abundance of $0.5 \pm 0.3$ solar, or a steep power-law model with $\Gamma = 3.4^{+0.4}_{-0.3}$ and Fe line emission with $650 \pm 20$ eV equivalent width. These spectral properties rule out the possibility that NNR 7 could be an accreting HMXB, since accreting HMXBs have harder power-law spectra and Fe I $K\alpha$ emission at 6.4 keV, typically with equivalent widths <100 eV (Torrejón et al. 2010). Elshamouty et al. (2016) found that, in quiescence, one neutron star HMXB, V0332+53, exhibits a soft spectrum ($\Gamma \approx 4$ or $kT_{BB} \approx 0.4$ keV) without prominent Fe lines. If this spectrum is typical of quiescent HMXBs, then we can also rule out the possibility
<figure><img src="image_11.png" /><figcaption>**Figure 9.** NuSTAR image of the region around NNR 8. The 3–10 keV band is shown in red, 10–20 keV in green, and 20–40 keV in blue. White contours show the radio continuum emission of the CTB 33 complex from Sarma et al. (1997). Green points denote the positions of NuSTAR sources. G337.0–0.1 is a confirmed SNR, while G336.9–0.2 is an H II region. It has been suggested that the magnetar, NNR 24, is associated with this SNR (Brogan et al. 2000). However, the extended emission of NNR 8 is clearly not coincident with G337.0–0.1, and its origin may be an unassociated PWN.</figcaption></figure>

that NNR 7 is a quiescent HMXB, given its hard spectrum and prominent Fe emission. The unabsorbed 0.5–10 keV flux of NNR 7 based on the combined NARCS and NuSTAR spectrum<fnref n="26" /> is $1.20_{-0.12}^{+0.04} \times 10^{-12} \text{ erg cm}^{-2} \text{ s}^{-1}$. Adopting the 0.5–10 keV flux ratio for NARCS 1278 and 1279 and the bolometric luminosities of their Wolf–Rayet counterparts calculated by Rahoui et al. (2014), we find that their respective X-ray luminosities are $5 \times 10^{33}$ and $1.2 \times 10^{34} \text{ erg s}^{-1}$, and they have $L_X/L_{bol} = 1.3 \times 10^{-6}$ and $8 \times 10^{-7}$, respectively.

Isolated high-mass stars are known to be X-ray emitters, but their spectra typically have $kT \sim 0.5$ keV, and their 0.5–10 keV luminosities follow the scaling relation $L_X/L_{bol} \approx 10^{-7}$ (e.g., Berghoefer et al. 1997; Sana et al. 2006). The harder X-ray emission and higher $L_X/L_{bol}$ exhibited by NNR 7 have been observed from the wind-wind shocks in CWBs (Zhekov & Skinner 2000; Portegies Zwart et al. 2002) and the magnetically channeled shocks of high-mass stars with ~kG fields (Gagné et al. 2005; Petit et al. 2013). For NNR 7, a CWB nature is more likely given the strength of the Fe line at 6.7 keV; magnetic high-mass stars tend to exhibit weak Fe XXV line emission (Schulz et al. 2000; Schulz et al. 2003), while the Fe XXV lines in CWB spectra can have equivalent widths as large as ~1–2 keV (Viotti et al. 2004; Mikles et al. 2006). The X-ray spectrum of NNR 7 exhibits substantial absorption corresponding to $N_H = 1.1 \pm 0.2 \times 10^{23} \text{ cm}^{-2}$, which is in excess of the integrated interstellar absorption along the line of sight ($N_{HI+H_2} = 7.8 \times 10^{22} \text{ cm}^{-2}$). The excess absorption measured in the X-ray spectrum of NNR 7 could be due to either inhomogeneities in the interstellar medium (ISM) or local absorption, which is observed in some CWBs, such as η Carinae (Hamaguchi

et al. 2007). Finally, X-ray variability is more common in CWBs than in isolated high-mass stars (Corcoran 1996). The X-ray flux variations displayed by CWBs are primarily associated with the orbital period of the binary and can be as large as a factor of ≈20 (Pittard et al. 1998; Corcoran 2005). Thus, the X-ray variability exhibited by NNR 7 provides further evidence of its CWB origin.

