Multiwavelength observations of serendipitous Chandra X-ray sources in the field of A 2390.

We present optical spectra and near-infrared imaging of a sample of 31 serendipitous X-ray sources detected in the field of Chandra observations of the A 2390 cluster of galaxies. The sources have 0.5–7 keV fluxes of (0.6–8)×10[sup -14] erg cm[sup -2] s[sup -1] and lie around the break in the 2–10 keV source counts. They are therefore typical of sources dominating the X-ray Background in that band. 12 of the 15 targets for which we have optical spectra show emission lines at a range of line luminosities, and half of these show broad lines. These active galaxies and quasars have soft X-ray spectra. Including photometric redshifts and published spectra, we have redshifts for 17 of the sources, ranging from z∼0.2 up to z∼3 , with a peak between z=1–2 . 10 of our sources have hard X-ray spectra indicating a spectral slope flatter than that of a typical unabsorbed quasar. Two hard sources that are gravitationally lensed by the foreground cluster are obscured quasars, with intrinsic 2–10 keV luminosities of (0.2–3)×10[sup 45] erg s[sup -1] , and absorbing columns of N[sub H]>10[sup 23] cm[sup -2] . Both of these sources were detected in the mid-infrared by ISOCAM on the Infrared Space Observatory, which when combined with radiative transfer modelling leads to the prediction that the bulk of the reprocessed flux emerges at ∼100 μm. [ABSTRACT FROM AUTHOR]