Swift J004427.3-734801- a probable Be/white dwarf system in the Small Magellanic Cloud

Abstract

Swift J004427.3-734801 is an X-ray source in the Small Magellanic Cloud (SMC) that was first discovered as part of the Swift S-CUBED programme in January 2020. It was not detected in any of the previous 3 years worth of observations. The accurate positional determination from the X-ray data have permitted an optical counterpart to be identified which has the characteristics of an O9V-B2III star. Evidence for the presence of an IR excess and significant I-band variability strongly suggest that this is an OBe type star. Over 17 years worth of optical monitoring by the OGLE project reveal periods of time in which quasi-periodic optical flares occur at intervals of ~21.5d. The X-ray data obtained from the S-CUBED project reveal a very soft spectrum, too soft to be that from accretion on to a neutron star or black hole. It is suggested here that this a rarely identified Be star-white dwarf binary in the SMC.

arXiv comment6 Pages, 8 Figures. Accepted by MNRAS 2020 June 8. Received 2020 June 5; in original form 2020 April 28

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• The CO White Dwarf + Intermediate Mass/Massive Star Binary Evolution: Possible Merger Origins for Peculiar Type Ia and II Supernovae
• Formation and Destiny of White Dwarf and Be Star Binaries
• eROSITA calibration and performance verification phase: High-mass X-ray binaries in the Magellanic Clouds
• Formation and Evolution of Accreting Compact Objects
• Populations of evolved massive binary stars in the Small Magellanic Cloud II: Predictions from rapid binary evolution
• The newly discovered Be/X-ray binary Swift J004516.6-734703 in the SMC: witnessing the emergence of a circumstellar disc
• Disentangling the neighbouring pulsars SXP 15.3 and SXP 305
• A stringent upper limit on Be star fractions produced by binary interaction
• X-ray emission mechanisms in accreting white dwarfs