NNR 14 shares many similarities with NNR 7 and is also likely to be a CWB. The near-IR spectrum of the counterpart of NNR 14 shows emission lines typical of a Wolf–Rayet star of spectral type WN7 in the K-band, but the H-band spectrum lacks the emission lines expected for this spectral type. Overall, the near-IR spectrum may be consistent with an O3I star (J. Corral-Santana et al. 2017, in preparation). Its X-ray spectrum is well fit by an apec thermal model with $kT = 2.1_{-0.5}^{+0.9}$ keV or a power law with $\Gamma = 4.1_{-0.9}^{+1.2}$ and Fe line emission centered at $6.59_{-0.06}^{+0.08}$ keV (consistent with Fe XXV 6.7 keV emission) with a very high equivalent width of $1.8 \pm 0.5$ keV, making it very similar to the CWB candidate CXO J174536.1–285638 (Mikles et al. 2006). Furthermore, NNR 14 exhibits a very high X-ray absorbing column ($N_H = 2.9_{-0.7}^{+0.9} \times 10^{23} \text{ cm}^{-2}$) that is well in excess of the integrated interstellar column density along the line of sight ($N_{HI+H_2} = 8 \times 10^{22} \text{ cm}^{-2}$); this amount of absorption local to the X-ray source is larger than that for NNR 7 but still within the range observed in CWBs (Hamaguchi et al. 2007). NNR 14 is coincident with G338.0–0.1, an H II region most likely located at a distance of 14.1 kpc (Wilson et al. 1970; Kuchar & Clark 1997; Jones & Dickey 2012). It would not be surprising for NNR 14 and G338.0–0.1 to be physically associated, since H II regions are photoionized by high-mass stars, and the extreme $N_H$ along the line of sight to NNR 14 indicates that it is likely located in the far Norma arm or beyond. Thus, adopting a distance of 14 kpc for NNR 14, its unabsorbed 3–10 keV luminosity is $10^{34} \text{ erg s}^{-1}$, which is within the typical range for CWBs.

### 6.1.2. Supernova Remnants and Pulsar Wind Nebulae

In addition to HESS J1640–465, there are three other extended sources in the NuSTAR Norma survey: NNR 5, 8, and 21.

Jakobsen et al. (2014) identified the Chandra counterpart of NNR 5 as a PWN candidate due to its bow shock, cometary morphology, and hard power-law spectrum. Although an AGN or LMXB origin cannot be ruled out, these possibilities were disfavored due to the lack of significant X-ray variability, both on short-term timescales during the NuSTAR observation and on long-term timescales between the Chandra and NuSTAR observations, separated by 3 yr. Our search for pulsations in the NuSTAR data did not yield a detection that would have secured a PWN origin, but our search was only sensitive to high pulsed fractions >45%. A joint spectral fit to the NuSTAR and Chandra data, covering the point source and extended emission in both data sets, yielded a higher $N_H$ value and steeper photon index than that measured by Jakobsen et al. (2014). Our best-fit photon index of $\Gamma = 2.3 \pm 0.3$ for a power-law model is possible for a pulsar/PWN ($\Gamma \sim 1–2$; Kargaltsev & Pavlov 2008), which is consistent with the earlier results, derived using Chandra and XMM-Newton data. However, the $N_H$ value we measure ($2.7_{-0.8}^{+1.0} \times 10^{23} \text{ cm}^{-2}$) is higher than the integrated interstellar absorption along the line of sight ($N_H = 8 \times 10^{22} \text{ cm}^{-2}$), indicating that NNR 5 is likely on the far side of the Galaxy and may be associated with the star-forming complexes located at ~10 kpc. This source may be subject to additional local absorption or lie within or behind molecular clouds.

