Abstract
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 position 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 H α 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 disc. Over 18 yr worth of optical monitoring by the OGLE project reveal a periodic modulation at a period of 413 d, 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 Γ = 0.93 ± 0.16. Assuming the known distance to the SMC, the flux corresponds to a luminosity ∼1035 erg s−1. 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.
Highlights
Be star-neutron star X-ray binary (BeXRB) are a large sub-group of the well-established category of High Mass X-ray Binaries (HMXB) characterised by being a binary system consisting of a massive mass donor star, normally an OBe type, and an accreting compact object, a neutron star
Reported here is the confirmation that Swift J004929.5-733107 is a BeXRB system in the Small Magellanic Cloud (SMC)
This has enabled the correct identification of the optical counterpart, resolving a long-standing confusion in a crowded stellar field
Summary
BeXRB are a large sub-group of the well-established category of High Mass X-ray Binaries (HMXB) characterised by being a binary system consisting of a massive mass donor star, normally an OBe type, and an accreting compact object, a neutron star They are prevalent in the Small Magellanic Cloud (SMC) which contains the largest known collection of BeXRBs. Whilst catalogues have been produced listing such systems in the SMC (for example Coe & Kirk (2015); Haberl & Sturm (2016)), it clear that there are many more systems yet to be identified. To resolve this confusion optical spectra of all three stars were obtained under excellent seeing conditions with the Southern African Large Telescope (SALT) These spectra combined with the new accurate X-ray position resolve the previous ambiguity over which star is the correct counterpart to Swift J004929.5-733107. It is proposed here that this is the correct optical counterpart to Swift J004929.5-733107 and it is a newly identified Be star in the SMC
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