Gemini spectroscopy of galactic bulge sources: A population of hidden accreting binaries revealed?

Abstract

We present Gemini spectroscopy for 21 candidate optical counterparts to X-ray sources discovered in the Galactic Bulge Survey (GBS). For the majority of the 21 sources, the optical spectroscopy establishes that they are indeed the likely counterparts. One of the criteria we used for the identification was the presence of an Hα emission line. The spectra of several sources revealed an Hα emission line only after careful subtraction of the F or G stellar spectral absorption lines. In a subclass of three of these sources, the residual Hα emission line is broad (≳ 400 km s-1) which suggests that it is formed in an accretion disc, whereas in other cases the line width is such that we currently cannot determine whether the line emission is formed in an active star/binary or in an accretion disc. GBS source CX377 shows this hidden-accretion behaviour most dramatically. The previously identified broad Hα emission of this source is not present in its Gemini spectra taken ~1 yr later. However, broad emission is revealed after subtracting an F6 template star spectrum. The Gemini spectra of three sources (CX446, CX1004, and CXB2) as well as the presence of possible eclipses in light curves of these sources suggest that these sources are accreting binaries viewed under a high inclination.