<footnote n="26">²⁶ The unabsorbed 0.5–10 keV flux reported here for NARCS 1278 and 1279 combined is higher than that reported in Rahoui et al. (2014) because we account for the absorption due to the X-ray derived $N_H$, while in Rahoui et al. (2014), only absorption attributed to the ISM is removed.</footnote>
The source NNR 8 is a region of extended emission with a centrally peaked morphology coincident with the CTB 33 supernova remnant (SNR) and H II complex located at a distance of ~11 kpc and visible at radio wavelengths (Sarma et al. 1997). While NNR 8 may be associated with this complex, it notably does not overlap nearby SNR G337.0–0.1, as shown in Figure 9. This hard X-ray diffuse emission was discovered in an *XMM-Newton* field containing the soft gamma-ray repeater (SGR) 1627–41 (NNR 24) and is attributed by Esposito et al. (2009) to either a galaxy cluster or a PWN. The joint *Chandra* and *NuSTAR* spectrum of NNR 8 is well fit by an absorbed power-law model with a typical pulsar/PWN index of Γ = 1.8 ± 0.2. In contrast, an absorbed bremsstrahlung model yields a temperature of $kT = 25_{-9}^{+22}$ keV in the 0.5–20 keV band, which is higher than expected for most galaxy clusters (Maughan et al. 2012). No pulsations were detected from NNR 8, but our search was only sensitive to periodic signals with very high pulsed fractions (>48%), leaving open the possibility of a pulsar embedded in diffuse PWN emission.

Assuming NNR 8 is a PWN, we can estimate the spin-down energy loss of the pulsar from correlations based on the PWN X-ray luminosity and photon index. Since the high $N_H$ ($1.4_{-0.5}^{+0.7} \times 10^{23}$ cm$^{-2}$) measured from the X-ray spectrum of NNR 8 indicates that it lies on the far side of the Galaxy, and it is reasonable to expect a PWN to be in the vicinity of star-forming regions, we adopt the 11 kpc distance of the far Norma arm and CTB 33 for NNR 8 and calculate its unabsorbed 2–10 keV luminosity to be $1.0 \times 10^{34}$ erg s$^{-1}$. Using the correlation between 2–10 keV luminosity and spin-down energy loss from Possenti et al. (2002), we estimate the pulsar $\dot{E} \approx 7 \times 10^{36}$ erg s$^{-1}$. The pulsar spin-down luminosity can also be estimated from the PWN photon index using correlations derived by Gotthelf (2003). The photon index of NNR 8 yields $\dot{E} \approx 1.4 \times 10^{37}$ erg s$^{-1}$, which is consistent with the value determined from the correlation of $L_X$ and $\dot{E}$ given the statistical uncertainties of the X-ray luminosity and photon index of NNR 8. The fact that these estimates of $\dot{E}$ are consistent provides additional support in favor of a PWN origin for this source.

The extended emission of NNR 21 is associated with SNR G337.2+0.1, located at a distance of ~14 kpc. Using *Chandra* observations, Jakobsen (2013) found that the radial profile of the SNR exhibits a central compact source, suggesting a pulsar powering a PWN, as well as excess emission at a radius of ≈1.8, attributable to the SNR shell. The dearth of *NuSTAR* photons from the central point source does not allow for a significant detection of a pulsar signal, so we cannot confirm the PWN origin of NNR 21. *XMM-Newton* observations of this SNR revealed that it has a nonthermal spectrum that steepens further from the central core (Combi et al. 2006, hereafter C06), as is seen in many plerionic SNRs (e.g., IC 443, 3C 58, and G21.5–0.9; Bocchino & Bykov 2001 and references therein). Spectral fitting of the *NuSTAR* and *Chandra* data results in a higher column density ($N_H = 2.6_{-0.7}^{+0.9} \times 10^{23}$ cm$^{-2}$) and steeper photon index ($\Gamma = 2.6_{-0.4}^{+0.5}$) than that measured by C06 for the pulsar/PWN (central source and extended emission combined). The *Chandra/NuSTAR*-derived photon index, while consistent at the 90% confidence level with the *XMM* measured value ($\Gamma = 1.82 \pm 0.45$), is steeper than expected for a pulsar/PWN. We find that the unabsorbed 2–10 keV luminosity of NNR 21 is $3 \times 10^{34}$ erg s$^{-1}$, and thus the $L_X-\dot{E}$ correlation from Possenti et al. (2002) yields a spin-down luminosity estimate of $\dot{E} \approx 1.5 \times 10^{37}$ erg s$^{-1}$. The spin-down luminosity that is

