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arXiv · 2104.02790

The Be/neutron star system Swift J004929.5-733107 in the Small Magellanic Cloud -- X-ray characteristics and optical counterpart candidates

M. J. Coe·J. A. Kennea·P. A. Evans·L. J. Townsend·A. Udalski·I. M. Monageng·D. A. H. Buckley
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Abstract

Swift J004929.5-733107 is an X-ray source in the Small Magellanic Cloud (SMC) that has been reported several times, but the optical counterpart has been unclear due to source confusion in a crowded region of the SMC. Previous works proposed [MA93] 302 as the counterpart, however we show here, using data obtained from the S-CUBED project, that the X-ray positio is inconsistent with that object. Instead we propose a previously unclassified object which has all the indications of being a newly identified Be star exhibiting strong HU emission. Evidence for the presence of significant I-band variability strongly suggest that this is, in fact, a Be type star with a large circumstellar disk. Over 18 years worth of optical monitoring by the OGLE project reveal a periodic modulation at a period of 413d, probably the binary period of the system. A SALT optical spectrum shows strong Balmer emission and supports a proposed spectral classification of B1-3 III-IVe. The X-ray data obtained from the S-CUBED project reveal a time-averaged spectrum well fitted by a photon index = 0.93 pm 0.16. Assuming the known distance to the SMC the flux corresponds to a luminosity 10E35 erg/s. All of these observational facts suggest that this is confirmed as a Be star-neutron star X-ray binary (BeXRB) in the SMC, albeit one with an unusually long binary period at the limits of the Corbet Diagram.

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M. J. Coe, J. A. Kennea, P. A. Evans, L. J. Townsend, A. Udalski, I. M. Monageng, D. A. H. Buckley. 2021-04-06. The Be/neutron star system Swift J004929.5-733107 in the Small Magellanic Cloud -- X-ray characteristics and optical counterpart candidates. https://doi.org/10.1093/mnras/stab972

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