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

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posted on 2021-09-13, 12:34 authored by MJ Coe, JA Kennea, PA Evans, LJ Townsend, A Udalski, IM Monageng, DAH Buckley
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.


The OGLE project has received funding from the National Science Centre, Poland, grant MAESTRO 2014/14/A/ST9/00121 to AU. PAE acknowledges UKSA support. IMM acknowledges support from the National Research Foundation (NRF) and the University of Cape Town (UCT). This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester. LJT acknowledges support from the National Research Foundation (NRF), South Africa and the SALT consortium. Some of the observations reported in this paper were obtained with the Southern African Large Telescope, as part of the Large Science Programme on transients 2018-2-LSP-001 (PI: Buckley).



Monthly Notices of the Royal Astronomical Society, Volume 504, Issue 1, June 2021, Pages 1398–1406,

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School of Physics and Astronomy


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Monthly Notices of the Royal Astronomical Society






1398 - 1406


Oxford University Press (OUP)





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