The new primary X-ray spectral component of IC4329A
confirmed with Suzaku and NuSTAR
Katsuma Miyake,1 Hirofumi Noda,2 Shin’ya Yamada,3 Kazuo Makishima4 and Kazuhiro Nakazawa1
1University of Tokyo, 2 Tohoku University, 3 Tokyo Metropolitan University, 4 RIKEN
E-mail(KM): miyake@juno.phys.s.u-tokyo.ac.jp
Abstract

The bright and highly variable Seyfert I active galactic nucleus, IC4329A, was observed simultaneouslywith Suzaku and NuSTAR for about 3 days. Within the observation, the 2–10 keV flux changed by afactor of 1.5 around the mean of ∼ 1.0 × 10−10 erg cm−2 s−1 . Using the model-independent spectral
decomposition method developed by Noda et al. (2011), the time-averaged Suzaku and NuSTAR spectra(3–70 keV) were consistently decomposed into a fast and a slowly varying components. While the former
can be modeled as a power-law (PL) with a photon index Γ = 2.1, the latter was interpreted as the sumof a harder primary PL (Γ = 1.4) and the reflection. By combining the Suzaku and NuSTAR data, thepresence of this new primary PL, first revealed in the Suzaku data (Miyake et al. 2016), has been reinforced.
Key words: galaxies: active — galaxies: individual (IC4329A) — galaxies: Seyfert — X-rays: galaxies

432Count rate (cnt/s)

050

100150 200
Time (ks)

250

300

0.1

Fig. 1. Background subtracted and dead-time corrected light curvesof IC4329A in 2–10 keV with FPMA (black) and XIS FI (gray).
Time 0 is 2012 August 13 UT 02:13:09.

ratio
0.05

2. Observation and Data Reduction
IC4329A is a bright and highly variable Seyfert I AGN(Perola et al. 1999). This source was observed in 2012
August 13 simultaneously with Suzaku and NuSTAR.
We had already analyzed this Suzaku data set, and confirmed the Γ = 1.4 PL in the 2–45 keV spectra (Miyakeet al. 2016). In the present work, we try to strengthen
the result by incorporating the NuSTAR data. We analyzed publicly available XIS 0, XIS 3, and HXD-PIN
data of Suzaku, and FPMA and B data of NuSTAR. XIS
0 and 3 data were co-added and are referred to as XIS
FI. On-source events of the XIS and FPM were extracted
from a circular region of 3′ radius, and background eventsfrom an annular region of the radii 4′ .8–7′ .8. From theHXD-PIN data, we subtracted Non X-ray Backgroundestimated by Fukazawa et al. (2009) and Cosmic X-ray
Background (Boldt 1987).

0.02

1. Introduction
X-ray spectra of Seyfert I active galactic nuclei (AGNs)
were considered to consist of a power-law (PL) like primary component, and its reflection component accompanied by an Fe Kα line (Fabian & Miniutti 2005). However, it is in reality diﬃcult to observationally extract theprimary PLs, because diﬀerent components are mixed in
an observed spectrum (Cerruti et al. 2011). Noda etal. (2011, 2013a) developed a model-independent spectral decomposition method, the “Count-Count Correlation with Positive Oﬀset” (C3PO) method. Using it, wediscovered that X-ray spectra of many AGNs harbor thetwo distinct primary PLs (Noda et al. 2013b, 2014).

51020
Energy (keV)

50Fig. 2. Background-subtracted time-averated spectra of IC4329A
obtained with FPMA (black), and XIS FI and the HXD (gray).

3.2. C3PO method
We applied the C3PO method to NuSTAR data for thefirst time. The 2–3 keV band of XIS FI was selected
as the reference band, because it exhibited the highest variability. Figure 3 shows examples of Count-Count
Plots (CCPs). Since all CCPs in finer bands from 3 to70 keV showed good linearity like figure 3, we applied theC3PO method to the present data. As shown in figure 4,
we successfully decomposed the time-averaged spectra ofIC4329A into fast (FS) and slowly variable spectra (SS).
The result of the decomposition was consistent betweenthe NuSTAR and Suzaku data. The FS has a PL shapewith Γ ∼ 2, while the SS has the Fe K line.
FPMA count rate (cnt/s)

1.2
3-5 keV0.6

15-50 keV

0.8
0.4
0.4

00.2

0.4

0
0.4
0.8
1.2
XIS 2-3 keV count rate (cnt/s)

0.8

1.2Fig. 3. CCPs between the reference XIS 2–3 keV and a broad FPMAband. The best-fitting straight line is shown in gray.

The C3PO method performs a simultaneous fitting using two model components denoted as model Fand model S. The FS is fitted with model F, the SSwith model S, and the time-averaged spectrum withmodel F+model S. As discussed in Miyake et al. (2016),
the SS of the Suzaku data requires not only the reflection but also another PL. Therefore, we adopted a PL
as model F, and again the sum of reflection and anotherPL as model S. As a result of the simultaneous fittingwith parameters common to the NuSTAR and Suzaku
data, the fit was successful with χ2 /d. o. f. = 427/368.

keV 2 (Photons cm -2 s -1 keV -1)

0.1
0.01
10−3
10−42

χ

3. Data Analysis3.1. Light curves and spectra
Figure 1 shows background subtracted and dead-timecorrected light curves in 2–10 keV obtained with NuSTAR and Suzaku, with net exposures of 162 and 102 ks,respectively. The source varied by a factor of ∼ 1.5
in the observation, around a mean 2–10 keV flux of∼ 1.0 × 10−10 erg cm−2 s−1 . In the present work, weutilize the NuSTAR data only over the period which issimultaneous with Suzaku; the net exposure is reduced
to 68 ks. Figure 2 shows background-subtracted timeaveraged spectra in the form of their ratios to a PL modelwith photon index Γ = 2. In addition to an approximately PL-like continuum with a cutoﬀ at ≥ 40 keV,they exhibit Fe Kα line and edge at 6.4 and 7.2 keV in
the rest frame, respectively.

0−2
51010

2020

5050

5510102050

Energyrgy(ke(keV)

Fig. 4. The time-averaged spectra (black) shown with the FSs (green)
and SSs (red), fitted with the sum of the fast variable PL (green),
the slowly variable PL (magenta), and the reflection (blue).

The PL of model F has Γ = 2.0, while model S consistsof another Γ = 1.5 PL, strongly-absorbed with a column
density NH = 3.7 × 1023 cm−2 , and reflection with theabundance fixed at 1 solar and the solid angle of 1.0π.
4. Discussion and Conclusion
The spectra of IC4329A was successfully decomposedwith the C3PO method, and they were well reproducedby considering two primary PL radiation as other AGNs.
So far, the NuSTAR spectrum of IC4329A has been
considered to include a relativistically smeared Fe line
(e.g. Brenneman et al. 2014). When the relativisticreflection was adopted instead of the slowly variable PL,
the fit became unaccepted with χ2 /d. o. f. = 648/367.
Thus, the advantage of our modeling over the relativistic
Fe line interpretation has been very much strengthened
by incorporating the NuSTAR data, because these modelings gave fit χ2 /d. o. f. of 200/168 and 205/169, respectively, when using only the Suzaku data. Therefore, our“two primary PLs” interpretation has been reinforced bycombining both the NuSTAR and Suzaku data.
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