Optical properties of High-Mass X-ray Binaries (HMXB) in the Small Magellanic Cloud

TL;DR

This study analyzes optical observations of High-Mass X-ray Binaries in the Small Magellanic Cloud over a decade, revealing their abundance, stellar companion types, and raising questions about stellar evolution and binary formation.

Contribution

It provides extensive optical data on HMXBs in the SMC, highlighting their unique properties and raising new questions about stellar evolution in low-mass galaxies.

Findings

SMC has a high number of HMXBs despite its small mass.

Most SMC HMXBs have Be star companions.

The absence of black hole systems in SMC HMXBs remains unexplained.

Abstract

The SMC represents an exciting opportunity to observe the direct results of tidal interactions on star birth. One of the best indicators of recent star birth activity is the presence of signicant numbers of High-Mass X-ray Binaries (HMXBs) - and the SMC has them in abundance! We present results from nearly 10 years of monitoring these systems plus a wealth of other ground-based optical data. Together they permit us to build a picture of a galaxy with a mass of only a few percent of the Milky Way but with a more extensive HMXB population. However, as often happens, new discoveries lead to some challenging puzzles - where are the other X-ray binaries (eg black hole systems) in the SMC? And why do virtually all the SMC HMXBs have Be starcompanions? The evidence arising from these extensive optical observations for this apparently unusual stellar evolution are discussed.