2002MNRAS.333..809C

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 A2390 cluster of galaxies. The sources have 0.5-7keV fluxes of (0.6-8)x10^–14 erg.cm^–2.s^–1 and lie around the break in the 2-10keV 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 2-10keV luminosities of (0.2-3)x10⁴⁵ erg.s^–1, and absorbing columns of N_H>10²³ cm^–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 ∼100µm.