Swift J011511.0-725611: S-CUBED detection of a possible Be/WD Binary in the SMC
ATel #14341; J. A. Kennea (Penn State), M. J. Coe (Southampton), P. A. Evans (Leicester), L. J. Townsend, Francois Van Wyk (SAAO) and A. Udalski (Warsaw).
on 21 Jan 2021; 15:43 UT
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)
Subjects: X-ray, Binary, Transient
We report the discovery of a new X-ray transient, Swift J011511.0-725611, in the Small Magellanic Cloud by the Swift SMC Survey, S-CUBED. S-CUBED is a weekly shallow X-ray and UV survey of the optical extent of the SMC utilizing the X-ray telescope and UV/Optical Telescope onboard NASA's Neil Gehrels Swift Explorer (e.g. Kennea et al. 2018).
S-CUBED has been performing weekly scans of the SMC since June 2016, however Swift J011511.0-725611 was first detected on December 29th, 2020. No previous detection of this point source has been reported. Initial brightness was found to be 0.05 (+0.07/-0.03) count/s in the short S-CUBED exposure, rising to 0.15(+0.10/-0.07) count/s and 0.19(+0.08/-0.06) count/s on Jan 5 2021 and Jan 12 2021 respectively. Target of Opportunity observations were requested, and observations between 2-4 ks were taken between Jan 13-17 2021. TOO observations show some evidence of short term variability, but no further brightening.
Utilizing these deeper target-of-opportunity observation of the new transient, we derived an accurate localization of the source, RA/Dec(J2000) = 18.79563, -72.93624, which is equivalent to
RA(J2000) = 01h 15m 10.95s,
Dec(J2000) = -72d 56m 10.5s,
with an error radius of 1.6 arc-seconds (90% confidence).
We note that this position is consistent with the location of a known star, [M2002] SMC 75919. This optical counterpart was observed at the SAAO 1.9m telescope on 18 January 2021 and the spectrum confirms the characteristics reported by Golden-Marx et al (2016), namely an early type star with infilled Balmer lines. Golden-Marx (2016) proposes a spectral type O9IIIe and Lamb et al (2013) propose O8-O9Ve.
The X-ray spectrum of this new transient is soft, in contrast to the hard spectra seen in most Be/X-ray binaries in the SMC, strongly implying a white dwarf compact object instead of a neutron star. Assuming a standard SMC absorption of 6 x 10^20 cm^-2, the spectrum is well fit by a black-body model with kT = 128 +/- 5 eV, although we note that the fit is significantly improved by the inclusion of a 0.87 keV edge, consistent with O VIII (Hydrogen like Oxygen).
Analysis of the OGLE III and IV data of the optical counterpart reveals a strong modulation at a period of 17.39d. We propose that this is the binary period of the system.
We note the similarity of this source to Swift J004427.3-734801, another proposed Be/WD system in the SMC discovered by S-CUBE (e.g. Coe et al., MNRAS, 2020, 497, 50). That source has a similarly soft X-ray spectrum and early type companion star (B0 IV-V; Coe et al., ATEL #14161). It also has a similar binary period of 21.5d.
We therefore suggest Swift J011511.0-725611 is also a Be/WD system, with very similar characteristics to Swift J004427.3-734801.
Observations with Swift and S-CUBED will continue, after a short period of constraint.
