Variability of Optical Counterparts in the Chandra Galactic Bulge Survey

TL;DR

This study presents optical lightcurves of variable stars associated with X-ray sources in the Galactic Bulge, revealing diverse variability types and suggesting most are true counterparts, enhancing understanding of X-ray binary populations.

Contribution

First comprehensive optical variability analysis of X-ray sources in the Galactic Bulge, identifying diverse variable star types and their potential as true counterparts.

Findings

87% of X-ray sources have potential optical counterparts.

24% of these counterparts are detectably variable.

Most variables are likely true counterparts, not random field stars.

Abstract

We present optical lightcurves of variable stars consistent with the positions of X-ray sources identified with the Chandra X-ray Observatory for the Chandra Galactic Bulge Survey. Using data from the Mosaic-II instrument on the Blanco 4m Telescope at CTIO, we gathered time-resolved photometric data on timescales from $\sim2$ hr to 8 days over the $\frac{3}{4}$ of the X-ray survey containing sources from the initial GBS catalog. Among the lightcurve morphologies we identify are flickering in interacting binaries, eclipsing sources, dwarf nova outbursts, ellipsoidal variations, long period variables, spotted stars, and flare stars. $87\%$ of X-ray sources have at least one potential optical counterpart. $24\%$ of these candidate counterparts are detectably variable; a much greater fraction than expected for randomly selected field stars, which suggests that most of these variables are real counterparts. We discuss individual sources of interest, provide variability information on candidate counterparts, and discuss the characteristics of the variable population.