SXP 5.05 = IGR J00569-7226 : using X-rays to explore the structure of a Be stars circumstellar disk

TL;DR

This paper reports the discovery and analysis of the first eclipsing Be/X-ray binary in the Small Magellanic Cloud, using X-ray observations to probe the structure of the Be star's circumstellar disk.

Contribution

It presents the first observational evidence of the size and clumpy structure of a Be star's circumstellar disk through X-ray occultation analysis.

Findings

Discovered a 5.05s X-ray pulsar in the SMC

Identified the system as an eclipsing Be/X-ray binary

Revealed the disk's size and clumpy structure via occultation

Abstract

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 multi-wavelength observations identified the system as new Be/X-ray binary system in the Small Magellanic Cloud. Follow-up X-ray observations by Swift and XMM-Newton revealed an X-ray pulse period of 5.05s and that the system underwent regular occulation/eclipse behaviour every 17d. 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 disk, 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 (approx 13 solar masses) is significantly different from that inferred from the spectroscopic classification of B0.2Ve (approx 16 solar masses) - an effect that has been noted for some other high mass X-ray binary (HMXB) systems.