Spectral distribution of Be/X-ray binaries in the Small Magellanic Cloud

TL;DR

This study investigates the spectral distribution of Be/X-ray binaries in the Small Magellanic Cloud, finding it similar to that in the Galaxy, suggesting angular momentum loss during binary evolution influences spectral characteristics regardless of metallicity.

Contribution

It provides the first detailed spectral distribution analysis of SMC Be/X-ray binaries, highlighting the role of angular momentum loss over metallicity in shaping their spectra.

Findings

Spectral distribution in SMC matches that of the Galaxy.

Metallicity has limited impact on spectral distribution.

Angular momentum loss influences binary spectral characteristics.

Abstract

The spectral distributions of Be/X-ray binaries in the Large Magellanic Cloud and Galaxy have been shown to differ significantly from the distribution of isolated Be stars in the Galaxy. Population synthesis models can explain this difference in spectral distributions through substantial angular momentum loss from the binary system. In this work we explore the spectral distribution of Be/X-ray binaries in the Small Magellanic Cloud (SMC) using high signal-to-noise spectroscopy of a sample of 37 optical counterparts to known X-ray pulsars. Our results show that the spectral distribution of Be/X-ray binaries in the SMC is consistent with that of the Galaxy, despite the lower metallicity environment of the SMC. This may indicate that, although the metallicity of the SMC is conducive to the formation of a large number of HMXBs, the spectral distribution of these systems is likely to be most strongly influenced by angular momentum losses during binary evolution, which are not particularly dependent on the local metallicity.