CXOU J005245.0-722844: discovery of a Be star/white dwarf binary system in the SMC via a very fast, super-Eddington X-ray outburst event

CXOU J005245.0-722844 is an X-ray source in the Small Magellanic Cloud (SMC) that has long been known as a Be/X-ray binary (BeXRB) star, containing an OBe main-sequence star and a compact object. In this paper, we report on a new very fast X-ray outburst from CXOU J005245.0-722844. X-ray observations taken by Swift constrain the duration of the outburst to less than 16 d and find that the source reached super-Eddington X-ray luminosities during the initial phases of the eruption. The XRT spectrum of CXOU J005245.0-722844 during this outburst reveals a supersoft X-ray source, best fit by an absorbed thermal blackbody model. Optical and ultraviolet follow-up observations from the Optical Gravitational Lensing Experiment (OGLE), Asteroid Terrestrial-impact Last Alert System (ATLAS), and Swift identify a brief $\sim$0.5 mag optical burst coincident with the X-ray outburst that lasted for less than 7 d. Optical photometry additionally identifies the orbital period of the system to be 17.55 d and identifies a shortening of the period to 17.14 d in the years leading up to the outburst. Optical spectroscopy from the Southern African Large Telescope (SALT) confirms that the optical companion is an early-type OBe star. We conclude from our observations that the compact object in this system is a white dwarf (WD), making this the seventh candidate Be/WD X-ray binary. The X-ray outburst is found to be the result of a very-fast, ultraluminous nova similar to the outburst of MAXI J0158-744.