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 Kα emission and the presence of a Compton-thick active galactic nucleus. Our estimate of the luminosity of the illuminating source (L_2-10keV>= 1 × 10⁴⁵ergs^-1), required to produce the observed Fe Kα 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^CL_bol~= 1 × 10⁴⁴ergs^-1) obtained from a thermal spectrum place this X-ray source on the L-T_X 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.