PASJ: Publ. Astron. Soc. Japan 51, 677–681 (1999)

ASCA Observation of a Radio-Loud Quasar
and a New Candidate for a Cluster of Galaxies
Fumie Akimoto,1 Akihiro Furuzawa,1 Yuzuru Tawara,1 Yuichi Kamata1,2
and
Koujun Yamashita1
1Department of Physics, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602
2
Research Center for Advanced Energy Conversion, Nagoya University
Furo-cho, Chikusa-ku, Nagoya 464-8603
E-mail (FA): akimoto@u.phys.nagoya-u.ac.jp(Received 1999 June 14; accepted 1999 August 19)

Abstract
During an X-ray observation of the distant galaxy cluster CL 0107+31 (z = 0.69) with ASCA, an
extended X-ray source was detected in the ﬁeld of view. Based on an image analysis in two diﬀerent energy
bands of 1–3 keV and 3–10 keV, we found that this source consists of a hard point-like component and an
additional soft diﬀuse X-ray source extended over ≥ 3 in radius. However, due to poor photon statistics,
we could not distinguish whether their X-ray spectra are thermal or non-thermal. The peak position of thecentral X-ray source coincides with a radio-loud quasar, NRAO 58. NRAO 58 is found to have a power-law
46
erg s−1 (in the quasar rest
spectrum with a photon index 1.6+0.3−0.2 and a (2–10 keV) luminosity of ∼ 4×10
frame). The extended component is likely to be a candidate for a nearby galaxy group which accidentallycoincides with the quasar.
Key words: galaxies: clusters: individual (CL 0107+31) — galaxies: individual (NRAO 58) — X-rays:
galaxies

1.

Introduction

NRAO 58 is a radio galaxy whose integrated ﬂux density at 4.75 GHz is 270 mJy (Neumann et al. 1994). The
optical counterparts were identiﬁed in both B (18.2 mag)and R (17.4 mag) images in APM and USNO cataloguesof the Space Telescope Science Institute (STScI), and located at the position of (αJ2000 = 01h 09m27.s9, δJ2000 =+31◦ 49 56 ). Laurent-Muehleisen et al. (1998) identiﬁed
this object as a radio-loud quasar, RGB J0109+318, with
redshift of 1.71.
An X-ray source, which coincides with the object,
was also detected during the ROSAT All Sky Survey(RX J0109.4+3149, Brinkmann et al. 1997). The position of the X-ray peak was determined to be (αJ2000 =01h 09m27.s7, δJ2000 = +31◦ 49 57 ), which coincides with
NRAO 58.
We report on the X-ray properties of this source basedon data obtained from an ASCA observation. This paper
comprises ﬁve sections. In section 2 we describe the details of the observation. The results and the discussions
are presented in section 3 and section 4, respectively. A
summary of this paper is given in section 5. Throughout
this paper we use the solar abundances given by Anders

and Grevesse (1989), H0 = 50 km s−1 Mpc−1 and q0 = 0,
and the quoted errors are the 90% conﬁdence level.
2.

Observations

Figure 1 shows an X-ray image obtained from ASCA
observations, emphasizing the relative position to a cluster of galaxies, CL 0107+31 (3C 34). As can be clearly
seen, the X-ray source including NRAO 58 (the westpeak) is much brighter than CL 0107+31 (the east peak).
This X-ray image was obtained during an observation of
CL 0107+31 on January 11–14 in 1997; when the centerof the ﬁeld of view (FOV) was located at the primarytarget CL 0107+31, the source including NRAO 58 was
detected near to the edge of the gas-imaging spectrometer (GIS) FOV, and was thus not in the solid-state imaging spectrometer (SIS), which had been operated with
1 CCD mode [see Ohashi et al. (1996) and Makishima etal. (1996), Burke et al. (1994) for details of GIS and SIS,
respectively. Regarding the X-ray telescopes of ASCA(XRTs), see Serlemitsos et al. (1995)].
A supplementary observation was carried out while
focusing on a bright X-ray source including NRAO 58,
6 months later, on July 21–22 in 1997, both with the678

F. Akimoto et al.

Fig. 1. ASCA GIS 2+GIS 3 X-ray contour in the 0.7–
10 keV band of both observations for a serendipitously detected X-ray source detected near the distant rich compact cluster of galaxies CL 0107+31
surrounding a bright radio galaxy; 3C 34 (z = 0.69).
The image has been smoothed with a Gaussian proﬁle having a width of σ = 0. 5, and the contour levels are logarithmically spaced with a lowest level of
2×10−4 cts s−1 arcmin−1 . The superposed grayscale shows a 1.4 GHz radio image obtained fromthe NRAO VLA Sky Survey (NVSS) project, whose
the beam size = 45 (FWHM) resolution

