The Small Magellanic Cloud
The Small Magellanic Cloud (SMC) is a gas-rich irregular dwarf galaxy and the second-nearest star forming galaxy after the Large Magellanic Could. Recent star formation was triggered by tidal interactions between the two galaxies and with the Milky Way. Because of the relatively short distance of about 60 kpc and low Galactic foreground extinction and absorption the SMC is an ideal target for population studies of objects which are not always accessible in the Milky Way. Understanding the emission components of nearby galaxies is required to understand the unresolved emission from more distant galaxies.
An X-ray survey of the Small Magellanic Cloud with XMM-Newton
Between May 2009 and March 2010 XMM-Newton carried out a deep X-ray survey of the SMC. Together with archival observations an area of 5.5 square degrees was covered including the bar and the eastern wing of the SMC. The images obtained from the European Photon Imaging Camera revealed more than 3000 X-ray sources. This allows to study the different X-ray source populations of the SMC, comprising supernova remnants, Be/X-ray binary systems and supersoft X-ray sources, and the hot thin plasma in the interstellar medium.
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).