Compton thick AGN in the Suzaku era

TL;DR

This paper presents Suzaku X-ray observations of two heavily obscured Seyfert 2 galaxies, including a Compton thick source, providing insights into their broad band X-ray spectra and iron line features.

Contribution

It offers new broadband X-ray spectral data of two Seyfert 2 galaxies, highlighting differences in their obscuration and reflection features, including potential relativistic iron lines.

Findings

Detection of broad band X-ray spectra up to tens of keV

Identification of Compton thick obscuration in one galaxy

Evidence of relativistic iron line in NGC 4992

Abstract

Suzaku observations of two hard X-ray (> 10 keV) selected nearby Seyfert 2 galaxies are presented. Both sources were clearly detected with the PIN Hard X-ray Detector up to several tens of keV, allowing for a fairly good characterization of the broad band X-ray continuum. Both sources are heavily obscured, one of which (NGC 5728) being Compton thick, while at lower energies the shape and intensity of the scattered/reflected continuum is very different. Strong iron Kalpha lines are detected in both sources. There are also hints for the presence of a broad relativistic iron line in NGC 4992.

Figures

Full text

Compton thick AGN in the Suzaku era

Andrea Comastri1

  

Roberto Gilli1

  

Cristian Vignali2

  

Giorgio Matt3

  

Fabrizio Fiore4

  

Kazushi Iwasawa5

1 Introduction

A fraction as high as 50% of Seyfert 2 galaxies in the nearby Universe are obscured in the X–ray band by column densities of the order of, or larger than the inverse of the Thomson cross-section ($N_{H}\geq\sigma_{T}^{-1}\simeq 1.5\times 10^{24}$ cm*-2*), hence dubbed Compton thick (CT). If the optical depth ($\tau=N_{H}\sigma_{T}$) for Compton scattering does not exceed values of the order of “a few”, X–ray photons with energies higher than 10–15 keV are able to penetrate the obscuring material and reach the observer. For higher values of $\tau$, the entire X–ray spectrum is depressed by Compton down scattering and the X–ray photons are effectively trapped by the obscuring material irrespective of their energy. The former class of sources (mildly CT) can be efficiently detected by X–ray instruments sensitive above 10 keV, while for the latter (heavily CT) their nature may be inferred through indirect arguments, such as the presence of a strong iron K$\alpha$ line over a flat reflected continuum. The search for and the characterization of the physical properties of CT AGN is relevant to understand the evolution of accreting Supermassive Black Holes (SMBHs). In particular, mildly CT AGN are the most promising candidates to explain the residual (i.e. not yet resolved) spectrum of the X–ray background around its 30 keV peak (Comastri 2004a; Worsley et al. 2005) but only a handful of them are known beyond the local Universe (see Comastri 2004b for a review). If this were the case, we may be missing a not negligible fraction of the accretion power in the Universe and of the baryonic matter locked in SMBH (Marconi et al. 2004). An unbiased census of extremely obscured AGN would require to survey the hard X–ray sky above 10 keV with good sensitivity. Such an argument is one of the key scientific drivers of the SimbolX mission (Ferrando et al. 2006), which will be hopefully launched in the next decade. For the time being one has to rely on the observations obtained by the high energy detectors on board BeppoSAX, INTEGRAL, Swift and, more recently, Suzaku. Though limited to bright and thus low redshift sources, they have proven to be quite successful in finding heavily obscured CT AGN. As a first step forward towards a census of CT AGN we have conceived a program with Suzaku to observe hard X–ray selected bright AGN from the INTEGRAL/IBIS (Beckmann et al. 2006) and Swift/BAT (Markwardt et al. 2005) catalogues. The goal of this program is to discover “new” CT AGN which are likely to be present among the already detected sources, but not recognized as such due to the poor counting statistics and/or the lack of information at lower energies. In order to select the most suitable candidates, we have considered the sources in the above mentioned AGN catalogues with a bright hard X–ray flux and tentative evidence of intrinsic absorption from observations at lower energies. For a few of them the column densities are estimated to be close to the CT threshold. Suzaku observations were obtained for NGC 5728 and NGC 4992.

2 The Suzaku observations

The reprocessed (v1.2) data were reduced using standard calibration products available in November 2006. Source spectra are obtained from the Front Illuminated XIS chips with an extraction radius of $\sim$3*′*, while background spectra are extracted from nearby regions with a larger radius to guarantee good statistics. The effective exposure time for both sources is of the order of 30 ksec. The pin hard X–ray source spectra were obtained taking into account both the instrumental background appropriate for each observation and the cosmic X–ray background. The pin/XIS intercalibration constant was fixed at 1.16. In the following, we report the basic results obtained from the analysis of the X–ray spectra of the two sources and refer to Comastri et al. (2007, in preparation) for a more exhaustive description of the data analysis and interpretation.

