Faint Galactic X-ray Binaries

TL;DR

This paper reviews the properties and classification of faint Galactic X-ray binaries, highlighting recent insights into their luminosity indicators, optical spectral features, and unusual source characteristics.

Contribution

It provides a concise overview of faint X-ray binaries, emphasizing new methods for estimating luminosity and clarifying misconceptions about spectral signatures.

Findings

Type I X-ray burst duration can indicate source luminosity.

Low hydrogen/helium emission lines do not necessarily mean ultra-compact systems.

Discussion of unusual X-ray binary candidates.

Abstract

We present a short overview of the properties of faint Galactic X-ray binaries. We place emphasis on current classification scenarios. One of the important parameters for the faint sources is their intrinsic luminosity. In the case of low-mass X-ray binaries it has recently been realised that besides a phase of radius expansion, the duration of type I X-ray bursts can be used as a primer for the source luminosity in some cases. Further, we show that a very low equivalent width of hydrogen and helium emission lines in the optical spectrum alone is not a tell-tale sign for an ultra-compact system. Finally, we list and discuss some unusual sources that could be X-ray binaries.