High-Mass X-ray binaries in the Small Magellanic Cloud
In high-mass X-ray binaries (HMXBs) an early-type star and a compact object are orbiting each other. The compact object is in most cases a neutron star, but can also be a black hole or a white dwarf. Many of the HMXBs show pulsations in their X-ray flux, which indicate the spin period of the neutron star. The SMC is peculiar with respect to its exceptionally high number of known HMXBs. So far only in one (well confirmed) case the optical counterpart is a super-giant star (SMC X-1), while for all other identified cases a Be star (with Balmer emission lines) was found, forming a Be/X-ray binary (BeXRB). Be stars loose mass via a still unknown mechanism, forming a circum-stellar disk in their equatorial plane. Accretion of matter from this disk by the compact object powers the X-ray emission and can lead to strong X-ray outbursts.
So far most of the multi-wavelength work on the BeXRBs in the SMC concentrated on the BeXRB pulsars. However, many (candidate) BeXRBs exist with X-ray and optical properties strongly suggesting a HMXB nature, only pulsations were not detected. After the XMM-Newton survey of the SMC about 120 HMXBs are known with relatively high confidence for being genuine HMXBs. Sixty-three of them are X-ray pulsars. A catalogue describing the properties of individual objects and statistical investigations of the HMXB population in the SMC was published in by Haberl & Sturm 2016 (see list of publications on the right).