Einstein Probe Discovery of EP J005245.1‑722843: A Rare Be–White Dwarf Binary in the Small Magellanic Cloud?
Liu, Y.; Haberl, F.; Ness, J. -U.; Chen, Y.; Rea, N.; Soria, R.; Tao, L.; Coti Zelati, F.; Kuulkers, E.; Jin, C.; Zhang, B.; Marino, A.; Maitra, C.; Guillot, S.; Yuan, W.; van den Heuvel, E. P. J.; Ji, L.; Chen, W.; Zhang, W.; Gendreau, K.; Feng, H.; Strohmayer, T.; Zhang, C.; Jaisawal, G. K.; Ng, M.; Yang, H. N.; Sun, H.; Weng, S. -S.; Zhang, G. B.; Pan, X.; Lv, Z.; Ling, Z. X.; Jia, S.; Cheng, H. Q.; Li, D. Y.
Spain, China, Germany, France, Denmark, Netherlands, Italy, Australia, United States
Abstract
On 2024 May 27, the Wide-field X-ray Telescope on board the Space Sciences, University of Chinese Academy of Einstein Probe (EP) mission detected enhanced X-ray emission from a new transient source in the Small Magellanic Cloud during its commissioning phase. Prompt follow-up with the EP Follow-up X-ray Telescope, the Swift X-ray Telescope. and NICER have revealed a very soft, thermally emitting source (kT ~ 0.1 keV at the outburst peak) with an X-ray luminosity of L ~ 4 × 1038 erg s‑1, labeled EP J005245.1‑722843. This supersoft outburst faded very quickly in a week's time. Several emission lines and absorption edges were present in the X-ray spectrum, including deep nitrogen (0.67 keV) and oxygen (0.87 keV) absorption edges. The X-ray emission resembles the supersoft source phase of typical nova outbursts from an accreting white dwarf (WD) in a binary system, despite the X-ray source being historically associated with an O9-B0e massive star exhibiting a 17.55 day periodicity in the optical band. The discovery of this supersoft outburst suggests that EP J005245.1‑722843 is a BeWD X-ray binary: an elusive evolutionary stage where two main-sequence massive stars have undergone a common envelope phase and experienced at least two episodes of mass transfer. In addition, the very short duration of the outburst and the presence of Ne features hint at a rather massive, i.e., close to the Chandrasekhar limit, Ne–O WD in the system.