S-CUBED detection of an outburst of SXP 59.0 in the SMC

ATel #10250; J. A. Kennea (PSU), P. A. Evans (Leicester) and M. J. Coe (Southampton)
on 8 Apr 2017; 03:37 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

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The Swift SMC Survey (S-CUBED) is a weekly shallow (60s exposure) survey consisting of 142 tiles aimed at covering the SMC to monitor SMC X-ray sources and search for transient outbursts. In S-CUBED observations taken on March 30th, 2017 at 13:00UT, in a single 60s exposure tile, a bright X-ray source was detected at the following location: RA/Dec(J2000) = 13.7355, -72.4487, which is equivalent to:

RA(J2000) = 00h 54m 56.53s,
Dec(J2000) = -72d 26’ 55.4s,

with an estimated uncertainty of 4.1 arc-seconds radius (90% confidence). This position lies close to (7.6 arc-seconds) the SMC HMXB SXP 59.0. The measured count rate in the 60s Photon Counting mode exposure is 0.9 +/- 0.2 c/s. A previous observation of this field taken on March 22nd, 2017 showed a marginal detection in 60s exposure, with a count rate of 0.2 +/- 0.1 c/s, previous observations resulted in upper limits on the source brightness of typically < 0.2 c/s per observation. Therefore this is the first detected outburst of this source, detected by S-CUBED, since observations of this region began on Jun 8th, 2016.

A 3ks Target of Opportunity observation of this source was taken by Swift starting at 14:41UT on April 7th, 2017 (there was a delay in follow-up due to observing constraints). XRT data were taken in Windowed Timing (WT) mode to improve timing accuracy and avoid pile-up. In these data we detect a bright point source, consistent with the PC mode localization. This source has brightened since the March 30th, 2017 observation, to an XRT count rate of 2.1 +/- 0.1 c/s.

An analysis of the timing mode data reveals the presence of pulsations at a period of 59.045s, unambiguously confirming that this source is SXP 59.0, which is entering a new bright outburst phase.

The spectrum can be well fit by an absorbed power-law model, with a photon index of 0.96 +/- 0.10. The fitted flux value is 8 x 10^-11 erg/s/cm² (0.5 - 10 keV), assuming an SMC distance of 61 kpc, this equates to a luminosity of 4 x 10³⁷ erg/s (0.5 - 10 keV).

Observations of SXP 59.0 by Swift and S-CUBED will continue.