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SWIFT J004427.3-734801 : discovery of a new Be/X-ray binary system in the Small Magellanic Cloud

ATel #13626; M. J. Coe (Southampton), J. A. Kennea (PSU), P. Evans (Leicester), A. Udalski (Warsaw)
on 10 Apr 2020; 13:03 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Malcolm Coe (mjcoe@soton.ac.uk)

Subjects: Optical, X-ray, Binary, Neutron Star, Star, Transient, Variables

Referred to by ATel #: 13709, 13714

The source SWIFT J004427.3-734801 has been detected by the S-CUBE SMC monitoring project (Kennea et al, 2018) since 22 January 2020, but not in any of the previous 3 years of monitoring. It is currently being detected at a flux of approx. 0.1 counts/s which indicates a luminosity of 3E-36 erg/s for a source in the SMC (assuming the standard interstellar extinction to the SMC and a power law model with a photon index of 2.0).

2MASS J00442806-7348031 lies 3.8 arcsec from the best S-CUBE position which has an uncertainty of 4.9 arcsec (90% uncertainty radius) and is the very likely optical counterpart with V=15.0. The 2MASS source is identified as having the colours of an OB star in the catalogue of Massey et al (2002).

OGLE III & IV data reveal a long term I band modulations around 0.2 mags and a 20 year peak brightness of I=14.82 around 3 years ago. Since then it has faded to a recent value of I= 14.92. Timing studies of the OGLE data reveal a clear modulation with a period of 21.4d which we infer is the orbital period of the system. Interestingly the OGLE data exhibit strong evidence for a sharp flare once per orbit.

Thus we propose this system as the new discovery of a Be/X-ray binary system in the SMC with a binary period of 21d, and exhibiting sharp flares in the light from the OB star probably produced by the periastron passage of the assumed neutron star companion.

More detailed monitoring observations of this system by Swift are underway.