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arxiv: 2606.06213 · v1 · pith:G2SJ5S7Gnew · submitted 2026-06-04 · 🌌 astro-ph.HE

Probing a new subclass of llGRB-SN transients: Insights from EP250304a and its associated supernova

L. Cotter , A. Martin-Carrillo , R. A. J. Eyles-Ferris , L. Izzo , D. B. Malesani , Y. Julakanti , G. Corcoran , A. Saccardi
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P. G. Jonker A. J. Levan F. Carotenuto P. T. O'Brien J. H. Gillanders J. N. D. van Dalen M. E. Ravasio S. Schulze N. Sarin F. E. Bauer M. Fraser J. Quirola-Vasquez A. P. C. van Hoof S. J. Smartt C. Gall A. Rest C. T. Murphey N. Tanvir T.-W. Chen S. Campana C. Ashall J. P. Anderson J. A. Chacon F. J. Cowie V. D'Elia L. Galbany C. P. Gutierrez D. H. Hartmann P. Jakobsson S. Kobayashi A. H. Kong P. Mazalli T. E. Muller-Bravo M. De Pasquale L. Rhodes A. Rossi J. Sanchez-Sierras J. Sollerman A. Andersson A. Aryan T. de Boer J. S. Bright K. C. Chambers M. Gromadzki M. E. Huber C. Inserra T. Lowe P. Minguez G. S. Narayan M. Nicholl G. S. H. Paek A. Sedgewick K. W. Smith J. W. Tweddle S. Yang
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classification 🌌 astro-ph.HE
keywords ep250304alow-luminosityassociatedconsistentemissioneventgamma-rayllgrb-sne
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With the advent of the Einstein Probe (EP) mission, we are entering a new era in the study of gamma-ray bursts (GRBs), enabling the detection of faint, low-luminosity transients that would previously have gone undetected. EP250304a was an event discovered by EP associated with the broad-lined type Ic supernova (SN) SN 2025fhm located at z = 0.2. Despite no gamma-ray emission being detected at the time of the EP trigger, we identify evidence for a relativistic outflow consistent with a GRB-like jet across multiple wavelengths. We present a detailed spectral and photometric analysis of EP250304a/SN 2025fhm, including multi-band light curve modelling performed with the Redback Python package. We find that this event closely resembles low-luminosity GRB-SNe (llGRB-SNe) such as GRB 060218/SN 2006aj, GRB 100316D/SN 2010bh, and GRB 171205A/SN 2017iuk, all of which exhibit early-time emission consistent with a thermal shocked cocoon. These similarities suggest that EP250304A/SN 2025fhm may belong to an emerging subclass of shocked cocoon-dominated llGRB-SNe, representing the low-luminosity end of a broader continuum of engine-driven GRB-SN explosions.

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