SXP 7.92: a recently rediscovered Be/X-ray binary in the Small Magellanic Cloud, viewed edge on

Bartlett, E.S., Coe, M.J., Israel, G., Clark, J.S., Esposito, P., D'Elia, V. and Udalski, A. (2017) SXP 7.92: a recently rediscovered Be/X-ray binary in the Small Magellanic Cloud, viewed edge on. Monthly Notices of the Royal Astronomical Society, 466 (4), 4659-4671. (doi:10.1093/mnras/stx032).

Abstract

We present a detailed optical and X-ray study of the 2013 outburst of the Small Magellanic Cloud Be/X-ray binary SXP 7.92, as well as an overview of the last 18 years of observations from OGLE (Optical Gravitational Lensing Experiment), RXTE, Chandra and XMM-Newton. We revise the position of this source to RA(J2000) = 00:57:58.4, Dec(J2000) = −72:22:29.5 with a 1σ uncertainty of 1.5 arcsec, correcting the previously reported position by Coe et al. by more than 20 arcmin. We identify and spectrally classify the correct counterpart as a B1Ve star. The optical spectrum is distinguished by an uncharacteristically deep narrow Balmer series, with the Hα line in particular having a distinctive shell profile, i.e. a deep absorption core embedded in an emission line. We interpret this as evidence that we are viewing the system edge on and are seeing self-obscuration of the circumstellar disc. We derive an optical period for the system of 40.0 ± 0.3 d, which we interpret as the orbital period, and present several mechanisms to describe the X-ray/optical behaviour in the recent outburst, in particular the ‘flares'and ‘dips’ seen in the optical light curve, including a transient accretion disc and an elongated precessing disc.

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Contributors

Author: M.J. Coe

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