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

Jonker, PG, Levan, AJ, Liu, X, Xu, D, Liu, Y, Xu, X ORCID: 0000-0001-5917-9031, Li, A, Sarin, N ORCID: 0000-0003-2700-1030, Tanvir, NR, Lamb, GP ORCID: 0000-0001-5169-4143, Ravasio, ME, Sánchez-Sierras, J, Quirola-Vásquez, JA, Rayson, BC, van Dalen, JND, Malesani, DB, van Hoof, APC, Bauer, FE, Chacón, J, Smartt, SJ ORCID: 0000-0002-8229-1731 et al EP250207b is not a collapsar fast X-ray transient. Is it due to a binary compact object merger? Monthly Notices of the Royal Astronomical Society. ISSN 0035-8711 (Accepted)

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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 γ-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 $\mathrm{M{r^\prime }=-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.

Item Type:	Article
Additional Information:	This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society Published by Oxford University Press
Uncontrolled Keywords:	0201 Astronomical and Space Sciences; Astronomy & Astrophysics; 5101 Astronomical sciences; 5107 Particle and high energy physics; 5109 Space sciences
Subjects:	Q Science > QB Astronomy Q Science > QC Physics
Divisions:	Astrophysics Research Institute
Publisher:	Oxford University Press (OUP)
Date of acceptance:	14 November 2025
Date of first compliant Open Access:	17 November 2025
Date Deposited:	17 Nov 2025 15:11
Last Modified:	17 Nov 2025 15:30
DOI or ID number:	10.1093/mnras/staf2021
URI:	https://researchonline.ljmu.ac.uk/id/eprint/27581

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