SXP 15.6: New outburst detection by S-CUBED, with NICER and SALT followup

ATel #15054; M. J. Coe (Southampton), P. A. Evans (Leicester), J. A. Kennea (PSU), Keith Gendreau, Zaven Arzoumanian (NASA/GSFC), Elizabeth Ferrara (NASA/GSFC/UMD), David Buckley (SAAO), Itumeleng Monageng (UCT/SAAO)
on 21 Nov 2021; 19:38 UT
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

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We report on the detection, by S-CUBED, of an outburst of the SMC Be/XRB SXP 15.6, AKA XMM J004855.5-734946 (ATEL #9197, #9229). S-CUBED is a weekly survey of the optical extent of the SMC with the Swift X-ray and UV/Optical Telescopes (Kennea et al., 2018). On November 9th, 2021 observation, SXP 15.6 was found to be at an XRT count rate of 0.43 +/- 0.10 count/s. The following week on Nov 16th observation, S-CUBED detected the source at a rate of 0.45 +/-0.15 count/s. In S-CUBED monitoring, which began June 2016, SXP 15.6 had never been detected at a brightness above 0.25 count/s, making this a significant outburst.

We requested and obtained a 4ks Swift Target-of-opportunity observation. Observations began at 15:44UT on Nov 19, 2021. Analysis of the XRT data finds the source at an average count rate of 0.70 +/- 0.03 count/s. The spectrum is well fit by a power-law model with a photon index = 0.88 +/- 0.10, with a N_H fixed at the SMC value of 6x10^20 cm^-2. The measured flux is 4.5 x 10^-11 erg/s (0.3-10 keV), which is equivalent to a luminosity of 2 x 10^37 erg/s (0.3 - 10 keV) assuming a SMC distance of 62 kpc.

We also requested a NICER TOO for SXP 15.6, and beginning 02:04UT on Nov 20, 2021, a 7.3ks observation was taken. Analysis of the NICER observation reveals the presence of a significant detection of a pulsar period at 15.64034s. This pulsar period is also weakly detected in the Swift/XRT data. We note that this pulsar period is the same as that reported in 2016 observations by Vasilopoulos et al (2016), suggesting no strong evolution of the spin period in the last 5 years.

Further NICER observations have been requested in order to monitor the flux and pulsar period evolution during this new outburst. Due to spacecraft constraints Swift monitoring will not be possible for the near future.

In addition we observed the optical counterpart of SXP 15.6 on Nov 19, 2021 at 19:51:45 UT with the Southern African Large Telescope (SALT) under the SALT transient follow-up programme (2021-2-LSP-001). The Robert Stobie Spectrograph was used with a PG2300 grating at an exposure time of 1200 secs covering the wavelength range 6090 - 6920 AA at a resolution of 1.5A. The H-alpha line is seen strongly in emission with a single peak morphology plus a hint of a red shoulder. The measured equivalent width was -8.2 +/- 0.3 Angstrom. This is compared to a historical measurement in December 2011 from ESO/NTT of -5.2 +/- 0.2 Angstrom. Further observations are scheduled with SALT to trace the evolution of the H-alpha emission line.