2 X-ray observations

2.1 Sample definition

The X-ray results reported in this paper are obtained from the XMM observation of the Lockman Hole field, centered on the sky position RA 10:52:43, Dec +57:28:48 (J2000). This is a region of extremely low Galactic Hydrogen column density, $N_{\rm H}=5.7 \times 10^{19}$ ${\rm cm}^{-2}$ (Lockman et al. 1986). The observation was performed in five separate revolutions (70, 71, 73, 74 and 81) during the period April 27-May 19, 2000 for a total exposure time of 190 ksec. Due to periods of high background and flares, the good time intervals added up to approximately 100 ksec.

The dataset, the cleaning procedure used, the source detection and the astrometric corrections are described in details in Paper I.

In this work, we use a sample of 192 sources with a likelihood value >10 (corresponding to $\sim$4$\sigma$; see Paper I), and extend the analysis to the whole Lockman Hole field of view (in Paper I only sources with off-axis angle <10 $\hbox {$^\prime $ }$ were considered). The flux limits of this sample in the [0.5-2], [2-10] and [5-10] keV bands are 0.31, 1.4 and $2.4 \times 10^{-15}$ erg cm^-2 s^-1, respectively. We have used only EPIC-pn data in this work. We restrict the X-ray spectral analysis to sources with more than 70 counts in the [0.5-7] keV band after background subtraction, for which a reasonable parameter fit can be obtained. This defines a sample of 98 sources, of which 76 within an off-axis angle of $10\hbox{$^\prime$ }$. The full sample includes 38 Type-1 AGN, 15 Type-2 AGN, 34 unidentified sources (mostly newly detected XMM-Newton sources), four extended sources, two normal galaxies and five stars.

2.2 The X-ray source catalogue

The catalogue of the 98 X-ray sources studied here is given in Table 2. We report in the first two columns the source number and the ROSAT number (if any); in the third column the classification scheme (see Schmidt et al. 1998): 1 = Type-1 AGN, 2 = Type-2 AGN, 3 = galaxy, 4 = group/cluster of galaxies, 5 = star, 9 = unidentified source; in Cols. 4 and 5 the X-ray source coordinates (J2000). Off-axis angles and observed counts in the [0.5-7] keV band are reported in Cols. 6 and 7. The X-ray flux is given in three different bands: [0.5-2], [2-10] and [5-10] keV (see Table 2 in Paper I for the energy conversion factors in the different bands). Columns 11, 12 and 13 give the R and $K^\prime$ magnitudes, and $R-K^\prime$ colour respectively. In Col. 14, we give the redshift based on low-resolution Keck spectra. The column density, $\log(N_{\rm H})$ (in excess to the galactic ones), and the spectral index $\Gamma$ as measured from spectral fitting are reported in Cols. 15 and 16. The errors correspond to $90\%$ confidence level for one interesting parameter ( $\Delta \chi ^2=2.706$). The last two columns give the absorbed X-ray luminosities, which are derived from the X-ray spectra in the rest-frame bands: [0.5-2] and [2-10] keV. We assume a critical density universe with H₀=50 km  ${\rm s}^{-1}$ ${\rm Mpc}^{-1}$ and $\Lambda=0$.