# Spectroscopic classification of X-ray sources in the Galactic Bulge Survey

## TL;DR

This study classifies 26 X-ray sources in the Galactic Bulge using optical spectroscopy and photometry, revealing diverse stellar and binary types, including CVs, young stars, and active stars, with some sources showing unique features.

## Contribution

The paper provides the first detailed optical classification of 26 X-ray sources in the Galactic Bulge Survey, identifying new candidate binaries and peculiar objects.

## Key findings

- Identification of various stellar and binary source classes.
- Discovery of a magnetic CV with X-ray pulsations.
- Detection of a young stellar object with unique variability.

## Abstract

We present the classification of 26 optical counterparts to X-ray sources discovered in the Galactic Bulge Survey. We use (time-resolved) photometric and spectroscopic observations to classify the X-ray sources based on their multi-wavelength properties. We find a variety of source classes, spanning different phases of stellar/binary evolution. We classify CX21 as a quiescent cataclysmic variable (CV) below the period gap, and CX118 as a high accretion rate (nova-like) CV. CXB12 displays excess UV emission, and could contain a compact object with a giant star companion, making it a candidate symbiotic binary or quiescent low mass X-ray binary (although other scenarios cannot be ruled out). CXB34 is a magnetic CV (polar) that shows photometric evidence for a change in accretion state. The magnetic classification is based on the detection of X-ray pulsations with a period of 81 $\pm$ 2 min. CXB42 is identified as a young stellar object, namely a weak-lined T Tauri star exhibiting (to date unexplained) UX Ori-like photometric variability. The optical spectrum of CXB43 contains two (resolved) unidentified double-peaked emission lines. No known scenario, such as an AGN or symbiotic binary, can easily explain its characteristics. We additionally classify 20 objects as likely active stars based on optical spectroscopy, their X-ray to optical flux ratios and photometric variability. In 4 cases we identify the sources as binary stars.