2007A&A...473...85P

We report on a 14ks XMM-Newton observation of the hyperluminous infrared galaxy IRAS 09104+4109, which harbors a type 2 quasar in its nucleus. Our analysis was aimed at studying the properties of the absorbing matter and the Fe K complex at 6-7keV in this source. We analyzed the spectroscopic data from the PN and the MOS cameras in the 0.4-10keV band. We also used an archival BeppoSAX 1-50keV observation of IRAS 09104+4109 to investigate possible variations of the quasar emission. The X-ray emission in the EPIC band is dominated by the intra-cluster medium thermal emission. We found that the quasar contributes ∼35% of the total flux in the 2-10keV band. Both a transmission- (through a Compton-thin absorber with a Compton optical depth of τ_C∼0.3, i.e. N_H∼5x10²³cm^–2) and a reflection-dominated (τ_C>1) model provide an excellent fit to the quasar continuum emission. However, the value measured for the EW of Fe Kα emission line is only marginally consistent with the presence of a Compton-thick absorber in a reflection-dominated scenario, which had been suggested by a previous, marginal (i.e. 2.5σ) detection with the hard X-ray (15-50keV), non-imaging BeppoSAX/PDS instrument. Moreover, the value of luminosity in the 2-10 keV band measured by the transmission-dominated model is fully consistent with that expected on the basis of the bolometric luminosity of IRAS 09104+4109. From the analysis of the XMM-Newton data we therefore suggest the possibility that the absorber along the line of sight to the nucleus of IRAS 09104+4109 is Compton-thin. Alternatively, the absorber column density could have changed from Compton-thick to -thin in the five years elapsed between the observations. If this is the case, then IRAS 09104+4109 is the first ``changing-look'' quasar ever detected.