Doubling the number of Be/X-ray binaries in the SMC

TL;DR

This study significantly increases the known population of Be/X-ray binaries in the Small Magellanic Cloud by identifying new candidates through optical and X-ray data correlation, revealing insights into their distribution and luminosity.

Contribution

The paper proposes up to 25 new Be/X-ray binary candidates in the SMC, more than doubling the known systems, based on correlation of X-ray sources with emission-line stars.

Findings

More than two thirds of Be/X-ray binaries are emission-line objects.

The new candidates are mainly low-luminosity systems.

The ratio of Be to OB supergiant X-ray binaries is higher than in the Milky Way.

Abstract

A correlation of X-ray source and Halpha emission-line object catalogues in the Small Magellanic Cloud (SMC) shows that more than two thirds of the optically identified Be stars in Be/X-ray binaries are found as emission-line objects in the catalogues. On the basis of this result we propose up to 25 X-ray sources mainly from recent ROSAT catalogues as new Be/X-ray binaries and give their likely optical counterparts. Also for the five yet unidentified X-ray pulsars in the SMC we propose emission-line stars as counterparts. This more than doubles the number of known high mass X-ray binary systems in this nearby galaxy. The spatial distribution of the new candidates is similar to that of the already identified Be/X-ray binaries with a strong concentration along the SMC main body and some systems in the eastern wing. The new candidates contribute mainly to the low-luminosity end of the X-ray luminosity distribution of Be/X-ray binaries. A comparison with the luminosity distribution in the Milky Way reveals no significant differences at the high-luminosity end and the large number of low-luminosity systems in the SMC suggests that many such systems may still be undetected in the Galaxy. The overall ratio of known Be to OB supergiant X-ray binaries in the SMC is an order of magnitude larger than in the Galaxy, however, might show spatial variations. While in the eastern wing the ratio is comparable to that in the Galaxy no supergiant X-ray binary is currently known in the main body of the SMC. Possible explanations include a different star formation history over the last ~15 My.