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Swift/XRT detection of an outburst from SXP 15.3

ATel #11030; L. Ducci (IAAT Tuebingen), P. Romano (OAB), J. A. Kennea (PSU), C. Malacaria (IAAT Tuebingen), S. Covino (OAB), A. Santangelo (IAAT Tuebingen), P. A. Evans (Leicester), M. J. Coe (Southampton), L. J. Townsend (UCT)
on 3 Dec 2017; 19:14 UT
Credential Certification: Pat Romano (romano@ifc.inaf.it)

Subjects: X-ray, Binary, Transient

During an observation of the Small Magellanic Cloud (SMC) carried out from December 2nd 2017, 11:12:03 UT to 19:30:52 UT (total exposure time of 7 ks), Swift/XRT detected renewed activity from the Be X-ray binary SXP 15.3.

S-CUBED, the Swift SMC Survey (ATel #10600), which utilizes Swift/XRT to survey the SMC every week with 142 tiled pointings of 60s exposure each, has been tracking the on-going outburst of SXP15.3.

S-CUBED first detected elevated emission from the source associated with the current outburst during its observation on October 10th, 2017. Since the observation on November 7th, 2017, the source has been in outburst at an average count rate of 1.6 c/s, including the most recent S-CUBED observation, taken on November 28th, 2017.

The source is detected in the observation of December 2nd 2017 at the very edge of the FoV at RA/Dec(J2000): 13.0608, -73.3223, which is equivalent to:
RA(J2000) = 00h 52m 14.58s,
Dec(J2000) = -73d 19' 20.1'',

with an estimated uncertainty of 2.2 arc-seconds radius (90% confidence). This position is 2.9 arc-seconds from the Simbad catalogued position of SXP 15.3. The count rate is comparable with the previous S-CUBED data, as it reached 1.7 counts/s. The XRT spectrum can be fit with an absorbed power-law model, with a power-law slope of 1.5 +/- 0.2 and a column density of (0.4 +/- 0.1)E22 cm^-2. The 0.3-10 keV absorbed flux obtained from the spectral fit is (6.7 +/- 0.6)E-11 erg/cm^2/s. Assuming a distance of 61 kpc and the unabsorbed flux, we obtain a (0.3-10 keV) X-ray luminosity of (3.9 +/- 0.3)E37 erg/s. Uncertainties are given at 90% c.l.

We performed a search for periodicities in the observation of December 2nd 2017 using the Z^2_1 test (Buccheri et al., 1983, A&A, 128, 245) and we found a significant (~4.1 sigma) signal at a period of about 15.25 s, in good agreement with the spin period of ~15.253s detected during the previous outburst (Kennea et al. 2017, ATel #10600).

S-CUBED monitoring of the SMC and SXP 15.3 will continue.