Gemini Spectroscopy of Galactic Bulge Sources: A Population of Hidden Accreting Binaries Revealed?

TL;DR

This study uses Gemini spectroscopy to identify and analyze hidden accreting binaries in the Galactic Bulge, revealing broad Halpha emission lines indicative of accretion disks and uncovering new binary candidates.

Contribution

It demonstrates a method to detect hidden accretion activity in X-ray sources through spectral subtraction, identifying new accreting binary candidates in the Galactic Bulge.

Findings

Detection of broad Halpha emission lines indicating accretion disks.

Identification of three high-inclination accreting binary candidates.

Revelation of hidden accretion behavior in previously known sources.

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 Ha emission line. The spectra of several sources revealed an Ha emission line only after careful subtraction of the F or G stellar spectral absorption lines. In a sub-class of three of these sources the residual Halpha emission line is broad (> 400 km/s) which suggests that it is formed in an accretion disk, 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 disk. GBS source CX377 shows this hidden accretion behaviour most dramatically. The previously-identified broad Ha emission of this source is not present in its Gemini spectra taken about 1 year 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.