The X-ray transient system in the Small Magellanic Cloud, Swift J005139.2-721704 - binary period and optical spectral class

ATel #12229; M. J. Coe (Southampton), J. A. Kennea (PSU), D. Buckley (SAAO), V. McBride (OAD/SAAO), A. Udalski (Warsaw), P. Evans (Leicester), P. Roche (Cardiff) and L. Townsend (UCT)
on 19 Nov 2018; 11:02 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Malcolm Coe (mjcoe@soton.ac.uk)

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A new X-ray transient, designated Swift J005139.2-721704, was reported in Atel #12209 by Coe et al. Subsequently this region was observed by NICER and a pulse period of 4.78 s reported (ATel #12222). Those authors (Strohmayer et al) proposed that this object was associated with a previously known RXTE pulsar (XTE 0052-723 = SXP 4.78) discovered by Corbet et al in 2001 (IAUC 7562 ). That source had never been successfully associated with an optical counterpart due to the lack of imaging from RXTE. However the Swift position of the current transient unambiguously associates Swift J005139.2-721704 with the OB star [M2002] SMC 20671.

Further analysis of the OGLE III and IV data on [M2002] SMC 20671 were carried out. In light of the proposed identification of this transient with a 4.78s pulsar it was decided to explore the period range below the 2 days that was previously reported. The result was a strong detection of a period at 1.805d, with a second somewhat weaker peak at the beat frequency between that period and the 1 day typical sampling. Folding the data at that period reveals a sinusoidal-like average lightcurve. If this is the true binary period, then it is so short there is no time for the circumstellar disk to generate the classic FRED-like modulation seen in much longer binary period systems.

The observed average colour and magnitude of [M2002] SMC 20671 determined from OGLE are V-I = 0.13 and I = 15.5. This suggests that the object is not a main sequence star in the SMC unless exceptional reddening (for objects in the SMC) is introduced. Without such exceptional reddening the colours are that of a late B-type, early A-type star.

A blue spectrum was obtained from SALT on 16 November 2018 to investigate the spectral classification of [M2002] SMC 20671. The spectral lines are somewhat narrower than one might expect from a dwarf (luminosity class V) star, but unlikely to be narrow enough to justify a supergiant classification. The V=15.6 makes it also unlikely to be a SG. Certainly the spectrum is not consistent with an early A-type - the lines of neutral helium are still strong enough in this object to constrain it to being a B star. Overall the SALT spectrum combined with the OGLE colours suggest most probably that of a considerably reddened early type B star in the SMC.

Assuming that this binary period of 1.8 d and the 4.78 s pulsations period are coming from the same system, then it can be located on the Corbet diagram. It sits in the region populated by the two Magellanic Cloud, Roche lobe overflow (RLOF) systems, LMC X-4 and SMC X-1. But we note that unlike the system that is the subject of this report, both of those systems have supergiant mass donor stars. It seems unlikely that [M2002] SMC 20671 is a SG, hence we have the first Be/X-ray binary possibly identified as a RLOF system.

New Swift observations taken on 18 November 2018 show that the system has doubled in brightness in just 3 days and hence further observations at all wavelengths are encouraged to try and understand this apparently unusual system.