GIS and the SIS. The GIS data were taken in the nominal PH mode throughout the observations with a circularﬁeld of 25 radius, while the SIS data were taken in the
2 CCD mode covering a FOV of 11 × 22 . The eﬀective exposure time of these two observations were ∼66 ks
during the ﬁrst period and ∼24 ks for the second period,
respectively.
In both observations, steady X-ray emission fromCL 0107+31 was detected with counting rates of∼ 0.002 cts s−1 in each GIS within a circle of 5 radiusin the 0.7–10 keV energy band and in each SIS within acircle of 3 radius in the 0.5–10 keV energy band. The
counting rate of the source including NRAO 58 was estimated to be ∼ 0.016 cts s−1 for the GIS within a circle of
3. 5 radius centered at the peak of the X-ray image in the
0.7–10 keV energy band in the ﬁrst observational period,and ∼ 0.018 cts s−1 in the second observational period.
The SIS counting rate was estimated to be ∼ 0.027 cts s−1
in the same region as that covered by the GIS in the 0.5–
10 keV energy band, which is consistent with that of the
GIS.

[Vol. 51,

Fig. 2. Radial proﬁle of the surface brightness per detector in the energy range of 1–3 keV, marked as
crosses. The error of each data point is 1σ, including the background uncertainty. The dashed andsolid lines represent proﬁles which are expected fromsimulations for a point source and a uniform diﬀuse
component, respectively.

3.

Results

3.1. Counterparts of the X-Ray Source
Two distinct X-ray sources are shown in ﬁgure 1.
The east weak X-ray source corresponds to the clusterof galaxies CL 0107+31 surrounding the central radiogalaxy 3C 34, while the position of the west bright X-ray
source is (01h 09m. 5, +31◦49 )J2000 , which coincides withthe radio-loud quasar, NRAO 58 within 1 , the typical
positional error of ASCA (Tanaka et al. 1994). No otheridentiﬁed sources have been found in this ﬁeld.
3.2. Radial Distribution of X-Ray Surface Brightness
Figure 2 shows the background-subtracted radial proﬁle of the X-ray source centered on NRAO 58 in the energy band of 1–3 keV. The background data were extracted from public-released blank-sky data maintainedby ASCA Guest Observer Facility at NASA/GSFC, andwere normalized by the eﬀective exposure time. Thisbackground is consistent with the background level,
which was estimated from the source-free region in ourdata.
For a comparison with the radial proﬁle, a model proﬁle of a point-source, that is, XRT Point Spread Function(PSF), was produced by an XRT raytracing simulation
assuming 2 keV monochromatic X-rays (Tsusaka et al.
1995). Within 3 radius from the peak, the radial proﬁleis well ﬁtted by a single point source.

No. 5]

NRAO 58 and a Cluster of Galaxies Candidate

On the other hand, excess extended emission can beseen at larger radii. This result is aﬀected by the background uncertainty, which consists of two origins. One
comes from the use of blank-sky data as background; theother comes from the diﬀerence between the simulated
PSF and the actual PSF. From an analysis of the background using 10 ks observations (Ikebe 1995), the uncertainty of the GIS background for our observation can be
estimated to be 0.04 cts/pix (a pixel size is 0. 25 × 0. 25).
The uncertainty of PSF at 5 apart from the X-ray peakis less than 20%. Then, the uncertainty of the surface
brightness is estimated to be 0.02 cts/pix. Since the surface brightness of the excess emission was obtained tobe 0.13 cts/pix at 5 oﬀ from the X-ray peak, the sourceshows a signiﬁcant excess over that of a point source. Unfortunately, the uncertainty of the SIS background is verylarge (factor of ∼2) and can not be estimated exactly.
The central point-like emission has a harder X-ray spectrum than the extended component (see subsection 3.4).
To avoid contamination by the hard component, the image obtained in the 1–3 keV energy band was used toinvestigate the extended component.
To describe the outer tail of the radial proﬁle, we ﬁtted
the measured proﬁle within a 7 radius to a model which
consists of a point-source and a diﬀuse component. Because of insuﬃcient photon statistics, the diﬀuse component was assumed to have a ﬂat brightness distribution
with a cutoﬀ radius of 7 . For a single point-source modela ﬁt to the radial proﬁle, using combined data of the ﬁrst
and second observations, resulted in a χ2 value of 196(d.o.f. = 54). Adding a diﬀuse component to the point
source model gives a much better ﬁt: the χ2 value reduces to 63 (d.o.f. = 53), and the resultant ﬂux of thediﬀuse component is 23 ± 5% of the total ﬂux.
The source was also detected accidentally at the edge of
the ROSAT PSPC (43 apart from the center) in a 27 ks
pointed observation, with a counting rate of 0.05 cts s−1 .
However, the uncertainties of the background level andthe point spread function at such large oﬀ axis angles
prevent us from examining the extent of the source. (Weobtained these data from the public archive through the
HEASARC on-line service provided by NASA/GSFC.)
3.3. Temporal Analysis
To investigate changes in the X-ray ﬂux between two
observational periods, we compared the ﬂuxes within acircle of 7 radius centered at the peak of the source including NRAO 58, taking into account vignetting corrections in each observation; also, the counting rates of the
background were estimated from the source-free regionsin the same ﬁeld of view. The X-ray ﬂux during thefollow up observation was 30% smaller than that of theﬁrst observation in the energy band of 0.7–10 keV, which
is signiﬁcant at the 4 σ level. Unfortunately, the poor

