Swift detection of active X-ray transients in the SMC

ATel #9299; J. A. Kennea (PSU), P. A. Evans (Leicester) and M. J. Coe (Southampton)
on 1 Aug 2016; 20:00 UT
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

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We report on the results of the the Swift SMC Survey (S-CUBED), a wide area/short exposure survey of the SMC in X-rays performed by the Swift X-ray Telescope (XRT). The survey consists of 142, 60 second exposure tiled pointings covering the SMC (including the "Wing"), performed approximately weekly. Observations for S-CUBED began on June 8th, 2016. In recent observations we have detected the turn on and brightening of several known transients in the SMC, which we report on below. As the exposures are low we do not attempt spectral analysis of these sources, fluxes are reported in XRT count/s. As a guide, for a Crab-like spectrum, 1mCrab = ~0.7 count/s for Swift/XRT.

SMC X-1: This HMXB source has been significantly detected in all observations taken as part of the survey, however in the three observations taken on July 6th, 10th, 15th, 2016, it brightened from a quiescent level of ~0.1 count/s to 30 +/- 3 count/s. The most recent observation taken July 29th, it had returned to ~0.1 count/s. These variations follow the well known ~55 day super-orbital period seen from this source, which can be seen in the BAT Transient Monitor web page:

Scaled Map Transient Analysis for SMC X-1

SXP 6.85: In the observations taken on July 29th, 2016 this Be/X-ray binary was significantly detected by XRT at a rate of 0.3 +/- 0.1 counts/s. In all previous S-CUBED observations SXP 6.85 was not detected, with typical 3-sigma upper limits of <0.13 count/s. The localization of this transient is consistent within errors with the catalogued position of SXP 6.85, suggesting that SXP 6.85 has entered a new outburst. This source was previous reported to be in outburst on April 29th-30th, 2015 by INTEGRAL (Nikolajuk et al, ATEL #7481). The short exposure time of survey observations do not allow us to make any analysis of the pulsar period in this source.

IGR J01217-7257: This was first seen by INTEGRAL in January 2014 (Coe et al., ATEL #5806). Based on a XMM-Newton observation revealing a 2.16s periodicity, this source has been identified as XTE J0119-731 (Haberl et al., ATEL #8305), and is a likely Be/X-ray binary. In the survey we detect this source significantly in observations taken on July 6th, 10th and 15th, 2016 at an average level of 0.31 +/- 0.07 count/s. The XRT derived coordinates of this transient are consistent with the XMM-Newton localization (Haberl et al., ATEL #8305). In the most recent observation taken on July 29th, 2016, the source is not detected with a 3-sigma upper limit of < 0.07 c/s, suggesting that the recent outburst was brief.

3XMM J004855.1-734946: This source was discovered to be in outburst in S-CUBED observations taken on June 8th and 16th, 2016 (Evans et al., ATEL #9197), and was reported to be a Be/X-ray binary pulsar (Vasilopoulos et al., ATEL #9229). Analyzing Swift observations taken on this object since the outburst was reported, it has risen from ~0.04 count/s on June 8th, 2016 to peak at 0.22 +- 0.02 count/s on June 28th, 2016. The source then fell again to ~0.04 count/s on July 12th, 2016, and in an observation taken on July 15th, 2016, was no longer detected with an 3-sigma upper limit of 0.1 c/s. In the most recent observation, on July 29th, 2016, the source was detected again, at a level of 0.30 +/- 0.06 count/s, the brightest level seen by Swift yet.

Further observations in the S-CUBED program take place on a weekly basis, barring observational constraints, and results will be reported via ATEL.