estimated using the $\Gamma-\dot{E}$ correlation from Gotthelf (2003) is in good agreement if it is based on the *XMM*-derived $\Gamma = 1.8$ ($\dot{E} \approx 1.4 \times 10^{37}$ erg s$^{-1}$), but it is at odds if the *Chandra/NuSTAR*-derived $\Gamma$ is adopted ($\dot{E} > 1.7 \times 10^{38}$ erg s$^{-1}$).<fnref n="27" />

Comparing our power-law fits of NNR 21 with the results of C06, the *Chandra/NuSTAR*-derived $N_H$ is statistically higher than the $N_H = 1.15 \pm 0.27 \times 10^{23}$ cm$^{-2}$ measured by C06 for the whole PWN, but it is consistent at better than 90% confidence with the value C06 measured for the outer region of the PWN ($N_H = 1.62 \pm 0.56 \times 10^{23}$ cm$^{-2}$), which excludes the central 12" radius region. This central region has a much lower column density of $5.9 \pm 1.5 \times 10^{22}$ cm$^{-2}$. Even if we compare the results of our *apex* model fits with C06, the *Chandra/NuSTAR*-derived $N_H$ is more consistent with the $N_H$ value that C06 measured for the outer region than for the whole PWN. One possible explanation for these spatial and temporal $N_H$ variations is that the outer region of the PWN is interacting with a molecular cloud. This scenario would naturally explain the higher $N_H$ measured in the outer region of the PWN compared to the central region by C06, and the increase in the average $N_H$ measured for the whole PWN between the 2004 *XMM* observation and the 2011 *Chandra* observation could be attributed to a larger fraction of the PWN interacting with the dense interstellar medium as the PWN expands. Additional X-ray observations to obtain spatially resolved spectroscopy of NNR 21 are required to better understand the origin of the spectral variations exhibited by this SNR.

### 6.1.3. Magnetars

A known magnetar and a magnetar candidate are present in the *NuSTAR* Norma survey. NNR 24 is a known soft gamma-ray repeater, SGR1627–41, which was discovered by the Burst and Transient Source Experiment (BATSE) when the source went into outburst in 1998 June (Woods et al. 1999). It has been suggested that this SGR is associated with the young SNR G337.0–0.1 in the CTB 33 complex (Hurley et al. 1999), shown in Figure 9. SGR 1627–41 last went into outburst in 2008 (Esposito et al. 2008), and it was found to have returned to quiescence by 2011 in NARCS observations (An et al. 2012). The cross-normalization constant from fitting the *NuSTAR* and *Chandra* spectra is consistent with 1.0 at 90% confidence, indicating that the magnetar persists in quiescence and has not significantly decreased in flux since 2011. We measure a photon index of $5.0_{-1.4}^{+2.2}$, which is steeper but still consistent with that measured by An et al. (2012) at 90% confidence. Assuming a distance of 11 kpc based on the association with the CTB 33 complex, we find that NNR 24 has unabsorbed luminosities of $2.3 \times 10^{33}$ erg s$^{-1}$ in the 3–10 keV band and $5.2 \times 10^{31}$ erg s$^{-1}$ in the 10–20 keV band.

