# Candidate H{\alpha} emission and absorption line sources in the Galactic Bulge Survey

## TL;DR

This paper compiles a catalog of H-alpha emission and absorption line sources, along with blue objects, in the Galactic Bulge Survey, aiding the identification of various stellar and extragalactic objects through multi-wavelength analysis.

## Contribution

It introduces a new catalog of H-alpha line candidates and blue objects in the Galactic Bulge, utilizing colour-colour diagrams for source selection and crossmatching with X-ray data.

## Key findings

- Identified 1337 H-alpha emission line candidates.
- Found 336 H-alpha absorption line candidates.
- Detected 389 blue objects likely containing compact systems.

## Abstract

We present a catalogue of candidate H{\alpha} emission and absorption line sources and blue objects in the Galactic Bulge Survey (GBS) region. We use a point source catalogue of the GBS fields (two strips of (l x b) = (6 x 1) degrees centred at b = 1.5 above and below the Galactic centre), covering the magnitude range 16 < r' < 22.5. We utilize (r'-i', r'-H{\alpha}) colour-colour diagrams to select H{\alpha} emission and absorption line candidates, and also identify blue objects (compared to field stars) using the r'-i' colour index. We identify 1337 H{\alpha} emission line candidates and 336 H{\alpha} absorption line candidates. These catalogues likely contain a plethora of sources, ranging from active (binary) stars, early-type emission line objects, cataclysmic variables (CVs) and low-mass X-ray binaries (LMXBs) to background active galactic nuclei (AGN). The 389 blue objects we identify are likely systems containing a compact object, such as CVs, planetary nebulae and LMXBs. Hot subluminous dwarfs (sdO/B stars) are also expected to be found as blue outliers. Crossmatching our outliers with the GBS X-ray catalogue yields sixteen sources, including seven (magnetic) CVs and one qLMXB candidate among the emission line candidates, and one background AGN for the absorption line candidates. One of the blue outliers is a high state AM CVn system. Spectroscopic observations combined with the multi-wavelength coverage of this area, including X-ray, ultraviolet and (time-resolved) optical and infrared observations, can be used to further constrain the nature of individual sources.