SXP 5.05 = IGR J00569-7226: using X-rays to explore the structure of a Be star's circumstellar disc

On MJD 56590-1 (2013 Oct 25–26), observations of the Magellanic Clouds by the INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) observatory discovered a previously unreported bright, flaring X-ray source. This source was initially given the identification IGR J00569-7226. Subsequent multiwavelength observations identified the system as new Be/X-ray binary system in the Small Magellanic Cloud (SMC). Follow-up X-ray observations by Swift and XMM–Newton revealed an X-ray pulse period of 5.05 s and that the system underwent regular occultation/eclipse behaviour every 17 d. This is the first reported eclipsing Be/X-ray binary system in the SMC, and only the second such system known to date. Furthermore, the nature of the occultation makes it possible to use the neutron star to ‘X-ray’ the circumstellar disc, thereby, for the first time, revealing direct observational evidence for its size and clumpy structure. Swift timing measurements allowed for the binary solution to be calculated from the Doppler-shifted X-ray pulsations. This solution suggests this is a low-eccentricity binary relative to others measured in the SMC. Finally, it is interesting to note that the mass determined from this dynamical method for the Be star (?13.0?M?) is significantly different from that inferred from the spectroscopic classification of B0.2Ve (?16.0?M?) – an effect that has been noted for some other high mass X-ray binary systems.