Serendipitous active galactic nuclei in the XMM-Newton fields of Markarian 205 and QSO 0130-403

Abstract

The X-ray spectra of serendipitously observed active galactic nuclei (AGN) in the XMM-Newton fields of Mrk 205 and QSO 0130-403 are analysed. The sample consists of 23 objects, none of which is detected at radio frequencies, with a median X-ray luminosity of ~4 × 10⁴⁴ erg s^-1 and redshifts ranging from ~0.1 to just over 3. The mean photon index was found to be 1.89 +/- 0.04. In contrast with past ASCA and ROSAT observations of high-redshift radio-loud quasars, we find little evidence for excess intrinsic absorption in these radio-quiet objects, with only three sources requiring a column density in excess of the Galactic value. Comparing the measured spectral indices over the redshift range, we also find there is no X-ray spectral evolution of quasi-stellar objects (QSOs) with time, up to redshift of 3. Within the sample there is no evidence for evolution of the optical to X-ray spectral index, α_ox, with redshift, the mean value being -1.66 +/- 0.04. However, upon comparing the values from the Bright Quasar Survey at low redshift (z < 0.5) and high-redshift QSOs detected by Chandra(z > 4), a slight steepening of α_ox is noted for the more distant objects. In most of the sources there is no significant requirement for a soft excess, although a weak thermal component (<=10 per cent of L_X) cannot be excluded. There is an indication of spectral flattening (by ΔΓ= 0.2) at higher energies (>3 keV, QSO rest frame) for the sample as a whole. This is consistent with the presence of a Compton reflection component in these radio-quiet AGN, with the scattering medium (such as an accretion disc or molecular torus) occupying a solid angle of 2π sr to the X-ray source.