Finding rare AGN: X-ray number counts of Chandra sources in Stripe 82

Abstract

We present the first results of a wide-area X-ray survey within the Sloan Digital Sky Survey Stripe 82, a 300 deg2 region of the sky with a substantial investment in multiwavelength coverage. We analysed archival Chandra observations that cover 6.2 deg2 within Stripe 82 ('Stripe 82 ACX'), reaching 4.5s flux limits of 1.2 × 10-15, 5.4 × 10-15 and 2.9 × 10-15 erg s-1 cm-2 in the soft (0.5-2 keV), hard (2-7 keV) and full (0.5-7 keV) bands, to find 480, 137 and 705 X-ray sources, respectively. 214 sources are detected only in the full band and 4 sources are detected solely in the soft band. Utilizing data products from the Chandra Source Catalog, we construct independent logN-log S relationships, detailing the number density of X-ray sources as a function of flux. The soft and full bands show general agreement with previous Chandra surveys; the hard band number counts agree among Stripe 82 ACX, XBoötes and XDEEP2, but all three are somewhat systematically lower than the counts derived from Chandra Multiwavelength Project. We compare the luminosity distribution of Stripe 82 ACX with the smaller, deeper Chandra Deep Field-South, Extended Chandra Deep Field-South and Chandra-COSMOS surveys to illustrate the benefit of wide-area surveys in locating high-luminosity and/or high-redshift active galactic nuclei (AGN). Finally, we compare the identified AGN with predictions from population synthesis models, noting that prior to any spectroscopic follow-up campaign, we have already located roughly half the high-luminosity quasars at high redshift expected to lie within the survey area. However, our data also suggest that refinements to population synthesis models will be required. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.