EP250207b is not a collapsar fast X-ray transient. Is it due to a compact object merger?
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.
Cite
@article{arxiv.2508.13039,
title = {EP250207b is not a collapsar fast X-ray transient. Is it due to a compact object merger?},
author = {P. G. Jonker and A. J. Levan and Xing Liu and Dong Xu and Yuan Liu and Xinpeng Xu and An Li and N. Sarin and N. R. Tanvir and G. P. Lamb and M. E. Ravasio and J. Sánchez-Sierras and J. A. Quirola-Vásquez and B. C. Rayson and J. N. D. van Dalen and D. B. Malesani and A. P. C. van Hoof and F. E. Bauer and J. Chacón and S. J. Smartt and A. Martin-Carrillo and G. Corcoran and L. Cotter and A. Rossi and F. Onori and M. Fraser and P. T. O'Brien and R. A. J. Eyles-Ferris and J. Hjorth and T. -W. Chen and G. Leloudas and L. Tomasella and S. Schulze and M. De Pasquale and F. Carotenuto and J. Bright and Chenwei Wang and Shaolin Xiong and Jinpeng Zhang and Wangchen Xue and Jiacong Liu and Chengkui Li and D. Mata Sanchez and M. A. P. Torres},
journal= {arXiv preprint arXiv:2508.13039},
year = {2025}
}
Comments
15 pages, 11 Figures (+2 in an Appendix), accepted for publication in MNRAS