A new X-ray transient system in the Small Magellanic Cloud, Swift J005139.2-721704

ATel #12209; M. J. Coe (Southampton), J. A. Kennea (PSU), D. Buckley (SAAO) and A. Udalski (Warsaw)
on 13 Nov 2018; 17:25 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Malcolm Coe (mjcoe@soton.ac.uk)

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Observations from the Niels Gehrels Swift telescope on 9 and 12 November 2018 revealed a new transient X-ray source in the SMC which we identify as Swift J005139.2-721704. Previous observations of the same field on the 6 November barely detected the source. By the 12 November it had reached 0.5 counts/s in the XRT telescope.

A follow-up 5ks Windows Timing measurements on 12 December did not detect any coherent pulsations. But we obtain a hard, photon index = 0.87 +/- 0.08 from the spectral fit - very similar to that obtained from the PC mode date of 0.87 +/- 0.09. The Swift WT count rates indicate an X-ray flux of 8E-11 erg/s/cm^2, corresponding to a luminosity of approximately 4E37 erg/s if the source is in the SMC.

The best position of the new source is RA 00 51 39.2, Dec -72 17 03.6 with an estimated uncertainty of 1.4 arc-seconds radius (90% confidence). This position is coincident with a V=15.6 B-type star [M2002] SMC 20671, and hence this is probably a new SMC HMXB transient system. The current Swift count rates indicate an X-ray luminosity of approximately 1E37 erg/s if the source is in the SMC.

OGLE III and IV data were examined for the proposed optical counterpart, and it is noted that the star is brighter in the I-band than it has been for the past ~6000 days. This suggest the recent growth, or increase in size, of a circumstellar disk feeding the X-ray outburst. Timing analysis of the OGLE data did not reveal any periodic behaviour in the 2 to 100 day range.

X-ray and optical observations of the field are scheduled to follow this outburst over the next few weeks.