Abstract
The explosion of a star is first marked by the shock wave breaking out of the stellar surface, producing a burst of ultraviolet and X-ray radiation. These events are observationally rare, despite likely accompanying the majority of supernovae (SNe). Here, we report on our multiwavelength observing campaign of the closest Einstein Probe fast X-ray transient (FXT) EP260321a at z = 0.0344. The thermal (kT = 130 eV) X-ray emission with peak luminosity of 1.0 × 10 45 erg s −1 points to a shock breakout origin. We demonstrate that EP260321a is accompanied by a broad-lined Type Ic supernova, SN 2026gzf. The supernova (SN) properties, including its spectral evolution, lightcurve evolution, and expansion velocities, are all typical of the energetic stripped-envelope SNe associated with gamma-ray bursts (GRBs). However, deep X-ray upper limits obtained with the Chandra X-ray Observatory do not detect an X-ray afterglow, and instead exclude the afterglow of known GRBs or FXTs. If the stellar explosion launched a successful relativistic jet, we require that it had both a low Lorentz factor of Γ 0 < 30 and a kinetic energy of E kin < 10 49 erg for a stellar wind density of A * ≳ 1. We propose that EP260321a originated from a mildly relativistic, weak outflow that was choked by the progenitor star. This scenario is capable of naturally explaining its low X-ray luminosity and lack of prompt gamma-ray emission. EP260321a bridges the gap between SN 2008D and low-luminosity GRBs, suggesting a greater diversity in the physical parameters of stripped stars as they undergo terminal collapse.
| Original language | English |
|---|---|
| Article number | L13 |
| Number of pages | 26 |
| Journal | The Astrophysical Journal Letters |
| Volume | 1006 |
| Issue number | 1 |
| Early online date | 14 Jul 2026 |
| DOIs | |
| Publication status | Published - 20 Jul 2026 |
Acknowledgements
The authors acknowledge the anonymous referee for their careful reading of the manuscript and for their constructive comments that led to a clearer presentation of our results. B.O. and X.J.H. thank Moses Mogotsi, Lee Townsend, and Danièl Groenewald for approving a SALT DDT request and for assistance in obtaining the observations. B.O. acknowledges the staff of the CXO, including Pat Slane, Dan Schwartz, Jack Steiner, and Vinay Kashyap, for approving and rapidly scheduling the Chandra observations. Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or the European Research Council Executive Agency. Neither the European Union nor the granting authority can be held responsible for them. This work used resources on the Vera Cluster at the Pittsburgh Supercomputing Center (PSC). Vera is a dedicated cluster for the McWilliams Center for Cosmology and Astrophysics at Carnegie Mellon University. We thank the PSC staff for their support of the Vera Cluster. The scientific results reported in this article are based on observations made by CXO. This research has made use of data obtained from the Chandra Data Archive provided by CXC. This Letter employs a list of Chandra datasets, obtained by the CXO, contained in the Chandra Data Collection DOI: 10.25574/cdc.617. This research has made use of software provided by CXC in the application package CIAO. This Letter contains data obtained at the Wendelstein Observatory of the Ludwig-Maximilians University Munich. Some of the observations reported in this Letter were obtained with SALT. This material is based upon work supported by the US Department of Energy (DOE), Office of Science, Office of HighEnergy Physics, under Contract No. DE-AC02–05CH11231, and by the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility under the same contract. The authors are honored to be permitted to conduct scientific research on I’oligam Du’ag (Kitt Peak), a mountain with particular significance to the Tohono O’odham Nation. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the US NSF, the US DOE, or any of the listed funding agencies. This material is based upon work supported in part by the National Science Foundation through Cooperative Agreements AST-1258333 and AST-2241526 and Cooperative Support Agreements AST-1202910 and 2211468 managed by the Association of Universities for Research in Astronomy (AURA), and the Department of Energy under Contract No. DE-AC02-76SF00515 with the SLAC National Accelerator Laboratory managed by Stanford University. Based on observations obtained with the Samuel Oschin Telescope 48 inch and the 60 inch Telescope at the Palomar Observatory as part of the ZTF project. ZTF is supported by the NSF under grant Nos. AST-1440341, AST-2034437, and currently Award AST-2407588. ZTF receives additional funding from the ZTF partnership. Current members include Caltech, USA; Caltech/IPAC, USA; University of Maryland, USA; University of California, Berkeley, USA; University of Wisconsin at Milwaukee, USA; Cornell University, USA; Drexel University, USA; University of North Carolina at