The Calán-Yale Deep Extragalactic Research (CYDER) Survey: Optical Properties and Deep Spectroscopy of Serendipitous X-ray Sources

We present the first results from the Calán-Yale Deep Extragalactic Research (CYDER) survey. The main goal of this survey is to study serendipitous X-ray sources detected by Chandra in an intermediate flux range (10 − 10 ergs s) that comprises most of the X-ray background. 267 X-ray sources spread over 5 archived fields were detected. The logN − log S distribution obtained for this sample is consistent with the results of other surveys. Deep V and I images were taken of these fields in order to calculate X-ray-to-optical flux ratios. Identifications and redshifts were obtained for 106 sources using optical spectroscopy from 8-m class telescopes to reach the optically faintest sources, to the same level as deeper X-ray fields like the Chandra Deep Fields, showing that the nature of sources detected depends mostly on the optical limit for spectroscopy. In general, sources optically classified as obscured Active Galactic Nuclei (AGNs) have redder optical colors than unobscured AGN. A rough correlation between fX/fopt and hard X-ray luminosity was found for obscured Department of Astronomy, Yale University, P.O. Box 208101,New Haven,CT 06520. Yale Center for Astronomy & Astrophysics, Yale University, P.O. Box 208121, New Haven, CT 06520 Departamento de Astronomı́a, Universidad de Chile, Casilla 36-D, Santiago, Chile. Institut d’Estudis Espacials de Catalunya/CSIC,Gran Capità 2-4, E-08034 Barcelona, Spain. Astronomical Institute “Anton Pannekoek”,University of Amsterdam,1098 SJ Amsterdam, The Netherlands NSF Astronomy and Astrophysics Postdoctoral Fellow