SWIFT J004427.3-734801: the identification of the mass donor counterpart in this proposed Be-White Dwarf system
ATel #14161; M. J. Coe (Southampton), I. Monageng (UCT/SAAO), L. J. Townsend (SAAO), M. Orio (INAF-Padova & Wisconsin), D. A.H. Buckley (SAAO), J. A. Kennea (PSU).
on 9 Nov 2020; 14:32 UT
Credential Certification: Malcolm Coe (mjcoe@soton.ac.uk)
Subjects: Optical, X-ray, Binary, Transient
Referred to by ATel #: 14341
In Coe et al 2020 (https://arxiv.org/abs/2005.02891) the newly discovered SMC luminous supersoft X-ray source Swift J004427.3-734801 was proposed to be a Be-white dwarf (WD) system. Binaries hosting a WD with a Be companion of spectral type later than, approximately, B3, are supposed to be very common. It is thought that mass loss and transfer allow even Be star companions of earlier types (e.g. Raguzova 2001, A&A, 367, 848). However these binaries are difficult to detect, and they are very interesting as possible progenitors of Type Ia supernovae.
We observed Swift J004427.3-734801 on 2020-07-20 at 01:46:67 UTC with the Southern African Large Telescope (SALT) using the Robert Stobie Spectrograph under the SALT transient follow-up programme (2018-2-LSP-001). We used a single exposure time of 1200 sec with the PG1300 VPH grating covering a wavelength range 3900 - 6000 Angstroms. The spectrum is dominated by a number of Balmer and helium lines in absorption, with the H-beta line showing weak emission, and is indicative of an early spectral type. The spectrum shows the HeII 4541 and He 4686 lines present, with the HeII 4200 line being weaker than the HeI 4143 line. This constrains the spectral type to a B0 based on the criteria in Evans et al. (2004). Using the distance modulus of the SMC we constrain the luminosity class as a subgiant or main sequence star. In summary, the spectral type of the donor is B0 IV-V.
The presence of such an early type Be star as the mass donor onto a white dwarf presents evolutionary challenges. Its spectral class implies that it is above 16 solar masses, much larger than the canonical ~8-10 solar masses star upper limit of single stars that end as WDs. If the companion is a WD, extremely high mass transfer must have occurred in the binary evolution. The X-ray properties are indeed consistent with a massive WD undergoing recurrent hydrogen burning episodes, possibly during the periastron passage. The only alternative scenario, of a black hole compact object, is much more difficult to reconcile with the large X-ray luminosity and extremely soft X-ray spectrum.
