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, vol. 545
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.
Authors 26
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Radboud University Nijmegen · Space Research Organisation Netherlands
Affiliation as printed
Radboud University Department of Astrophysics/IMAPP, , NL-6525 AJ Nijmegen ,
SRON, Netherlands Institute for Space Research , Niels Bohrweg 4, NL-2333 CA Leiden ,
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Radboud University Nijmegen · University of Warwick
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Radboud University Department of Astrophysics/IMAPP, , NL-6525 AJ Nijmegen ,
University of Warwick Department of Physics, , Gibbet Hill Road, Coventry CV4 7AL ,
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Beijing Normal University , No. 19, Xinjiekouwai St, Haidian District, Beijing 100875 ,
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Institute of Astronomy, University of Cambridge , Madingley Road, CB3 0HA , Cambridge,
Kavli Institute for Cosmology, University of Cambridge , Madingley Road, CB3 0HA , Cambridge,
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School of Physics and Astronomy, University of Leicester , University Road, LE1 7RH , Leicester,
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Liverpool John Moores University
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Astrophysics Research Institute, Liverpool John Moores University , IC2 Liverpool Science Park, 146 Brownlow Hill, Liverpool L3 5RF ,
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Affiliation as printed
Radboud University Department of Astrophysics/IMAPP, , NL-6525 AJ Nijmegen ,
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Affiliation as printed
Radboud University Department of Astrophysics/IMAPP, , NL-6525 AJ Nijmegen ,
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Affiliation as printed
Radboud University Department of Astrophysics/IMAPP, , NL-6525 AJ Nijmegen ,
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Affiliation as printed
School of Physics and Astronomy, University of Leicester , University Road, LE1 7RH , Leicester,
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Affiliation as printed
Radboud University Department of Astrophysics/IMAPP, , NL-6525 AJ Nijmegen ,
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University of Copenhagen · Radboud University Nijmegen
Affiliation as printed
Cosmic Dawn Center (DAWN) ,
Niels Bohr Institute, University of Copenhagen , Jagtvej 128, DK-2200 Copenhagen N ,
Radboud University Department of Astrophysics/IMAPP, , NL-6525 AJ Nijmegen ,
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Affiliation as printed
Radboud University Department of Astrophysics/IMAPP, , NL-6525 AJ Nijmegen ,
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Affiliation as printed
Instituto de Alta Investigación, Universidad de Tarapacá , Casilla 7D, 1010000, Arica ,
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Queen's University Belfast · University of Oxford
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Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University , Belfast BT7 1NN ,
University of Oxford Department of Physics, , Keble Road, Oxford OX1 3RH ,
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School of Physics and Centre for Space Research, University College Dublin , Belfield, Dublin 4, D04 P7W1 ,
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School of Physics and Centre for Space Research, University College Dublin , Belfield, Dublin 4, D04 P7W1 ,
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INAF-Osservatorio Astronomico d’Abruzzo , via M. Maggini snc, I-64100 Teramo ,
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School of Physics and Centre for Space Research, University College Dublin , Belfield, Dublin 4, D04 P7W1 ,
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School of Physics and Astronomy, University of Leicester , University Road, LE1 7RH , Leicester,
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Affiliation as printed
School of Physics and Astronomy, University of Leicester , University Road, LE1 7RH , Leicester,
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DARK, Niels Bohr Institute, University of Copenhagen , Jagtvej 155A, DK-2200 Copenhagen ,
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DTU Space · Technical University of Denmark
Affiliation as printed
Technical University of Denmark DTU Space, Department of Space Research and Space Technology, , Elektrovej 327, DK-2800 Kgs. Lyngby ,
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Osservatorio Astronomico di Padova · National Institute for Astrophysics
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Istituto Nazionale di Astrofisica INAF Osservatorio Astronomico di Padova via dell’Osservatorio 8 , I-36012 Asiago ,
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Affiliation as printed
Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), Northwestern University , 1800 Sherman Ave., Evanston, IL 60201 ,
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University of Messina MIFT Department, , via F. S. D’Alcontres 31, I-98166 Messina ,
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