Einstein Probe discovery of EP J005245.1-722843: a rare BeWD binary in the Small Magellanic Cloud?

A. Marino; H. Yang; F. Coti Zelati; N. Rea; S. Guillot; G. K. Jaisawal; C. Maitra; J. -U. Ness; F. Haberl; E. Kuulkers; W. Yuan; H. Feng; L. Tao; C. Jin; H. Sun; W. Zhang; W. Chen; E. P. J. van den Heuvel; R. Soria; B. Zhang; S. -S. Weng; L. Ji; G. B. Zhang; X. Pan; Z. Lv; C. Zhang; Z. Ling; Y. Chen; S. Jia; Y. Liu; H. Q. Cheng; D. Y. Li; K. C. Gendreau; M. Ng; T. E. Strohmayer

arXiv · 2407.21371

Einstein Probe discovery of EP J005245.1-722843: a rare BeWD binary in the Small Magellanic Cloud?

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Abstract

On May 27 2024, the Wide-field X-ray Telescope onboard the Einstein Probe (EP) mission detected enhanced X-ray emission from a new transient source in the Small Magellanic Cloud (SMC) 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$\sim$0.1 keV at the outburst peak) with an X-ray luminosity of $L\sim4\times10^{38}$ erg s$^{-1}$, labelled EP J005245.1-722843. This super-soft outburst faded very quickly in a week 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 SSS 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 days periodicity in the optical band. The discovery of this super-soft 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.

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A. Marino, H. Yang, F. Coti Zelati, N. Rea, S. Guillot, G. K. Jaisawal, C. Maitra, J. -U. Ness, F. Haberl, E. Kuulkers, W. Yuan, H. Feng, L. Tao, C. Jin, H. Sun, W. Zhang, W. Chen, E. P. J. van den Heuvel, R. Soria, B. Zhang, S. -S. Weng, L. Ji, G. B. Zhang, X. Pan, Z. Lv, C. Zhang, Z. Ling, Y. Chen, S. Jia, Y. Liu, H. Q. Cheng, D. Y. Li, K. C. Gendreau, M. Ng, T. E. Strohmayer. 2024-07-31. Einstein Probe discovery of EP J005245.1-722843: a rare BeWD binary in the Small Magellanic Cloud?. https://arxiv.org/abs/2407.21371

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