An XMM - Newton hard X-ray survey of ultraluminous infrared galaxies

Abstract

XMM - Newton observations of 10 ultraluminous infrared galaxies (ULIRGs) from a 200-ks mini-survey programme are reported. The aim is to investigate in hard X-rays a complete ULIRG sample selected from the bright IRAS 60-μm catalogue. All sources are detected in X- rays, five of which for the first time. These observations confirm that ULIRGs are intrinsically faint X-ray sources, their observed X-ray luminosities being typically L 2-10keV≤ 1042-1043 erg s-1, whereas their bolometric (mostly infrared) luminosities are Lbol > 1045 erg s-1. In all sources we find evidence for thermal emission from hot plasma with a rather constant temperature kT ≃ 0.7 keV, dominating the X-ray spectra below 1 keV, and probably associated with a nuclear or circumnuclear starburst. This thermal emission appears uncorrelated with the far-infrared luminosity, suggesting that, in addition to the ongoing rate of star formation, other parameters may also affect it. The soft X-ray emission appears to be extended on a scale of ∼30 kpc for Mrk 231 and IRAS 19254-7245, possible evidence of galactic superwinds. In these two sources, IRAS 20551-4250 and 23128-5919, we find evidence for the presence of hidden active galactic nuclei (AGNs), while a minor AGN contribution may be suspected also in IRAS 20100-4156. In particular, we have detected a strong (EW ∼ 2 keV) Fe K line at 6.4 keV in the spectrum of IRAS 19254-7245 and a weaker one in Mrk 231, suggestive of deeply buried AGNs. For the other sources, the X-ray luminosities and spectral shapes are consistent with hot thermal plasma and X-ray binary emissions of mainly starburst origin. We find that the 2-10 keV luminosities in these sources, most probably due to high-mass X-ray binaries, are correlated with LFIR: both luminosities are good indicators of the current global star formation rate in the Galaxy. The composite nature of ULIRGs is then confirmed, with hints for a predominance of the starburst over the AGN phenomenon in these objects even when observed in hard X-rays.