NNR 10, a transient source, may also be a magnetar. The long-term variability and spectral analysis of this source is described in detail in T14, and our spectral analysis yields consistent results. The flux of NNR 10 varies by more than a factor of 20 over a 3 week period, with the peak of activity lasting between 11 hr and 1.5 days and having a soft spectrum with $\Gamma = 4.1_{-0.8}^{+0.9}$ or $kT = 3_{-1}^{+2}$ keV for a bremsstrahlung model. The high $N_H$ measured from the X-ray spectrum of NNR 10 suggests that this source is located at $\gtrsim 10$ kpc and thus has a peak $L_X \gtrsim 10^{34}$ erg s$^{-1}$ in the 2–10 keV band. As argued by T14, NNR 10 is most likely either a shorter-than-average

<footnote n="27">²⁷ The $\Gamma-\dot{E}$ correlation is only valid for $\Gamma < 2.36$, so we can only provide a lower bound on $\dot{E}$ for the *Chandra/NuSTAR*-derived $\Gamma = 2.6_{-0.4}^{+0.5}$.</footnote>
outburst from a magnetar or an unusually bright flare from a chromospherically active binary.

### 6.1.4. Black Hole Binary Candidate

Among the remaining NuSTAR Norma sources not discussed in Sections 6.1.1–6.1.3, NNR 15 stands out as the only source showing clear short-timescale variability in the NuSTAR band and also having the lowest median energy. As can be seen in Figure 5, NNR 15 displays flaring behavior in the 3–20 keV band; during one flare lasting about 15 ks, the source flux increases by a factor of >6, and, during a smaller flare lasting about 7 ks, the flux increases by a factor of >2. This source also shows variability on year-long timescales, since the 3–10 keV flux measured in 2013 NuSTAR observations is a factor of 2 higher than the Chandra flux measured from 2011 observations. The NuSTAR and Chandra spectra are well fit by an absorbed power-law model with very low $N_H$, indicating that the source must reside within a few kpc, and $\Gamma = 2.6 \pm 0.4$ (or $kT = 2.9_{-0.7}^{+1.0}$ keV for a bremsstrahlung model). No Fe line is visible in the spectrum, but, due to the limited photon statistics, we can only constrain the equivalent width of a potential Fe line feature to be <1.7 keV, a loose constraint that does not help to distinguish between different types of X-ray sources. Assuming a distance of 2 kpc, NNR 15 has an average unabsorbed 3–20 keV luminosity of $1.5 \times 10^{32}$ erg s⁻¹. Its optical/infrared counterpart has been identified as a mid-GIII star (Rahoui et al. 2014).

Based on these properties, we identify NNR 15 as a BH LMXB candidate in quiescence, although an active binary (AB) or CV origin cannot be entirely ruled out. In quiescence, ABs typically have $L_X = 10^{29-31.5}$ erg s⁻¹ and $kT < 2$ keV (Dempsey et al. 1993), but they can exhibit flares with peak luminosities of ~$10^{32}$ erg s⁻¹ and $kT \approx 10$ keV (Franciosini et al. 2001). However, AB flares tend to have very short rise times and long decay times (Pandey & Singh 2012), whereas the flares seen in NNR 15 appear to have more symmetric profiles. CVs have $L_X = 10^{29-33}$ erg s⁻¹ and $kT = 1-25$ keV (e.g., Eracleous et al. 1991; Muno et al. 2004), with magnetic CVs being more luminous and spectrally harder than nonmagnetic CVs (Barlow et al. 2006; Landi et al. 2009), so their properties are consistent with those of NNR 15. However, the flaring exhibited by NNR 15 is not typically seen in CVs. Nonmagnetic CVs have outbursts that last several days and recurrence times of weeks to months; intermediate polars (IPs) have outbursts of similar duration but that are very rare (Hellier et al. 1997; Szkody et al. 2002). Polars exhibit flares with ~hour-long durations, but they tend to be very soft ($kT < 1$ keV; Choi et al. 1999; Still & Mukai 2001; Traulsen et al. 2010). The properties of NNR 15 are reminiscent of the quiescent state of V404 Cyg, a well-known LMXB hosting a BH (Makino et al. 1989; Casares et al. 1992; Shahbaz et al. 1996). Recent NuSTAR observations of V404 Cyg in quiescence show that, in the 3–25 keV band, its power-law spectrum has $\Gamma = 2.35 \pm 0.2$ and it exhibits flux variations of up to a factor of 10 over periods of a few hours (Rana et al. 2016). Given the similarities between the X-ray spectra and light curves of NNR 15 and V404 Cyg, NNR 15 is most likely a BH LMXB, although it may be a CV or an AB. To order-of-magnitude accuracy, it is estimated that ~1000 quiescent BH LMXBs reside in the Galaxy (Tanaka 1996); the primary source of uncertainty in this estimate is our limited knowledge of the typical recurrence timescale of BH transients. Making the simplifying assumption that quiescent BH binaries trace the stellar mass distribution of the

