Abstract
ABSTRACT We report on the discovery of Swift J004516.6–734703, a Be/X-ray binary system by the Swift SMC Survey, S-CUBED. Swift J004516.6–734703, or SXP 146.6, was found to be exhibiting a bright (∼1037 erg s−1) X-ray outburst on 2020 June 18. The historical UV and IR light-curves from OGLE and Swift/UVOT showed that after a long period of steady brightness, it experienced a significant brightening beginning around 2019 March. This IR/UV rise is likely the signature of the formation of a circumstellar disc, confirmed by the presence of strong an H α line in SALT spectroscopy, that was not previously present. Periodicity analysis of the OGLE data reveals a plausible 426 d binary period, and in X-ray a pulsation period of 146.6 s is detected. The onset of X-ray emission from Swift J004516.6–734703 is likely the signature of a Type-I outburst from the first periastron passage of the neutron star companion through the newly formed circumstellar disc. We note that the formation of the circumstellar disc began at the predicted time of the previous periastron passage, suggesting its formation was spurred by tidal interaction with the neutron star.
Highlights
Be/X-ray Binaries (BeXRBs) consist of a Be star, a massive typically main-sequence star with B spectral type showing strong Balmer and other emission lines (Porter & Rivinius 2003), and a compact object, which is most often a neutron star (NS), but sometimes a white dwarf (WD) (Coe et al 2020) or a black hole (Munar-Adrover et al 2014)
The S-CUBED survey, a weekly survey of the Small Magellanic Cloud (SMC) in X-ray and UV performed by Swift, discovered a previously unknown X-ray transient in the SMC, Swift J004516.6−734703, with a pulsar period of 146.6 s
Examination of historical and catalogued data on this star revealed that it has a B-type optical companion, and post-outburst observations with South African Large Telescope (SALT) revealed characteristic Balmer and other line emission in the spectrum, conclusively showing that this system was a BeXRB
Summary
Be/X-ray Binaries (BeXRBs) consist of a Be star, a massive typically main-sequence star with B spectral type showing strong Balmer and other emission lines (Porter & Rivinius 2003), and a compact object, which is most often a neutron star (NS), but sometimes a white dwarf (WD) (Coe et al 2020) or a black hole (Munar-Adrover et al 2014). Less frequently BeXRBs show large, sometimes super-Eddington, outbursts (e.g. as in the recent outburst of SMC X-3, Townsend et al 2017), which can last for several orbital periods (Reig 2011) Given the low background of the XRT, S-CUBED is sensitive to outbursts of accreting pulsars of > 1 − 2% Eddington luminosity This makes S-CUBED a powerful tool for both early detection of outbursts and regular monitoring of BeXRB sources. We present results from both long term monitoring of this new source with S-CUBED data from the Swift XRT and UVOT, along with post outburst followup observations with Swift and the South African Large Telescope (SALT; Buckley et al 2006). Discussions regarding the apparent onset of Be activity, and the later onset of X-ray emission are presented
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