Multi - wavelength 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 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 −14 erg cm −2 s −1 and lie around the break in the 2-10 keV source counts. They therefore are typical of sources dominating the X-ray Background in that band. Twelve of the fifteen targets for which we have optical spectra show emission lines, most of which have soft X-ray spectra. Including photometric redshifts and published spectra we have redshifts for seventeen of the sources, ranging from z ∼ 0.2 up to z ∼ 3 with a peak between z = 1 − 2. Ten 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 45 erg s −1 and absorbing columns of N H > 10 23 cm −2. Both were detected by ISO, showing that the absorbed flux emerges in the far-infrared.