The XMM-Newton/2dF survey - VII. Is there any X-ray absorption in optically selected QSOs?

We explore the X-ray properties of optically selected quasi-stellar objects (QSOs) spectroscopically identified in the course of the 2dF QSO survey (2QZ). Our main goal is to expand to higher redshifts, previous findings suggesting the presence of a fraction of X-ray obscured sources among the low-redshift, optically selected, broad line active galactic nucleus (AGN) population. The X-ray data are from the wide field (~2.5 deg²) shallow [f(0.5-8 keV) ~× 10^-14 erg cm^-2s^-1]XMM-Newton/2dF survey. A total of 96 2QZ QSOs overlap with the area covered by our X-ray survey. 66 of them have X-ray counterparts while 30 remain undetected in our X-ray survey. The 66 X-ray detected QSOs have a mean photon index of ~2 suggesting little or no X-ray obscuration for most of these sources. Individual X-ray spectral fittings reveal only one source (intrinsic L_X(0.5-8 keV) ~ 10⁴⁴ erg s^-1 at z= 0.82) that is likely to be obscured (N_H~ 10²³ cm^-2) at the 90 per cent confidence level. Additionally, there are nine 2QZ sources that show evidence for moderate absorption (mean observed N_H of ~10²¹ cm^-2). For the 30 QSOs that remain undetected in our X-ray survey, we use stacking analysis to estimate a mean hardness ratio (HR) of -0.59 +/- 0.11 suggesting that the bulk of this population has N_H consistent with the Galactic value. However, we cannot exclude the possibility that some of these sources have enhanced photoelectric absorption that is not revealed in the mean stacked spectrum. We estimate a lower limit to the fraction of optically selected QSO with X-ray absorption of approximately 10 per cent (10 out of 96).