679

Fig. 3. (a) GIS 2+GIS 3 spectrum from a circular region with a radius of 3. 5 obtained during two observation periods. In the top panel, we show the data
and the best ﬁt power law model (with photon index
1.60 and nH 2 × 1021 cm−2 ). The lower panels areresiduals from the best-ﬁt model in units of σ. (b)SIS 0+SIS 1 spectrum obtained during the second
observation period, ﬁtted with the same model.

photon counts do not allow us to distinguish whether thechange originates from the central source or the extended
source.
3.4. Spectral Analyses
For spectral analyses, we accumulated GIS 2+GIS 3
spectra for three regions: within a circle of 3. 5 and 7. 0
radius centered on the X-ray peak, and an annulus of 3. 5–
7. 0 radius, while we made SIS 0+SIS 1 spectra within acircle of 3. 5 radius. The data in the regions outside the7 circle were used to estimate the background spectrum.
In spite of the existence of extended emission, these
spectra show no signiﬁcant emission lines. Therefore, wecannot estimate the redshift of this source from the Xray data. We then ﬁtted the GIS and SIS spectra simultaneously, assuming a single power law and absorption hydrogen column density nH with galactic value(6.1 × 1020 cm−2 ) (Heiles 1975). The best-ﬁt parameters
are summarized in table 1. At a z = 1.71, the luminosities in the observed 2–10 keV band are also listed. The
resulting spectra are shown in ﬁgure 3. The photon index of 1.5–2.0 is typical among AGNs. It is also notedthat the photon index of the spectrum within r < 3. 5
is smaller than that in the outer region. This trend is
not due to an inappropriate choice of the amount of absorption, since it held even when we used the galactic
absorption value (see the last column of table 1): The
inner region seems to be harder than the outer region.

680

F. Akimoto et al.

[Vol. 51,

Table 1. Best-fit parameters of power law model.

∗

χ2ν

Photon index∗

46
6.1 +2.6−1.8 ×10

0.39

1.7+0.1−0.2−12
1.2+0.5−0.5 ×1046
5.2 +2.2−2.2 ×10

0.29

1.5+0.1−0.1−13
4.4+1.5−1.3 ×1046
1.9 +0.7−0.6 ×10

0.1 6

2.0+0.5−0.5RadiusarcminPhoton index

nH
10 cm−2

FX2−10keV
erg s−1 cm−2

LX2−10keV
erg s−1

0–7.0

1.8+0.2−0.2

0.19+0.23
−0.19

−12
1.4+0.6−0.4 ×10

0–3.5

1.6+0.3−0.2

0.17+0.16−0.14

3.5–7.0

2.0+0.5−0.5

0.061 fixed

22

Value at nH = 6.1×1020 cm−2 .

4.

Discussion

4.1.

The Point Source NRAO 58; A Radio-Loud Quasar
NRAO 58 was detected in the ROSAT survey and at
the edge of the detector during a ROSAT PSPC pointing
observation. The ROSAT ﬂux in the 0.1–2.4 keV energy
band was estimated to be 1.60 ×10−12 (Neumann et al.
1994) and (1.41 ± 0.33) ×10−12 erg s−1 cm−2 (LaurentMuehleisen et al. 1998; Brinkmann et al. 1997).
The ASCA total ﬂux in the observed 2–10 keV band−12
erg s−1 cm−2 in r < 7 (see in tais 1.4+0.6−0.4 ×10
ble 1). The X-ray ﬂux in the 0.1–2.4 keV energy band−12
erg s−1 cm−2 from an
is estimated to be 1.8+0.8−0.5 ×10
extrapolation of the ASCA X-ray spectrum assuming asingle power law with an photon index of 1.8.
The X-ray spectrum of the point-like component was
reproduced by a single power law with a photon index
of 1.6 +0.3
−0.2 , which dominates the X-rays coming fromthe inner region of the X-ray source, which is a typical value for AGNs. The X-ray ﬂux and the luminosity
in the 1–3 keV energy band (r < 7 ) are estimated to be−13
46
5.5+1.8
erg s−1 cm−2 and 2.4+1.3
erg s−1 ,−2.9 × 10−0.8 × 10
respectively, which correspond to 72–82% of the total
ﬂux within 7 . Even with this reduced X-ray ﬂux, theobtained ﬂux ratio log(fX /fV ) of ∼ −3 is typical for aradio-loud quasar.
4.2.

