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T. Tavenner (CTIO REU/U. Wash.), D. W. Hoard, S. Wachter (CTIO)
Although discovered nearly 40 years ago, the cosmic X-ray background (XRB) is still not fully understood. A substantial fraction of the soft XRB may be composed of discrete sources within the Galaxy (and their counterparts in other galaxies). Likely candidates for these objects are interacting binary stars (e.g. cataclysmic variables, X-ray binaries, etc.). Population synthesis studies imply that a large fraction of as yet undiscovered cataclysmic variables reside in the Galaxy. A ROSAT/PSPC survey for X-ray point sources in the Small Magellanic Cloud (SMC) by Kahabka et al. revealed 248 objects, 25% of which display X-ray properties consistent with those expected for interacting binary stars (i.e. weak X-ray emission at the hard end of the \lesssim2 keV ROSAT sensitivity range). Thus, they are good candidates for the ``missing'' population of such objects that, by analogy, may contribute to the XRB in our Galaxy.
On 2000 November 19--23 UT, we used the CTIO 1.5-m telescope to obtain deep UBVRI images (mag. limits of \approx23 in U, \approx24.5 in BVR, and \approx24 in I) at the positions of 13 ROSAT sources in the SMC. We selected our targets from the Kahabka et al. X-ray sources with weak, hard (in the ROSAT energy range) emission and the smallest X-ray position error circles (r\rm err\leq15''). The spectral energy distributions determined from calibrated multi-color photometry of all objects inside 1.25r\rm err will be used to identify likely optical counterparts of the X-ray sources for future follow-up investigation. We present here the preliminary results of this multi-color optical mini-survey of SMC X-ray source counterparts.
This research was conducted in part during the 2000 Research Experiences for Undergraduates (REU) Program at Cerro Tololo Inter-American Observatory (CTIO). The CTIO REU program is funded by the National Science Foundation (NSF). CTIO is operated by AURA, Inc. under cooperative agreement with the NSF.