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S-CUBED detection of SXP 15.3 in outburst

ATel #10600; J. A. Kennea (PSU), P. A. Evans (Leicester), M. J. Coe (Southhampton), A. Udalski (Warsaw University)
on 27 Jul 2017; 05:12 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

Subjects: X-ray, Transient

Referred to by ATel #: 11030

The Swift SMC Survey (S-CUBED) is a wide field shallow survey of the SMC in X-rays performed weekly by NASA's Swift Mission. S-CUBED consists of approximately 150 pointings with an exposure of 60s each, which cover the SMC to search for new and recurrent X-ray transients.

During an S-CUBED observation on July 25th, 2017 at 16:32UT, a source consistent with the localization of the Be/X-ray binary SXP 15.3 was detected at a brightness of 0.6 +/- 0.1 XRT count/s. Previously the source has been frequently detected in S-CUBED observations, but at a much lower count rates of between 0.05-0.15 XRT count/s. Swift performed a 2ks follow-up observations of the source starting at 14:34UT on July 26th, 2017. The source is detected in this observation at RA/Dec(J2000): 13.0558, -73.3218, which is equivalent to:

RA(J2000) = 00h 52m 13.38s,
Dec(J2000) = -73d 19' 18.4'',

with an estimated uncertainty of 3.5 arc-seconds radius (90% confidence). This position is 2.6 arc-seconds from the Simbad catalogued position of SXP 15.3, and is consistent within errors. Period analysis of the data show evidence of periodicity around 15.3s. However, by coincidence, the source was located inside the field of view of other Swift observations taken on July 25th and July 26th, and by combining those data, we find a strongly detected period in the data at 15.253s, confirming that this is indeed a new outburst of SXP 15.3. Over July 25th and 26th, the source maintains an average brightness of 0.73 +/- 0.02 XRT count/s, with no evidence of fading or brightening over that time period.

The spectrum is well fit by an absorbed power-law model with a photon index of 1.3 +/- 0.1. The absorption corrected flux is 5.5 x 10-11 erg/s/cm-2 (0.5 - 10 keV), which at an assumed SMC distance of 61kpc, corresponds to a 0.5-10 keV luminosity of 2.4 x 1037 erg/s.

OGLE IV monitoring of the optical counterpart to SXP 15.3, [MA 93] 552, show the I band flux to be at its brightest for several years with I=14.2 on 26 July 2017. This is 0.7 magnitudes brighter than the lowest state seen in OGLE IV some 5-6 years ago.