Modelling the observable behaviour of SXP 5.05

Abstract

SXP 5.05 is a Be/X-ray binary with a neutron star companion located in the Small Magellanic Cloud. It was first detected in 2013, and later that year, SXP 5.05 underwent a massive optical and X-ray outburst. This outburst dwarfs any other optical event that has been observed for this system during the last 5 years. The large increase in optical brightness of the system implies an increase in the size and density of the Be star's circumstellar disc. The X-ray data show two occultations of the neutron star per orbit and is not consistent with a neutron star passing behind the Be star, and hence the disc is responsible for these occultations. In this paper, we model the outburst of Be/neutron star binary SXP 5.05 as being due to a large increase in mass ejection by the Be star. The neutron star passes directly through the growing disc, and it is shown that the resulting obscuration can qualitatively explain the observed X-ray behaviour of the system. We find the only way to reproduce the timescales of the observed optical behaviour of the system is to increase the mass ejection substantially for a short time (<2 orbits) and to decrease the viscosity during the event. The general behaviour of the observed X-ray and Ha line emission are also reproduced by the model. However, the inferred mass ejection and viscosity needed to produce a sufficiently rapid increase of disc size are both higher than suggested by previous works.

arXiv comment10 pages, 10 figures. Accepted by MNRAS

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• Modelling decretion discs in Be/X-ray binaries
• The frequency of Kozai-Lidov disc oscillation driven giant outbursts in Be/X-ray binaries
• The unusual behaviour of the young X-ray pulsar SXP 1062 during the 2019 outburst
• A major optical & X-ray outburst from the Magellanic Bridge source RX J0209.6-7427