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SMC X-3 in outburst: Swift XRT Timing and Spectroscopy

ATel #9370; J. A. Kennea (PSU), M. J. Coe (Southampton) and P. A. Evans (Leicester)
on 11 Aug 2016; 19:49 UT
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

Subjects: X-ray, Neutron Star, Transient

Referred to by ATel #: 9677

As a result of the detection of SMC X-3 having entered outburst by S-CUBED (Kennea et al., ATEL #9362) and MAXI (Negoro et al., ATEL #9362), at 22:10UT on August 10, 2016, Swift began a 5ks Target of Opportunity observation. Swift's X-ray Telescope (XRT) performed observations in Windowed Timing (WT) mode, in order to obtain accurate timing and spectroscopy of the source.

Analysis of the WT light-curve shows that the source has an average rate of 13.7 +/- 0.2 count/s, The WT spectrum is well fit by a power-law model with photon index of 1.10 +/- 0.02. The model fitted flux is 8.3 +/- 0.1 x 10-10 erg/s/cm2 (0.5-10 keV), which at a standard SMC distance of 61 kpc relates to an X-ray luminosity of L_X = 3.7 x 1038 erg/s (0.5 - 10 keV), which is equivalent to ~2x Eddington Luminosity of a 1.4 Solar Mass Neutron star.

This appears to be the brightest outburst ever observed for SMC X-3. RXTE archival data showed the source never became brighter than 1037 erg/s (Klus et al., 2014, MNRAS 437, 3863). When first discovered, SMC X-3 was 0.7 x 1038 erg/s (Clark et al., 1978, ApJL, 221, L37).

In Chandra data taken in 2002 (e.g. Edge et al., ATEL #225), SMC X-3 had a pulsar period of 7.781s. We performed a z22 search (Buccheri et al., 1983, A&A, 128, 245) of the barycentrically corrected WT data around this period, and found a significant detected periodicity at P = 7.812749 +/- 0.000012 s (1-sigma error estimated using the Monte-Carlo method of Gotthelf et al, 1999, ApJ, 522, L49.)

The pulse profile is double peaked, with a pulse fraction of 31%. Compared with the Chandra period of 7.781s measured in data taken on July 20, 2002, the pulsar spun down with a period change of 0.032s. Over the time between observations, this gives P_dot = 7.2 x 10-11 s s-1, close to the value of ~ 8 x 10-11 s s-1 measured by Klus et al. (2014) from RXTE monitoring. The spin-down of SMC X-3 between outbursts has been previously noted by Galache et al., 2008, ApJS, 177, 189.

Further monitoring observations of SMC X-3 during this new outburst with Swift/XRT have been approved, and weekly monitoring will continue as part of the S-CUBED program.