2.1 NGC 5728

The Suzaku spectrum of NGC 5728 is shown in Fig. 1 (left panel). The source is clearly detected by the pin detector up to about 50 keV. The primary X–ray continuum is absorbed by Compton thick gas ($N_{H}\simeq 2.1\pm 0.2\times 10^{24}$ cm*-2*). The power law slope has been fixed at $\mathchar 28928\relax$ = 1.9 due to the narrow energy range (20–40 keV) over which the continuum is free from obscuration effects. At lower energy, the continuum can be represented by a two component model: a flat one responsible for most of the X–ray flux in the $\sim$ 2–6 keV energy range and a steep one taking over below 2 keV. The former may be ascribed to reflection of cold material presumably from the inner wall of the torus, while the latter has a power law shape and can be identified as primary emission scattered by off–nuclear gas into the line of sight, or unresolved soft X–ray emission lines, as commonly observed in Seyfert 2 galaxies (Guainazzi & Bianchi 2007). The scattered/reflected flux accounts for 1–2 % of the total unabsorbed flux ($\sim 5\times 10^{-11}$ erg s*-1* cm*-2*) in the 2–10 keV band. The 2–50 keV unabsorbed luminosity is 2.3 $\times$ 1043 erg -1, typical of a bright Seyfert galaxy. It is interesting to note that the hard ($>$ 10 keV) X–ray flux as measured by the pin detector is consistent within 20% with the Swift/BAT measurement in the overlapping energy range. A zoom of the $\sim$ 5–8 keV unfolded spectrum is shown in Fig. 1 (right panel). The iron line complex is best fitted with two gaussian lines: a strong (EW $\simeq 1.0\pm 0.3$ keV) iron K$\alpha$ line at $\sim$ 6.4 keV and a K$\beta$ (EW $\sim 130\pm 70$ eV) at $\sim$ 7 keV. The relative ratio is consistent with that expected from cold neutral gas. The addition of a Compton shoulder parameterized by a Gaussian profile centered at 6.3 keV and $\sigma$ = 40 eV (Matt 2002), though not statistically required, accounts for some 10% of the K$\alpha$ line flux, in reasonably good agreement with the value expected for reflection from Compton thick matter.

2.2 NGC 4992

The Seyfert 2 galaxy NGC 4992 is detected by Suzaku up to about 30 keV with a flux consistent (within 10%) with that reported by INTEGRAL. The continuum (a power law with $\mathchar 28928\relax$=1.9) is heavily obscured ($N_{H}\sim 4.5\pm 0.5\times 10^{23}$ cm*-2*) but not Compton thick. The high energy spectrum is best fitted by adding a strong, absorbed, disk reflection component to the primary power law. The quality of the data is not such to tightly constrain the intensity of the reflection component. The 90% lower limit ($R>$ 5) indicate a reflection dominated spectrum which is similar to that reported by Miniutti et al. (2007) from the analysis of the XMM-Newton data of IRAS 13197-1627. The source is extremely weak below 3–4 keV. The addition of an unabsorbed reflection spectrum only marginally improves the fit (Fig. 2, left panel). A zoom of the residuals in the 4.5–7.5 keV range, wrt the best fit continuum model, is shown in Fig. 2 (right panel). The shape of the residuals suggests the presence of a broad line. Indeed the best fit to the line emission is obtained with a diskline model. Leaving only the line flux and the disk inclination angle as free parameters, the line equivalent width is $\sim 750\pm 200$ eV and the inclination angle is $<$ 40 degrees (at 90% confidence). The best fit EW is consistent with a reflection dominated nature of the broad band spectrum. The absorption corrected 2–50 keV luminosity is $\sim 6\times 10^{43}$ erg s*-1*.

2.3 Epilogue

Relatively shallow Suzaku observations of two hard X–ray selected (with INTEGRAL/IBIS and Swift/BAT) nearby Seyfert 2 galaxies have revelead a wealth of spectral complexity in their X–ray spectra. The good sensitivity over a broad X–ray energy range makes Suzaku very efficient to study the most obscured sources in the nearby Universe and will eventually allow us to establish the AGN absorption distribution at high column densities.

Acknowledgements

We thank G. Miniutti for extremely useful discussions. Support from the Italian Space Agency (ASI) under the contract ASI-INAF I/023/05/0 is acknowledged.