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

ABSTRACT Fast X-ray transients (FXTs) are short-lived extragalactic X-ray sources. Recent progress through multiwavelength 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 Very Large Telescope/Multi Unit Spectroscopic Explorer 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 $z=0.082$, implying a peak observed absolute magnitude for the optical counterpart of ${M_{\mathrm{ r}^\prime }=-14.5}$. 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 extragalactic transients, including afterglow and kilonova models. The data can be fitted with a slightly off-axis viewing angle afterglow. However, the late-time (${\sim} 30$ d) optical/near-infrared 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 multiwavelength 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 whether the source could originate in a higher redshift host galaxy.

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Publication Details

Journal
Monthly Notices of the Royal Astronomical Society
Published
2025-11-14
DOI
https://doi.org/10.1093/mnras/staf2021
Citations
1
Primary Topic
Nuclear Physics and Applications
Type
article
Field-Weighted Citation Impact
1.88

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article

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

L. Tomasella, M. Fraser, Jesús Chacón, Rob A. J. Eyles-Ferris et al.
1 citations
Monthly Notices of the Royal Astronomical Society
Nuclear Physics and Applications
1.88
article

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

L. Tomasella, M. Fraser, Jesús Chacón, Rob A. J. Eyles-Ferris, P. G. Jonker, F. E. Bauer, A. J. Levan, A. Martín-Carrillo, P. T. O’Brien, F. Onori, D. Malesani, J. Quirola-Vásquez, Laura Cotter, J. Sánchez-Sierras, Xinpeng Xu, Gavin P. Lamb, M. De Pasquale, S. J. Smartt, Steve Schulze, A. De, G. Leloudas, M. E. Ravasio, Nikhil Sarin, Joyce N. D. van Dalen, Agnes P. C. van Hoof, B. C. Rayson, Yuan Liu, T. W. Chen, Xing Liu, A. Rossi, J. Hjorth, N. R. Tanvir, G Corcoran, An Li
article en
1 citations

Abstract

ABSTRACT Fast X-ray transients (FXTs) are short-lived extragalactic X-ray sources. Recent progress through multiwavelength 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 Very Large Telescope/Multi Unit Spectroscopic Explorer 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 $z=0.082$, implying a peak observed absolute magnitude for the optical counterpart of ${M_{\mathrm{ r}^\prime }=-14.5}$. 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 extragalactic transients, including afterglow and kilonova models. The data can be fitted with a slightly off-axis viewing angle afterglow. However, the late-time (${\sim} 30$ d) optical/near-infrared 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 multiwavelength 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 whether the source could originate in a higher redshift host galaxy.

Monthly Notices of the Royal Astronomical SocietyVol. 545(2)
University College Dublin (IE), Northwestern University (US), University of Messina (IT), University of Copenhagen (DK), Queen's University Belfast (GB), Radboud University Nijmegen (NL), University of Leicester (GB), Universidad de La Laguna (ES), Pontificia Universidad Católica de Chile (CL), University of Tarapacá (CL), Chinese Academy of Sciences (CN), National Central University (TW), Beijing Normal University (CN), Astronomical Observatory of Rome (IT), Department of Space (IN), University of Warwick (GB), University of Oxford (GB), Istituto di Astrofisica Spaziale e Fisica Cosmica di Bologna (IT), Breakthrough (US), DAWN Center for Independent Living (US), Institute of Astronomy (RU), Collurania Teramo Observatory (IT), Instituto de Astrofísica de Canarias (ES), Institute of High Energy Physics (CN), Astronomy and Space (AU), Osservatorio Astronomico di Padova (IT), National Astronomical Observatories (CN), University of Chinese Academy of Sciences (CN), Space Research Organisation Netherlands (NL), Liverpool John Moores University (GB), National Institute for Astrophysics (IT)
National Science Foundation, U.S. Department of Energy, Villum Fonden, Gruber Foundation, Leverhulme Trust, European Commission, National Research Foundation, Danmarks Grundforskningsfond, Ministero dell’Istruzione, dell’Università e della Ricerca, Korea Astronomy and Space Science Institute, Ministry of Education, India, Ministério da Ciência, Tecnologia, Inovações e Comunicações, Agencia Nacional de Investigación y Desarrollo, National Science and Technology Council, Science and Technology Facilities Council, Lawrence Berkeley National Laboratory
Openalex Percentile: Top 13%
Nuclear Physics and Applications
1.88
DOIOpen Access PDFOpenAlex
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