Investigating ChaMPlane X-ray sources in the Galactic Bulge with Magellan LDSS2 spectra

TL;DR

This study analyzes optical and X-ray data of 136 Galactic-bulge X-ray sources, discovering various stellar and binary populations, including new cataclysmic variables, and discusses an unexpected excess of CVs.

Contribution

It provides the first comprehensive optical and X-ray spectral analysis of a large sample of Galactic-bulge X-ray sources, revealing new CVs and stellar populations.

Findings

Discovery of new X-ray detected cataclysmic variables.

Identification of a population of stellar coronal sources.

Observation of an excess of CVs compared to simple models.

Abstract

We have carried out optical and X-ray spectral analyses on a sample of 136 candidate optical counterparts of X-ray sources found in five Galactic-bulge fields included in our Chandra Multi-wavelength Plane Survey. We use a combination of optical spectral fitting and quantile X-ray analysis to obtain the hydrogen column density towards each object, and a three-dimensional dust model of the Galaxy to estimate the most probable distance in each case. We present the discovery of a population of stellar coronal emission sources, likely consisting of pre-main sequence, young main sequence and main sequence stars, as well as a component of active binaries of RS CVn or BY Dra type. We identify one candidate quiescent low-mass X-ray binary with a sub-giant companion; we note that this object may also be an RS CVn system. We report the discovery of three new X-ray detected cataclysmic variables (CVs) in the direction of the Galactic Center (at distances ~2kpc). This number is in excess of predictions made with a simple CV model based on a local CV space density of <~ 10^-5 pc^-3, and a scale height ~200pc. We discuss several possible reasons for this observed excess.