X-rays from the Type II quasar in the hyperluminous infrared galaxy IRAS F15307+3252

Abstract

We report the detection of X-ray emission from the hyperluminous infrared galaxy IRAS F15307+3252 at z=0.93 and its properties obtained from XMM-Newton observations. Although the X-ray emission is very faint and the data are noisy, a prominent line-like feature in the observed 3-4 keV range is inferred from both photometric and spectroscopic techniques. It indicates an X-ray spectrum dominated by 6.4 keV Fe K emission and the presence of a Compton-thick AGN. Our estimate of the luminosity of the illuminating source (L(2-10keV)>=1e45 erg/s), required to produce the observed Fe K emission in reflection from cold matter, means that the hidden quasar nucleus accounts for a significant fraction of the large bolometric luminosity. The soft X-ray emission below 2 keV is found to be spatially extended and probably of a separate origin. The temperature and bolometric luminosity (kT=2 keV and L(bol,CL)=1e44 erg/s) obtained from a thermal spectrum place this X-ray source on the L-Tx relation of galaxy clusters. The possible association with a galaxy cluster can be added to the list of remarkable similarities between IRAS F15307+3252 and another hyperluminous infrared galaxy IRAS 09104+4109 (z=0.44), both of which have bolometric luminosities dominated by hidden quasar nuclei. Our result on IRAS F15307+3252 illustrates how difficult it is to detect Compton thick Type II quasars at z>=1, particularly if their bolometric outputs do not rival the hyperluminous population.