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F. Harrison, P. Mao (Caltech), D. Helfand (Columbia University)
The raison d'etre for the Chandra Observatory when it was proposed 25 years ago was to resolve the X-ray background. It is not clear whether this goal will literally be achieved by Chandra, but it is already apparent from the first moderately deep pointings that new contributing populations are being discovered. To date, however, published surveys include fewer than 175 2-10 keV sources below the ASCA limit of ~1 x 10-13 erg cm-2 s-1 (at which, 23% of the XRB is resolved). Less than three-quarters of these sources have secure optical identifications or interesting (R>23) upper limits, and less than a quarter have spectroscopically confirmed redshifts. Furthermore, the total surveyed area amounts to less than 0.4 deg2, rendering impossible the simplest goal of constructing a global logN - logS, let alone individual number-flux distributions for various classes of background contributors.
Recognizing the need for a large, unbiased survey of optically identified extragalactic X-ray sources over a broad range of observed fluxes, we have begun a project to identify and classify a large sample (>1000) of objects detected serendipitously in high-latitude Chandra fields. This Serendipitous Extragalactic X-ray Source Identification (SEXSI) program is utilizing the optical and infrared facilities of the MDM, Palomar, and Keck Observatories to obtain imaging and spectroscopy in selected Chandra fields in order to identify (or tightly constrain the properties of) optical counterparts for all X-ray sources with 1.0 x 10-13 erg cm-2 s-1 > fx(2-10 keV) > 3.0 x 10-16 erg cm-2 s-1. We report here our initial results from thirteen Chandra fields which increase by more than a factor of two the number of identified sources reported to date.