Volume 555, July 2013
|Number of page(s)||16|
|Published online||27 June 2013|
X-ray view of four high-luminosity Swift/BAT AGN: Unveiling obscuration and reflection with Suzaku
1 INAF/Istituto di Astrofisica Spaziale e Fisica cosmica, via Gobetti 101, 40129 Bologna, Italy
2 Dipartimento di Astronomia, Università di Bologna, via Ranzani 1, 40127 Bologna, Italy
3 NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA
Received: 23 April 2012
Accepted: 4 March 2013
Aims. A complete census of obscured active galactic nuclei (AGN) is necessary to reveal the history of the super massive black hole (SMBH) growth and galaxy evolution in the Universe given the complex feedback processes and the fact that much of this growth occurs in an obscured phase. In this context, hard X-ray surveys and dedicated follow-up observations represent a unique tool for selecting highly absorbed AGN and for characterizing the obscuring matter surrounding the SMBH. Here we focus on the absorption and reflection occurring in highly luminous, quasar-like AGN, to study the relation between the geometry of the absorbing matter and the AGN nature (e.g. X-ray, optical, and radio properties), and to help to determine the column density dependency on the AGN luminosity.
Methods. The Swift/BAT nine-month survey observed 153 AGN, all with ultra-hard X-ray BAT fluxes in excess of 10-11 erg cm-2 s-1 and an average redshift of 0.03. Among them, four of the most luminous BAT AGN (44.73 < Log LBAT < 45.31) were selected as targets of Suzaku follow-up observations: J2246.0+3941 (3C 452), J0407.4+0339 (3C 105), J0318.7+6828, and J0918.5+0425. The column density, scattered/reflected emission, the properties of the Fe K line, and a possible variability are fully analyzed. For the latter, the spectral properties from Chandra, XMM-Newton and Swift/XRT public observations were compared with the present Suzaku analysis, adding an original spectral analysis when none was available from the literature.
Results. Of our sample, 3C 452 is the only certain Compton-thick AGN candidate because of i) the high absorption (NH ~ 4 × 1023 cm-2) and strong Compton reflection; ii) the lack of variability; iii) the “buried” nature, i.e. the low scattering fraction (<0.5%) and the extremely low relative [OIII] luminosity. In contrast 3C 105 is not reflection-dominated, despite the comparable column density, X-ray luminosity and radio morphology, but shows a strong long-term variability in flux and scattering fraction, consistent with the soft emission being scattered from a distant region (e.g., the narrow emission line region). The sample presents high (>100) X-to-[OIII] luminosity ratios, with an extreme value of RX[OIII] ~ 800 for 3C 452, confirming the [OIII] luminosity to be affected by residual extinction in presence of mild absorption, especially for “buried” AGN such as 3C 452. Three of our targets are powerful FRII radio galaxies, which is shown by their high luminosity and absorption; this makes them the most luminous and absorbed AGN of the BAT Seyfert survey despite the inversely proportional NH − LX relation.
Key words: X-rays: galaxies / galaxies: active
© ESO, 2013
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