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arXiv · 2606.06213

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·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·Q. Wu·Y. Yin·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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Abstract

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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L. Cotter, A. Martin-Carrillo, R. A. J. Eyles-Ferris, L. Izzo, D. B. Malesani, Y. Julakanti, G. Corcoran, A. Saccardi, 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, Q. Wu, Y. Yin, 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. 2026-06-04. Probing a new subclass of llGRB-SN transients: Insights from EP250304a and its associated supernova. https://arxiv.org/abs/2606.06213

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