SXP 304: S-CUBED detection of an outburst, possible orbital period
ATel #12236; J. A. Kennea (PSU), M. J. Coe (Southampton) and P. A. Evans (Leicester)
on 21 Nov 2018; 22:51 UT
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)
Subjects: X-ray, Neutron Star
S-CUBED, the Swift Magellanic Cloud survey, performs a shallow (60s exposure) 142 tile exposure of the optical extent of the SMC, to search for transient and regular outbursts from the large population of Be/X-ray binaries in the SMC (Kennea et al., 2018, ApJ, 868, 47).
Long term monitoring of the source SXP 304 by S-CUBED has detected two outburst from this source. During the first outburst, the source was detected by S-CUBED between MJD 57736.97 - 57764.57 (over 27.6 days starting Dec 14th, 2016), peaking at a flux of (1.0 +/- 0.4) x 10^-11 erg/s/cm^2 (0.5 - 10 keV). The second outburst is currently ongoing and was first detected on MJD 58407.54 (Oct 16th, 2018) peaking at (1.5 +/- 0.5) x 10^-11 erg/s/cm^2 (0.5 - 10 keV) during the most recent observation on MJD 58442.34 (Nov 20th, 2018). Outside of these two outbursts, no X-rays have been detected from SXP 304.
The reported optically derived orbital period of SXP 304 is P = 520 +/- 12 days (Schmidtke and Cowley, 2006, AJ,132, 919), although the detection of that period was weak. Subsequently Bird et al (2012, MNRAS 423, 366) failed to find any convincing evidence for a binary period in the OGLE III & IV data sets.
If the two outbursts seen by S-CUBED are due to Type-1 outbursts (i.e. the periastron passage of the Neutron Star), then this would be incompatible with the purported 520 day period. The two outbursts are separated by ~662 days, and we suggest this as a candidate orbital period of SXP 304.
