Suzaku Observation of 1H0707-495: Puzzling

Progress of Theoretical Physics Supplement No. 169, 2007

269

Suzaku Observation of 1H0707-495: Puzzling Spectral Drop around 7 keV Kiyoshi Hayashida,1,∗) Naohisa Anabuki,1 Luigi Gallo,2 Kazushi Iwasawa,2 Yasuo Tanaka,2 Giovanni Miniutti,3 Andy C. Fabian,3 James N. Reeves,4 Tahir Yaqoob,4 Takashi Okajima,4 Chiho Matsumoto,5 Yoshito Haba,5 Yuichi Terashima,6 Aya Kubota,7 Yoshihiro Ueda8 and the Suzaku Team 1 Department

Suzaku observation of the narrow line Seyfert 1 galaxy 1H0707-495 reveals a spectral drop around 7.3 keV, deepest ever observed. We propose a new model for the spectral drop in which absorption in corona is considered, alternative to the previously proposed partial covering model or the relativistically blurred disk line model.

§1.

Introduction

X-ray emission from active galactic nuclei is basically characterized with a power law spectrum, indicating non-thermal nature of the emission. However, detailed observations have revealed various spectral features imprinted on the power law continuum, such as soft X-ray excess emission, iron-K emission lines, absorption features by ionized matter, and a Compton hump. Reprocessing and absorption of the power law continuum which make these features are extensively studied. 1H0707-495 is one of the narrow line Seyfert 1 (NLS1) galaxies, of which redshift is 0.0411. This source has the X-ray properties of the NLS1s, enhanced soft excess component and rapid large amplitude variability, as their extreme case. Moreover, the XMM-Newton observation of this source on 2000 revealed a sharp spectral drop around 7 keV, which led extensive discussions on its nature.1) The spectral drop was detected again in the XMM observation of this source on 2002, but the drop energy shifted from 7 keV to 7.5 keV.2) Similar spectral drop has been found in other sources later, for example in the NLS1 IRAS13224-38093) but at 8 keV.

∗)

E-mail: [email protected]

Downloaded from http://ptps.oxfordjournals.org/ by guest on December 15, 2016

of Earth & Space Science, Osaka University, Toyonaka 560-0043, Japan 2 Max-Planck-Institut f¨ ur extraterestrische Physik, Garching, Germany 3 Institute of Astronomy, Cambridge, Cambridge, UK 4 Exploration of the Universe Division, NASA GSFC, Greenbelt, USA 5 Department of Physics, Nagoya University, Nagoya 464-8602, Japan 6 Department of Physics, Ehime University, Matsuyama 790-8577, Japan 7 Institute of Physical and Chemical Research (RIKEN), Wako 351-0198, Japan 8 Department of Physics, Kyoto University, Kyoto 606-8502, Japan

270

K. Hayashida et al. §2.

Observation and data analysis

1H0707-495 was observed with Suzaku on December 3–6 2005 for about 88 ksec. In this paper, we report the results from the XIS4) using ver. 1.2 data products and HEASOFT 6.1.1-6.1.2. Figure 1(a) shows the X-ray light curves of the source for 0.2–0.4 keV, 0.4–1 keV, and 1–7 keV bands, with hardness ratio of 1–7 keV to 0.4–1 keV. X-ray variability of nearly one order of magnitude are evident. It should be noted that small tail components in the XIS instrumental response enable us to pick up 0.2–0.4 keV band with little contamination from the higher energy band. The averaged spectra of the source are displayed in Fig. 2(a). The spectra is approximated with a power law plus a disk black body component suffered by Galactic absorption. However, it is apparent that the spectra has a cutoff above 7 keV. This spectral (a) 0.2–0.4 keV. drop is fitted with an absorption edge at 7.31±0.07 keV (source rest frame), τ = 4.9±1.2. Further inspection of the residual suggests the emission line components at 1.5 keV and absorption features around 1keV. The absorption features around 1 keV, most likely absorption by ionized Fe-L and/or Ne, are stronger when the X-ray intensity is high. Expansion of the spectrum (the sum of 3 FI CCDs) for 5–10 keV range is dis(b) 0.4–1 keV, 1–7 keV, hardness ratio played in Fig. 2(b). Compared with the (1–7 keV)/(0.4–1 keV). spectral model derived from the XMMNewton observations on 2000 and 2002, Fig. 1. X-ray light curve of 1H0707-495. the Suzaku 2005 observation shows the deepest, nearly one order of magnitude, drop. According to the XMM-Newton observations, the width of the drop was narrower than 0.2 keV, but the statistics of the Suzaku spectrum is too low to constrain the width. Note that the uncertainty in the background subtraction in our Suzaku analysis is about 10−4 counts s−1 keV−1 in the scale of Fig. 2. The origin of the spectral drop at 7–7.5 keV observed with XMM-Newton is interpreted with two different models. One is a partial covering model.5) If the deep drop is due to iron K absorption, strong iron K emission line should appear in some geometry. As there is no apparent iron K emission line (the EW is smaller than 140 eV in the case of Suzaku), geometry like patchy absorber is required. The Suzaku spectra fitted with the partial covering model are shown in Fig. 3(a). Covering fraction is as high as 0.79 ± 0.06. The iron over abundance factor is not constrained, Bin time:

2048.

