The Chandra Large Area Synoptic X-ray Survey (CLASXS) of the Lockman Hole-Northwest: The X-ray catalog

摘要

We present the X-ray catalog and basic results from our Chandra Large Area Synoptic X-ray Survey (CLASXS) of the Lockman Hole-Northwest field. Our nine ACIS-I fields cover a contiguous solid angle of similar to0.4 deg(2) and reach fluxes of 5 x 10(-16) ergs cm(-2) s(-1) (0.4-2 keV) and 3 x 10(-15) ergs cm(-2) s(-1) (2-8 keV). Our survey bridges the gap between ultradeep pencil-beam surveys, such as the Chandra Deep Fields (CDFs), and shallower, large-area surveys, allowing a better probe of the X-ray sources that contribute most of the 2-10 keV cosmic X-ray background (CXB). We find a total of 525 X-ray point sources and four extended sources. At similar to10(-1)4 ergs cm(-2) s(-1) (2-8 keV), our number counts are significantly higher than those of several noncontiguous, large-area surveys. Such a large difference is an indication of clustering in the X-ray sources. On the other hand, the integrated flux from the CLASXS field, combined with ASCA and Chandra ultradeep surveys, is consistent with results from other large-area surveys, within the variance of the CXB. We see spectral evolution in the hardening of the sources at fluxes below 10(-1)4 ergs cm(-2) s(-1), which agrees with previous observations from Chandra and XMM-Newton. About one third of the sources in the CLASXS field have multiple observations, which allow variability tests. Above 4 x 10(-14) ergs cm(-2) s(-1) (0.4-8 keV), similar to60 of the sources are variable. We also investigated the spectral variability of the variable sources. While most show spectral softening with increasing flux, or no significant spectral change, there are a few sources that show a different trend. We observed four extended sources in CLASXS, which is consistent with the previously measured log N-log S relation for galaxy clusters. Using X-ray spectra and optical colors, we argue that three of the four extended sources are galaxy clusters or galaxy groups. We report the discovery of a gravitational lensing arc associated with one of these sources. Using red-sequence and brightest cluster galaxy methods, we find that the redshifts of the extended sources are in the range z similar to 0.5-1. The inferred masses within the Einstein radii are consistent with the mass profiles of local groups scaled to the same virial radii.