Identification of twenty-three accreting binaries in the Galactic Bulge Survey

We are undertaking a survey to characterize the X-ray sources found with the Chandra X-ray observatory in a strip of fields at −3◦ < l < 3◦, b = +1.5◦ and −3◦ < l < 3◦, b = −1.5◦. This so-called Galactic Bulge Survey (GBS) targets X-ray emitting binaries in the bulge with the primary purpose of finding quiescent X-ray binaries. The aims of this survey are to quantify dynamically the mass of compact objects in these X-ray binaries in order to constrain the neutron star equation of state and to test black hole formation models. In addition, using the survey number counts of various sources we aim to test models for binary formation and evolution. Here, we present the identification of optical counterparts to twenty-three GBS X-ray sources. We report their accurate coordinates and medium resolution optical spectra acquired at the Very Large Telescope and Magellan. All sources are classified as accreting binaries according to their emission line characteristics. To distinguish accreting binaries from chromospherically active objects we develop and explain criteria based on Hα and Hei λλ5786, 6678 emission line properties available in the literature. The spectroscopic properties and photometric variability of all the objects are discussed and a classification of the source is given where possible. Among the twenty-three systems, at least nine of them show an accretion-dominated optical spectrum (CX28, CX63, CX70, CX128, CX142, CX207, CX522, CX794 CX1011) and another six show photospheric lines from a late-type donor star in addition to accretion disc emission (CX44, CX93, CX137, CX154, CX377 and CX1004) indicating that they are probably accreting binaries in quiescence or in a low accretion rate state. Two sources are confirmed to be eclipsing: CX207 and CX794. CX207 shows a broad asymmetric Hα profile blue-shifted by > 300 km s−1. Such line profile characteristics are consistent with a magnetic (Polar) cataclysmic variable. CX794 is an eclipsing nova-like cataclysmic variable in the period gap. Time-resolved photometry and the large broadening of the Hα emission lines in CX446 (2100 km s−1 Full-Width at Half Maximum; FWHM) suggest that this is also an eclipsing or high-inclination accreting binary. Finally, the low-accretion rate source CX1004 shows a double-peaked Hα profile with a FWHM of 2100 km s−1. This supports a high inclination or even eclipsing system. Whether the compact object is a white dwarf in an eclipsing cataclysmic variable or a black hole primary in a high-inclination low-mass X-ray binary remains to be established.