The 172 ks Chandra exposure of the LALA Bootes field: X-ray source catalog

We present an analysis of a deep, 172 ks Chandra observation of the Large Area Lyman Alpha survey (LALA) Bootes field, obtained with the Advanced CCD Imaging Spectrometer (ACIS-I) on board the Chandra X-Ray Observatory. This is one of the deepest Chandra images of the extragalactic sky; only the 2 Ms Chandra Deep Field North (CDF-N) and 1 Ms Chandra Deep Field South (CDF-S) are substantially deeper. In this paper we present the X-ray source catalog obtained from this image, along with an analysis of source counts and optical identifications. The X-ray image is composed of two individual observations obtained in 2002 and reaches 0.5-2.0 and 2.0-10.0 keV flux limits of 1.5 × 10^-16 and 1.0 × 10^-15 ergs cm^-2 s ^-1, respectively, for point sources near the aim point. A total of 168 X-ray sources were detected: 160 in the 0.5-7.0 keV band, 132 in the 0.5-2.0 keV band, and 111 in the 2.0-7,0 keV band. The X-ray source counts were derived and compared with those from other Chandra deep surveys; the hard X-ray source density of the LALA Bootes field is 33% higher than that of CDF-S at the flux level of 2.0 × 10^-15 ergs cm^-2 s^-1, confirming the field-to-field variances of the hard-band source counts reported by previous studies. The deep exposure resolves ≳72% of the 2.0-10.0 keV X-ray background. Our primary optical data are R-band imaging from the NOAO Deep Wide-Field Survey (NDWFS), with a limiting magnitude of R = 25.7 (Vega, 3 σ, and 4″ diameter aperture). We have found optical counterparts for 152 of the 168 Chandra sources (90%); 144 of these are detected in the R-band image, and eight have optical counterparts in other bands (either B_W, V, I, or z′). Among the R-band nondetected sources, not more than 11 of them can possibly be at z > 5, based on the hardness ratios of their X-ray emission and nondetections in bluer bands (B_W V). The majority (∼76%) of the X-ray sources are found to have log(f_X/f _R) within 0.0 ± 1; these are believed to be AGNs. Most of the X-ray-faint/optically bright sources [log(f_X/f_R) < -1.0] are optically extended; these are low-z normal galaxies or low-luminosity AGNs. There is also a population of sources that are X-ray overluminous for their optical magnitudes [log(f_X/f_R) > 1.0], which are harder in X-ray and are probably obscured AGNs.