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SMC X-3: S-CUBED detection of turn-on, brightening and identification with MAXI J0058-721

ATel #9362; J. A. Kennea (PSU), M. J. Coe (Southampton), P. A. Evans, A. P. Beardmore (U Leicester), H. A. Krimm (CRESST/GSFC/USRA), P. Romano (INAF-IASFPA), K. Yamaoka (AGU), M. Serino (RIKEN) and H. Negoro (Nihon U.)
on 10 Aug 2016; 20:39 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

Subjects: X-ray, Neutron Star, Transient

Referred to by ATel #: 9370, 9677, 9731

The Swift SMC Survey (S-CUBED) is a shallow weekly X-ray survey of the SMC performed over ~140 sixty second exposure tiles by Swift (e.g. Kennea et al., ATEL #9299). S-CUBED has detected the turn-on and rise of the known transient source SMC X-3 (AKA SXP 7.78). The Swift/XRT localization is consistent within errors with the Chandra position for SMC X-3 reported by Edge et al (ATEL #225), although with such a short exposure we cannot confirm the pulsar period of the source.

In S-CUBED observations taken between June 8th and July 16th, 2016, the source was not detected, with a typical upper limit on the XRT count rate of <0.14 c/s. In the observation taken on July 30th, 2016 at 04:17UT the source was clearly detected at a count rate of 0.23 +/- 0.08 c/s. Follow-up observations on August 3rd, 2016 at 16:41UT and August 10th, 2016 at 14:24 UT show the source rising to 0.35 +/- 0.09 c/s and 4.5 +/- 0.5 c/s (uncorrected for pile-up) respectively.

The XRT spectrum is well fit by a hard (photon index = 0.5 +/- 0.7) power-law model, and the flux in the first two detections are 2.3 +/- 0.8 x 10-11 erg/s/cm2 and 3.5 +/- 0.9 x 10-11 erg/s/cm2 respectively, in the range of 0.5 - 10 keV. The August 10th, 2016 observation shows at least an order of magnitude brightening from the observation on August 3rd, 2016. We await the full dataset, and the results of a requested target-of-opportunity Windowed Timing mode observation in order to determine the flux, spectrum and pulsar period of SMC X-3 accurately.

We note that this rise is also detected by the BAT Transient Monitor (Krimm et al., 2013, ApJSS 209,14), which shows an increase in count rate from SMC X-3 in the 15-50 keV band. Starting on 2016 Aug 7 (MJD 57607), the source was detected at 0.0048 +/- 0.0009 ct/s/cm2 (~20 mCrab). The count rate continues to increase, and on Aug. 10, the rate is 0.010 +/- 0.002 ct/s/cm2 (~45 mCrab). This represents the brightest detection in the BAT monitor since the start of the Swift mission in 2004.

The center of the error circle of MAXI J0058-721 (Negoro et al, ATEL #9348) lies 31.6 arc-minutes from SMC X-3, inside the MAXI error circle, and the Swift XRT count rate seen on August 10th, 2016 is consistent with the reported 9 +/- 3 mCrab flux for the MAXI transient. Therefore we conclude that MAXI J0058-721 is in fact a new outburst of SMC X-3.

Swift TOO 7-point tiled observations of the MAXI error circle were taken on August 9th, 2016, however they did not cover SMC X-3, and detected no other bright transients. SXP 202A (Coe et al., ATEL #9307) was detected within the MAXI error circle, although it is fainter than at the peak brightness seen by S-CUBED on July 16, 2016, so we do not believe that this source is related to MAXI J0058-721. The only other source detected in the August 9th observations was the X-ray bright supernova remnant E0102.

Weekly monitoring of SMC X-3 by S-CUBED will continue.