Suzaku and SWIFT-BAT observations of a newly discovered Compton-thick AGN

¹ INAF - Osservatorio Astronomico di Brera, via Brera 28, 20121 Milano, Italy
e-mail: paola.severgnini@brera.inaf.it
² Department of Physics and Astronomy, Leicester University, Leicester, LE1 7RH, UK
³ Dipartimento di Astronomia, Universita’ degli Studi di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁴ INAF – Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁵ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica di Palermo, via U. La Malfa 153, 90146 Palermo, Italy

Received: 12 August 2010
Accepted: 6 October 2010

Abstract

Context. Obscured AGN are fundamental to understand the history of supermassive black hole growth and their influence on galaxy formation. However, the Compton-thick AGN (N_H  >  10²⁴ cm^-2) population is basically unconstrained, with less than a few dozen confirmed Compton-thick AGN found and studied so far. A way to select heavily obscured AGN is to compare the X-ray emission below 10 keV (which is strongly depressed) with the emission from other bands that are less affected by the absorption, like the IR band. To this end, we have cross-correlated the 2XMM catalogue with the IRAS Point Source catalogue and, by using the X-ray to infrared flux ratio and X-ray colours, we selected a well defined sample of Compton-thick AGN candidates at z < 0.1.

Aims. The aim of this work is to confirm the nature and to study one of these local Compton-thick AGN candidates, the nearby (z = 0.029) Seyfert 2 galaxy IRAS 04507+0358, by constraining the amount of intrinsic absorption (N_H) and thus the intrinsic luminosity.

Methods. To this end we obtained deep (100 ks) Suzaku observations (AO4 call) and performed a joint fit with SWIFT-BAT data. We analysed XMM-Newton, Suzaku and SWIFT-BAT data and present here the results of this broad-band (0.4−100 keV) spectral analysis.

Results. We found that the broad-band X-ray emission of IRAS 04507+0358 requires a large amount of absorption (higher than 10²⁴ cm^-2) to be well reproduced, thus confirming the Compton-thick nature of this source. In particular, the most probable scenario is that of a mildly (N_H  ~  1.3−1.5  ×  10²⁴ cm^-2, L(2−10 keV)  ~  5−7  ×  10⁴³ erg s^-1) Compton-thick AGN.