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

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 ~0.4 deg² and reach fluxes of 5×10^-16 ergs cm^-2 s^-1 (0.4-2 keV) and 3×10^-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 ~10^-14 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^-14 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 4×10^-14 ergs cm^-2 s^-1 (0.4-8 keV), ~60% 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 logN-logS 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~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.