The Diﬀuse Component; A Group or Cluster of
Galaxies?
Since no signiﬁcant emission line was detected in the
X-ray spectrum, it has not been determined whether theemission is thermal or non-thermal. The spatial extent of
the X-ray source indicates that it neither originates in the
X-ray halo of a relatively nearby galaxy nor is a galactic source, such as a supernova remnant. We searched
for counterparts of the diﬀuse X-ray source using the
SIMBAD database with 7 radius of the X-ray center, andcould ﬁnd no candidate, except for a catalog object corresponding to the central radio object, RX J0109.4 − 3149,
RGB J0109 + 318, and EF B0166 + 3134. The high galactic latitude of −30.◦ 9 suggests only a small possibility ofa chance coincidence with galactic objects. Thus a groupor a cluster of galaxies is a likely candidate of this X-ray
source. At z = 1.71, the X-ray luminosity in the 2–
10 keV energy band, temperature, and metal abundance
are estimated to be 5+13
−2 keV, 0.8 (< 3) solar abundance,46
erg s−1 in the rest frame. This X-ray
and 1.0+0.2−0.1 ×10
luminosity is similar to that of the largest rich clusters.
Optical imaging observations (V , R, I bands) of thisregion have been conducted at Kitt Peak Observatory, byNakahira et al. (1999, unpublished). They claimed that,though there is no evidence for the existence of a richgalaxy cluster, there are several small concentrations of
nearby galaxies (z ∼ 0.1) in the ﬁeld of view. According
to this result, a large redshift of 1.71 is unlikely.
For z = 0.1, the temperature and the X-ray luminos+2.2
42erg s−1 , reity would be 2.9+5.8−1.8 keV and 8.5−1.7 ×10
spectively. In this case, the temperature and the X-ray
luminosity are typical for a group of galaxies or a poorcluster of galaxies. It is suggested that the diﬀuse X-ray
component is from a nearby group of galaxies or a poorcluster of galaxies (z ∼ 0.1) and accidentally coincides
with the position of NRAO 58.
5.

Summary

A diﬀuse X-ray source was found in the ASCA GIS
FOV during an observation in 1997 January. The position of the source was determined to be (αJ2000 =01h 09m28s , δJ2000 = +31◦ 49 21 ). Within the ASCA error circle of the position, it is coincident with a radio-loudquasar, NRAO 58.
The surface-brightness distribution within a circle of7 radius can be reproduced by the combination of apoint-like component and a diﬀuse component (23% of
the total X-ray ﬂux). The central point-like source isconsistent with a radio-loud quasar from the SED. On
the other hand, we could not distinguish whether thediﬀuse component is of thermal or non-thermal origin.
Due to the absence of any optical counterparts in this region, except for the central object and the high galactic
latitude, the diﬀuse X-ray source is a candidate of Xray emitting cluster of galaxies. Several concentrations

No. 5]

NRAO 58 and a Cluster of Galaxies Candidateof nearby galaxies (z ∼ 0.1) in this region during the optical photometry at Kitt Peak Observatory support thishypothesis.
Future X-ray missions with better spatial and energy
resolutions, such as Astro-E, AXAF, and XMM, are expected to clarify whether the diﬀuse component is truly
a group or cluster of galaxies. Similarly, optical redshifts
are desired to determine the cluster properties.
We thank the ASCA team for their daily operationof ASCA and SimASCA software team. We also thankY. Kumai, M. Tanaka, K. Ohta, M. Matsumura, M.
Arnaud, and H. Watarai for their useful advice andhelp. This research is made possible with the use ofﬁve databases; the NASA/IPAC Extragalactic Databasewhich is maintained by the Jet Propulsion Laboratory,
California Institute of Technology, the Digitized Sky Survey database provided by the Space Telescope Science
Institute (STScI), the National Radio Astronomy Observatory VLA Sky Survey database, SIMBAD astronomical
database maintained by CDS, Strasbourg, and ROSAT
image database of the HEASARC on-line service provided by NASA/GSFC.

681

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