Optical spectroscopy of 20 Be/X-ray Binaries in the Small Magellanic Cloud

TL;DR

This study analyzes optical spectra of 20 Be/X-ray binaries in the Small Magellanic Cloud, confirming known spectral distributions and highlighting differences from Galactic Be stars, with implications for circumstellar disk interactions.

Contribution

It provides new classifications for 5 sources and expands the statistical analysis of Be/X-ray binaries in the SMC, comparing them with other galaxies.

Findings

Be-XRBs follow a different distribution than Be stars in the Galaxy.

SMC Be stars have larger Halpha equivalent widths than Be-XRBs.

Results support the relation between orbital period and Halpha equivalent width.

Abstract

We present a large sample (20 in total) of optical spectra of Small Magellanic Cloud (SMC) High-Mass X-ray Binaries obtained with the 2dF spectrograph at the Anglo-Australian Telescope. All of these sources are found to be Be/X-ray binaries (Be-XRBs), while for 5 sources we present original classifications. Several statistical tests on this expanded sample support previous findings for similar spectral-type distributions of Be-XRBs and Be field stars in the SMC, and of Be-XRBs in the Large Magellanic Cloud and the Milky Way, although this could be the result of small samples. On the other hand, we find that Be-XRBs follow a different distribution than Be stars in the Galaxy, also in agreement with previous studies. In addition, we find similar Be spectral type distributions between the Magellanic Clouds samples. These results reinforce the relation between the orbital period and the equivalent width of the Halpha line that holds for Be-XRBs. SMC Be stars have larger Halpha equivalent widths when compared to Be-XRBs, supporting the notion of circumstellar disk truncation by the compact object.