XMM- Newton observations of 10 Ultra-Luminous Infrared Galaxies ( ULIRGs ) from a 200 ksec mini-survey program are reported .
The aim is to investigate in hard X-rays a complete ULIRG sample selected from the bright IRAS 60 \mu m catalogue .
All sources are detected in X-rays , 5 of which for the first time .
These observations confirm that ULIRGs are intrinsically faint X-rays sources , their observed X-ray luminosities being typically L _ { 2 - 10 keV } \leq 10 ^ { 42 - 43 } erg / s , whereas their bolometric ( mostly IR ) luminosities are L _ { bol } > 10 ^ { 45 } erg / s .
In all sources we find evidence for thermal emission from hot plasma with a rather costant temperature kT \simeq 0.7 keV , dominating the X-ray spectra below 1 keV , and likely associated with a nuclear or circumnuclear starburst .
This thermal emission appears uncorrelated with the far-IR 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 \sim 30 kpc for Mkn 231 and IRAS 19254-7245 , possible evidence of galactic superwinds .
In these two sources , in IRAS 20551-4250 and IRAS 23128-5919 we find evidence for the presence of hidden AGNs , while a minor AGN contribution may be suspected also in IRAS 20100-4156 .
In particular , we have detected a strong ( EW \sim 2 keV ) Fe-K line at 6.4 keV in the spectrum of IRAS19254-7245 and a weaker one in Mkn 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 likely due to high-mass X-ray binaries , are correlated with L _ { FIR } : 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 .