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arXiv · 2508.13039

EP250207b is not a collapsar fast X-ray transient. Is it due to a compact object merger?

P. G. Jonker·A. J. Levan·Xing Liu·Dong Xu·Yuan Liu·Xinpeng Xu·An Li·N. Sarin·N. R. Tanvir·G. P. Lamb·M. E. Ravasio·J. Sánchez-Sierras·J. A. Quirola-Vásquez·B. C. Rayson·J. N. D. van Dalen·D. B. Malesani·A. P. C. van Hoof·F. E. Bauer·J. Chacón·S. J. Smartt·A. Martin-Carrillo·G. Corcoran·L. Cotter·A. Rossi·F. Onori·M. Fraser·P. T. O'Brien·R. A. J. Eyles-Ferris·J. Hjorth·T. -W. Chen·G. Leloudas·L. Tomasella·S. Schulze·M. De Pasquale·F. Carotenuto·J. Bright·Chenwei Wang·Shaolin Xiong·Jinpeng Zhang·Wangchen Xue·Jiacong Liu·Chengkui Li·D. Mata Sanchez·M. A. P. Torres
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Abstract

Fast X-ray Transients (FXTs) are short-lived extra-galactic X-ray sources. Recent progress through multi-wavelength follow-up of Einstein Probe discovered FXTs has shown that several are related to collapsars, which can also produce gamma-ray bursts (GRBs). In this paper we investigate the nature of the FXT EP250207b. The VLT/MUSE spectra of a nearby (15.9 kpc in projection) lenticular galaxy reveal no signs of recent star formation. If this galaxy is indeed the host, EP250207b lies at a redshift of z=0.082, implying a peak observed absolute magnitude for the optical counterpart of M_r=-14.5. At the time when supernovae (SNe) would peak, it is substantially fainter than all SN types. These results are inconsistent with a collapsar origin for EP250207b. The properties favour a binary compact object merger driven origin. The X-ray, optical and radio observations are compared with predictions of several types of extra-galactic transients, including afterglow and kilonova models. The data can be fit with a slightly off-axis viewing angle afterglow. However, the late-time (~30 day) optical/NIR counterpart is too bright for the afterglow and also for conventional kilonova models. This could be remedied if that late emission is due to a globular cluster or the core of a (tidally disrupted) dwarf galaxy. If confirmed, this would be the first case where the multi-wavelength properties of an FXT are found to be consistent with a compact object merger origin, increasing the parallels between FXTs and GRBs. We finally discuss if the source could originate in a higher redshift host galaxy.

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P. G. Jonker, A. J. Levan, Xing Liu, Dong Xu, Yuan Liu, Xinpeng Xu, An Li, N. Sarin, N. R. Tanvir, G. P. Lamb, M. E. Ravasio, J. Sánchez-Sierras, J. A. Quirola-Vásquez, B. C. Rayson, J. N. D. van Dalen, D. B. Malesani, A. P. C. van Hoof, F. E. Bauer, J. Chacón, S. J. Smartt, A. Martin-Carrillo, G. Corcoran, L. Cotter, A. Rossi, F. Onori, M. Fraser, P. T. O'Brien, R. A. J. Eyles-Ferris, J. Hjorth, T. -W. Chen, G. Leloudas, L. Tomasella, S. Schulze, M. De Pasquale, F. Carotenuto, J. Bright, Chenwei Wang, Shaolin Xiong, Jinpeng Zhang, Wangchen Xue, Jiacong Liu, Chengkui Li, D. Mata Sanchez, M. A. P. Torres. 2025-11-14. EP250207b is not a collapsar fast X-ray transient. Is it due to a compact object merger?. https://doi.org/10.1093/mnras%2Fstaf2021

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