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

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 alpha emission and the presence of a Compton-thick active galactic nucleus. Our estimate of the luminosity of the illuminating source (L2-10 keV >= 1 x 10(45) erg s(-1)), required to produce the observed Fe K alpha 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 similar or equal to 2 keV and L-bol(CL) similar or equal to 1 x 10(44) erg s(-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.