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 9 ACIS-I fields cover a contiguous solid angle of ∼ 0.4 deg and reach fluxes of 5×10−16 ergs cm s (0.4−2 keV) and 3×10−15 ergs cm s (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 4 extended sources. At ∼ 10 ergs cm s (2− 8 keV), our number counts are significantly higher than those of several non-contiguous, 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 ergs cm s , which agrees with previous observations from Chandra and XMMNewton. About 1/3 of the sources in the CLASXS field have multiple observations, allowing variability tests. Above 4 × 10 ergs cm s (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. Four extended sources in CLASXS is consistent with the previously measured LogNLogS of galaxy clusters. Using X-ray spectra and optical colors, we argue that 3 of the 4 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 Department of Astronomy, University of Maryland, MD College Park 20742 Laboratory for High Energy Astrophysics, NASA Goddard Space Flight Center, Code 662, Greenbelt, MD 20770 Department of Astronomy, University of Wisconsin, Madison, WI 53706 Department of Physics and Astronomy, University of Hawaii, Honolulu, HI 96822 Institute for Astronomy, University of Hawaii, Honolulu, HI 96822