The European Large Area ISO Survey - VII. ROSAT observations of ELAIS sources

We present a cross-correlation between the European Large Area ISO Survey (ELAIS) 15-mum ISO survey with the ROSAT(0.1-2 keV)all-sky survey and the pointed observations WGACAT source catalogue. The resulting sample contains 15 objects. Optical spectroscopic identifications exist for 13 objects: six broad-line quasi-stellar objects (QSOs), four narrow-line galaxies (NLGs) or type 2 active galactic nuclei (AGN) and three stars. We have used both the X-ray to infrared (IR) luminosity flux ratio f(x)/f(IR) and the X-ray hardness ratios diagnostics to estimate the amount of obscuration in these objects. The X-ray spectrum of the narrow-line galaxies does not present strong evidence for obscuration; however, the low f(x)/f(IR) ratio combined with the high X-ray luminosities suggest that at least one of the NLGs is associated with an obscured Seyfert nucleus. Four out of the six QSOs present high f(x)/f(IR) ratios and steep X-ray spectra with Gamma > 2. One QSO (ELAISC15-J133442+375736) at a redshift of z = 1.89, has an abnormally low X-ray/IR flux ratio, with its infrared luminosity approaching that of an hyperluminous galaxy (similar to10(12.98) h(-2) L-circle dot). Finally, one radio-loud QSO is the hardest X-ray source in our sample, presenting strong evidence for a high absorbing column (N-H similar to 10(22) cm(-2)).