s

0.2−0.4keV

0.01 0.02 0.03 0.04 0

0

5×104

105

1.5×105

2×105

2.5×105

Time (s) Start Time 13707 4:48:40:530 Stop Time 13710 2:12:56:530

1H 0707−495

512.0

s

0.6 0.4 0.2

0.3 0.2

2 1

(1−7keV)/(0.4−1keV)

0

0.1

1−7keV

0.4 0

0.4−1keV

0.8

Bin time:

0

5×104

105

1.5×105

2×105

Time (s)

Start Time 13707 4:36:56:530

Stop Time 13710 2:18:16:530

2.5×105


1H 0707−495

Suzaku Observation of 1H0707-495 1H0707−495 Suzaku

0.01 10−3

10−4

10−4

10−3

0.01

normalized counts s−1 keV−1

0.1

0.1

1H0707−495 Suzaku


271

0.2

0.5

1

2

5

10

Energy (keV)

0.2

0.5

1

2

5

10

Energy (keV)

(a) Average X-ray spectra obtained with Suzaku XIS on 2005. XIS1 (BI) spectrum and summed spectrum of XIS0,2,3 (FI) are displayed.

Fig. 2. Average spectra obtained with Suzaku.

Relativistically Blurred Reflection from Ionized Disk

0.2

0.01 10−4

10−3


0.01 10−3 10−4


0.1

0.1

Partial Covering

0.5

1

2

5

Energy (keV)

(a) Partial covering model.

10

0.2

0.5

1

2

5

10

Energy (keV)

(b) Relativistically blurred disk line model.

Fig. 3. Partial covering model and relativistically blurred disk line model.

i.e., the higher gives the better fit. For example, if we set 50 solar to be the hard limit, the 90% lower limit is 18 solar. Another interpretation is the relativistically blurred disk line model.6) In this model, the spectral drop is regarded not as absorption edge but as the blue side of the iron K emission line. The emission line comes from the vicinity of the black hole, where Doppler boosting and relativistic effects are extreme, as a reflection from ionized accretion disk. Fitting this model to our data makes some residual around the spectral drop, although further investigations of parameter space is needed. In the current best fit parameters, over abundance of ∼ 5 solar is needed.


(b) 5–10 keV range spectrum obtained with Suzaku on 2005, compared with the modeled spectra from XMM observations on 2000, and on 2002.

272

K. Hayashida et al. kTe=100keV / A(Fe)=0.0,0.01,0.1,0.3,1.0

1000

104

Counts/0.05keV

1000

104

Counts/0.05keV

105

105

kTe=10keV / A(Fe)=0.0,0.01,0.1,0.3,1.0

1

2

5

10

Ex(keV)

(a) kTe = 10 keV.

20

1

2

5

10

20

Ex(keV)

(b) kTe = 100 keV.

§3.

Low temperature corona or not-fully ionized corona model

We here propose an alternative model for the spectral drop around 7 keV. In the two models we have tested above, it is considered that how the primary power law component is affected by absorption or reflection. We consider a case that the primary emission is not a power law, specifically, the primary emission is due to inverse Compton process of soft X-ray photons in accretion corona of which electron temperature (kTe) is significantly lower than usually assumed value of 100 keV. If absorption by not-fully ionized iron is effective in such a corona by some reasons, e.g., low kTe or inhomogeneity of the corona, a sharp spectral drop with weak iron emission line might be reproduced. Preliminary and simplified simulation is performed by employing the absorption coefficient is zero below the iron K edge and the same as neutral one above the edge. We parameterize the absorption effect by the fraction of the iron which contribute the absorption (fabs ) multiplied by the abundance of the iron relative to the solar (ZF e ), fabs × ZF e . We set the initial soft photons of 1 keV energy start from the center of spherical corona with the radius of 5 Thomson depth. Fluorescent yield is set to be 0.324. Figure 4 displays the results. The left panel shows kTe = 10 keV case, and the right panel shows the kTe = 100 keV case. For each case, we tested fabs × ZF e =0, 0.01, 0.1, 0.3, and 1.0 for each case. For kTe = 10 keV case, the EW is less than 200 eV for all the fabs × ZF e parameters. Spectral drop of factor of more than 3 is realized with fabs × ZF e is greater than 0.3. It is apparent that we need more sophisticated simulation in which ionization stages of the iron and absorption coefficients, edge energies, and fluorescence yield determined by them are considered. Nevertheless, our preliminary simulation at least suggests that it is worth reconsidering how the primary power law is produced, in this source and in other sources.


Fig. 4. Simulation of Comptonization in Corona. Emergent spectra for fabs × ZF e = 0, 0.01, 0.1, 0.3, and 1.0 are plotted in back, red, orange, blue, and purple. (See the online edition for the color version of Fig. 4).

Suzaku Observation of 1H0707-495

273

References 1) 2) 3) 4) 5) 6)

Th. Boller et al., Mon. Not. R. Astron. Soc. 329 (2002), 1. L. C. Gallo et al., Mon. Not. R. Astron. Soc. 353 (2004), 1064. Th. Boller et al., Mon. Not. R. Astron. Soc. 343 (2003), L89. K. Koyama et al., Publ. Astron. Soc. Jpn. 59 (2007), S23. Y. Tanaka et al., Publ. Astron. Soc. Jpn. 56 (2004), L9. A. C. Fabian et al., Mon. Not. R. Astron. Soc. 353 (2004), 1071.