Galaxy, and using the estimate of the stellar mass enclosed in the Norma survey area by F14, we would expect ~4 BH LMXBs to reside in the survey area. Thus, it is at least plausible that one BH binary would be detected in the NuSTAR Norma survey.

### 6.1.5. Cataclysmic Variables and Active Galactic Nuclei

Based on the NARCS results, we expect that the majority of the NuSTAR Norma sources should be a mixture of CVs and AGNs. CVs typically have thermal spectra with $kT \approx 1-30$ keV, although IPs can display even higher temperatures ($kT \approx 30-50$ keV; Landi et al. 2009), while AGNs exhibit power-law spectra with $\Gamma \approx 1.5-2$ (Tozzi et al. 2006; Sazonov et al. 2008). The remaining 17 tier 1 sources (NNR 4, 6, 9, 11–13, 16–20, 22, 23, and 25–28) have bremsstrahlung temperatures and photon indices consistent with being either CVs or AGNs. With the NuSTAR, Chandra, and infrared data available for these sources, there are three primary ways to distinguish CVs and AGNs:

i. If the absorbing column density inferred from X-ray spectral fitting or Chandra quantiles is significantly lower than the integrated interstellar $N_H$ along the line of sight to the source, it is a Galactic source.

ii. If the source does not have a point-like infrared counterpart with >98% reliability in the VVV survey, it may be an AGN or a Galactic source with a K or M main-sequence companion, which would fall below the VVV sensitivity limits ($K_s < 18$ mag) if located at $\gtrsim 2$ kpc. Since the energy bands used by the Wide-Field Infrared Survey Explorer (WISE; Wright et al. 2010) can be more useful than the J, H, or K bands for identifying AGNs (Mateos et al. 2012; Stern et al. 2012), we also searched for counterparts to the NuSTAR sources in the AllWISE catalog (Cutri et al. 2013). The BH binary candidate NNR 15 and four tier 2 sources (NNR 29, 30, 31, and 38) have WISE matches located within the 95% positional uncertainty of their Chandra counterparts. The counterparts of NNR 15, 29, and 30 have been identified as low-mass stars through spectroscopic follow-up (Rahoui et al. 2014), and the other WISE counterparts have $W1 - W2 < 0.1$, far below the typical value of $W1 - W2 \ge 0.8$ for X-ray luminous AGNs (Stern et al. 2012). Furthermore, the near-IR spectra of the counterparts of NNR 31 and 38 indicate that they are Galactic sources (J. Corral-Santana et al. 2017, in preparation). Thus, none of the NNR sources with WISE counterparts are AGNs, but we cannot rule out the possibility that some AGNs are undetected by WISE. For instance, in the NuSTAR serendipitous survey, which has sensitivity limits comparable to those of the NuSTAR Norma survey, about 25% of the NuSTAR sources at Galactic latitudes $|b| > 10^\circ$, which are likely to be AGNs, do not have a WISE counterpart (Lansbury et al., submitted).

iii. If the source exhibits strong unshifted Fe emission, it is more likely to be a CV than an AGN. Both magnetic and nonmagnetic CVs often exhibit Fe emission; in some sources, individual Fe lines at 6.4, 6.7, and 6.97 keV with equivalent widths of 100–200 eV can be seen, while in others, a broad component centered around 6.7 keV with an equivalent width of up to a few keV is seen, likely resulting from the blending of multiple Fe lines due to low energy resolution (e.g., Mukai & Shiokawa 1993; Ezuka & Ishida 1999; Baskill et al. 2005; Bernardini