Einstein Probe Discovery of EP J005245.1−722843: A Rare Be–White Dwarf Binary in the Small Magellanic Cloud?
A. Marino, Haonan Yang, F. Coti Zelati, N. Rea, Sébastien Guillot, Gaurava K. Jaisawal, Chandreyee Maitra, Jan‐Uwe Ness, F. Haberl, E. Kuulkers, Weimin Yuan, H. Feng, Lian Tao, Chichuan Jin, Hui Sun, Wenda Zhang, W. Chen, E. P. J. van den Heuvel, Roberto Soria, Bing Zhang, Shanshan Weng, Long Ji, Guobao Zhang, Xin Pan, Zhipeng Lv, Chongjie Zhang, Z. X. Ling, Yong Chen, S. M. Jia, Yang Liu, Haiyang Cheng, Dongyue Li, Keith C. Gendreau, Mason Ng, Tod E. Strohmayer
Abstract
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 × 10 38 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.