A Suzaku observation of the ULIRG IRAS19254-7245: disclosing the AGN component

TL;DR

This study uses Suzaku X-ray observations to reveal the obscured active galactic nucleus in IRAS 19254-7245, measuring its intrinsic luminosity and spectral features, confirming its AGN nature despite heavy absorption.

Contribution

First detection of IRAS 19254-7245 above 10 keV, unveiling the intrinsic AGN luminosity and spectral properties in a heavily obscured ULIRG.

Findings

Detected IRAS 19254-7245 above 10 keV with flux ~5x10^-12 erg cm^-2 s^-1

Measured intrinsic AGN luminosity in the QSO regime (~3x10^44 erg s^-1)

Spectral analysis indicates reflection/scattering from ionized matter dominates below 10 keV

Abstract

We discuss here a long Suzaku observation of IRAS 19254-7245 (also known as the Superantennae), one of the brightest and well studied Ultra Luminous Infrared Galaxies in the local Universe. This long observation provided the first detection of IRAS 19254-7245 above 10 keV, and measured a 15-30 keV flux of ~5x10^(-12) erg cm^-2 s^-1. The detection above 10 keV has allowed us to unveil, for the first time, the intrinsic luminosity of the AGN hosted in IRAS 19254-7245, which is strongly absorbed (Nh ~ 3x10^(24) cm^-2) and has an intrinsic luminosity in the QSO regime (L(2-10 keV) ~ 3 x 10^(44) erg s^-1). The 2-10 keV spectrum of IRAS 19254-7245 is remarkably hard (Gamma~1.2), and presents a strong iron line (EW ~0.7 keV), clearly suggesting that below 10 keV we are seeing only reprocessed radiation. Since the energy of the Fe K emission is found to be at ~6.7 keV, consistent with He-like Fe, and its EW is too high to be explained in a starburst dominated scenario, we suggest that the 2--10 keV emission of IRAS 19254-7245 is dominated by reflection/scattering from highly ionized matter. Indeed, within this latter scenario we found that the photon index of the illuminating source is Gamma=1.87 (+0.11,-0.28), in excellent agreement with the mean value found for radio quiet unobscured AGN.