Abstract
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 2020 January. It was not detected in any of the previous 3 yr worth of observations. The accurate positional determination from the X-ray data has permitted an optical counterpart to be identified that has the characteristics of an O9V−B2III star. Evidence for the presence of an infrared excess and significant I-band variability strongly suggests that this is an OBe-type star. Over 17 yr worth of optical monitoring by the OGLE (Optical Gravitational Lensing Experiment) project reveals periods of time in which quasi-periodic optical flares occur at intervals of ∼21.5 d. 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 is a rarely identified Be star–white dwarf binary in the SMC.
Highlights
It has been predicted for a while, from binary system evolutionary models, that there should be a large number of Be/white dwarf (BeWD) systems compared to the number of Be/neutron (BeNS) star systems - see, for example Raguzova (2001) who predicts 7 times more BeWD systems that BeNS systems
It is possible that some observational constraints may be making such BeWD systems harder to detect, For example, the optical channel offers no obvious route to separating BeWD and BeNS systems
4 CONCLUSIONS Reported in this work is one of the few systems proposed as a White Dwarf (WD) with a massive OB star companion in the Small Magellanic Cloud (SMC)
Summary
It has been predicted for a while, from binary system evolutionary models, that there should be a large number of Be/white dwarf (BeWD) systems compared to the number of Be/neutron (BeNS) star systems - see, for example Raguzova (2001) who predicts 7 times more BeWD systems that BeNS systems. In the X-ray a possible distinguishing features may be very high luminosity above 1036 erg/s in the soft X-ray range (below 0.8 keV) indicating a nuclear burning white dwarf - that of a very soft spectrum for a BeWD compared to that of a BeNS. Such SSS have been long identified as a characteristic of accretion and. In addition historical optical photometric data are reported here from the OGLE project showing an occasional, quasi-periodic, rapid flaring behaviour of the counterpart over the last ∼17 years This behaviour is almost certainly related to a binary period of 21d. One of a handful in the Magellanic Clouds as a whole
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