|
Description
Recent results have revised upwards the total X-ray background (XRB) intensity below ~10 keV, therefore an accurate determination of the source counts is needed. There are also contradictory results on the clustering of X-ray selected sources. The authors have studied the X-ray source counts in four energy bands: soft (0.5 - 2 keV), hard (2 - 10 keV), XID (0.5 - 4.5 keV) and ultra-hard (4.5 - 7.5 keV) in order to evaluate the contribution of sources at different fluxes to the X-ray background. They have also studied the angular clustering of X-ray sources in those bands. AXIS (An XMM International Survey) is a survey of 36 high Galactic latitude XMM observations covering 4.8 square degrees in the Northern sky and containing 1433 serendipitous X-ray sources detected with 5-sigma significance. This survey has similar depth to the XMM catalogs and therefore serves as a pathfinder to explore their possibilities. The authors in their paper combined this survey with shallower and deeper surveys, and fitted the source counts with a Maximum Likelihood technique. Using only AXIS sources they studied the angular correlation using a novel robust technique. The AXIS source counts results are compatible with most previous samples in the soft, XID, ultra-hard and hard bands. This study has improved on previous results in the hard band. The fractions of the XRB resolved in the surveys used in this work are 87%, 85%, 60% and 25% in the soft, hard, XID and ultra-hard bands, respectively. Extrapolation of the source counts to zero flux is not sufficient to saturate the XRB intensity. Only galaxies and/or absorbed AGN could contribute the remaining unresolved XRB intensity. These results are compatible, within the errors, with recent revisions of the XRB intensity in the soft and hard bands. The maximum fractional contribution to the XRB comes from fluxes within about a decade of the break in the source counts (~10<sup>-14</sup> cgs), reaching ~50% of the total in the soft and hard bands. Angular clustering (widely distributed over the sky and not confined to a few deep fields) is detected at 99-99.9% significance in the soft and XID bands, with no detection in the hard and ultra-hard band (probably due to the smaller number of sources). The authors cannot confirm the detection of significantly stronger clustering in the hard-spectrum hard sources. Medium-depth surveys such as AXIS are essential to determine the evolution of the X-ray emission in the Universe below 10 keV. Included here are the basic data for the 2560 X-ray sources in the reference paper which satified the selection criteria of having an emldetect detection likelihood >= 10 (the default value) in at least one band, namely: (i) XMM-Newton pn count-rates in four XMM-Newton Science Analysis System (SAS) bands (band 2: 0.5 - 2 keV, band 3: 2 - 4.5 keV, band 4: 4.5 - 7.5 keV, band 5: 7.5 - 12 keV); (ii) spectral photon indices in the 0.5 - 4.5 keV band, the 2 - 10 keV band and the 0.5 - 10 keV band; (iii) fluxes in the soft (0.5 - 2 keV), hard (2 - 10 keV), XID (0.5 - 4.5 keV), ultra-hard (4.5 - 7.5 keV) and "total" (0.5 - 10 keV) bands; and (iv) flags describing to which of the samples discussed in the paper (soft, hard, XID or ultra-hard) each source belongs. There is no spectral or flux information given for the sources not belonging to any of the samples, but the count-rates of such sources are given for completeness. This table was created by the HEASARC in August 2007 based on <a href="https://cdsarc.cds.unistra.fr/ftp/cats/J/A+A/469/27">CDS catalog J/A+A/469/27</a> file table23.dat. This is a service provided by NASA HEASARC .
|
