Champlane Deep Galactic Bulge Survey. I. Faint Accretion-driven Binaries in the Limiting Window

Abstract

We have carried out a deep X-ray and optical survey with Chandra and HST of low-extinction regions in the Galactic bulge. Here we present the results of a search for low-luminosity (L(X) less than or similar to 10(34) erg s(-1)) accreting binaries among the Chandra sources in the region closest to the Galactic center, at an angular offset of 1.degrees 4, that we have named the Limiting Window. Based on their blue optical colors, excess H alpha fluxes, and high X-ray-to-optical flux ratios, we identify three likely accreting binaries; these are probably white dwarfs accreting from low-mass companions (cataclysmic variables; CVs) although we cannot exclude that they are quiescent neutron-star or black-hole low-mass X-ray binaries. Distance estimates put these systems farther than greater than or similar to 2 kpc. Based on their H alpha-excess fluxes and/or high X-ray-to-optical flux ratios, we find 22 candidate accreting binaries; however, the properties of some can also be explained if they are dMe stars or active galaxies. We investigate the CV number density toward the bulge and find that the number of observed candidate CVs is consistent with or lower than the number expected for a constant CV-to-star ratio that is fixed to the local value. Our conclusions are limited by uncertainties in the extinction (for which we see a similar to 30% variation in our 6.'6 x 6.'6 field) and selection effects. The X-ray properties of two likely CVs are similar to those of the faint, hard X-ray sources in the Galactic center region that have been suggested to be (mainly) magnetic CVs. If our candidates belong to the same population, they would be the first members to be optically identified; optical or infrared identification of their Galactic center analogs would be impossible due to the higher obscuration. We speculate that all Galactic hard X-ray sources in our field can be explained by magnetic CVs.