Chapel Hill, USA; Institute of Science and Technology, Austria; National Central University, Taiwan, and OKC, University of Stockholm, Sweden. Operations are conducted by Caltech’s Optical Observatory (COO), Caltech/IPAC, and the University of Washington at Seattle, USA. The Babamul alerts broker and BOOM software infrastructure (T. Jegou du Laz et al. 2026) is codeveloped by the California Institute of Technology and the University of Minnesota. This research is based on data obtained from the Astro Data Archive at NSF NOIRLab. NOIRLab is managed by AURA under a cooperative agreement with the US National Science Foundation. This project used data obtained with the DECam, which was constructed by the DES Collaboration. Funding for the DES projects has been provided by the US DOE, the US NSF, the Ministry of Science and Education of Spain, the Science and Technology Facilities Council of the United Kingdom, the Higher Education Funding Council for England, the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign, the Kavli Institute of Cosmological Physics at the University of Chicago, Center for Cosmology and Astro-Particle Physics at the Ohio State University, the Mitchell Institute for Fundamental Physics and Astronomy at Texas A&M University, Financiadora de Estudos e Projetos, Fundacao Carlos Chagas Filho de Amparo, Financiadora de Estudos e Projetos, Fundacao Carlos Chagas Filho de Amparo a Pesquisa do Estado do Rio de Janeiro, Conselho Nacional de Desenvolvimento Cientifico e Tecnologico and the Ministerio da Ciencia, Tecnologia e Inovacao, the Deutsche Forschungsgemeinschaft and the Collaborating Institutions in the DES. The Collaborating Institutions are Argonne National Laboratory, the University of California at Santa Cruz, the University of Cambridge, Centro de Investigaciones Energeticas, Medioambientales y Tecnologicas-Madrid, the University of Chicago, University College London, the DES-Brazil Consortium, the University of Edinburgh, the Eidgenossische Technische Hochschule (ETH) Zurich, Fermi National Accelerator Laboratory, the University of Illinois at Urbana-Champaign, the Institut de Ciencies de l’Espai (IEEC/CSIC), the Institut de Fisica d’Altes Energies, Lawrence Berkeley National Laboratory, the Ludwig Maximilians Universitat Munchen and the associated Excellence Cluster Universe, the University of Michigan, NSF NOIRLab, the University of Nottingham, the Ohio State University, the University of Pennsylvania, the University of Portsmouth, SLAC National Accelerator Laboratory, Stanford University, the University of Sussex, and Texas A&M University. This paper contains data from observations obtained with HET, which is a joint project of the University of Texas at Austin, the Pennsylvania State University, Ludwig-Maximilians-Universität München, and Georg-August Universität Göttingen. The HET is named in honor of its principal benefactors, William P. Hobby and Robert E. Eberly. We acknowledge the Texas Advanced Computing Center (TACC) at The University of Texas at Austin for providing highperformance computing, visualization, and storage resources that have contributed to the results reported within this paper. The LRS2 was developed and funded by the University of Texas at Austin, McDonald Observatory, Department of Astronomy, and Pennsylvania State University. We thank the Leibniz-Institut für Astrophysik Potsdam (AIP) and the Institut für Astrophysik Göttingen (IAG) for their contributions to the construction of the IFUs. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester. This research has made use of the XRT Data Analysis Software (XRTDAS) developed under the responsibility of the ASI Science Data Center (ASDC), Italy. This research has made use of data and/or software provided by the High Energy Astrophysics Science Archive Research Center (HEASARC), which is a service of the Astrophysics Science Division at NASA/GSFC. This research has made use of the Astrophysics Data System, funded by NASA under Cooperative Agreement 80NSSC21M00561.Funding
B.O. is supported by the McWilliams Postdoctoral Fellowship in the McWilliams Center for Cosmology and Astrophysics at Carnegie Mellon University. J.H. acknowledges support from NASA under award No. 80GSFC24M0006. C.M. I. is supported by the JST FOREST Program (JPMJFR2136) and the JSPS Grant-in-Aid for Scientific Research (20H05639, 20H00158, 23H01169, 23H04900, 26K07149). Support was provided by Schmidt Sciences, LLC for K. Malanchev. A.F. was funded by the European Union ERC-2022-STG— BOOTES—101076343. Support for this work was provided by NASA through Chandra award No. GO6-27037X, issued by the CXC, which is operated by the Smithsonian Astrophysical Observatory for and on behalf of NASA under contract NAS8-03060. Funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany’s Excellence Strategy—EXC-2094/2—390783311. Additional support for DESI was provided by the US National Science Foundation (NSF), Division of Astronomical Sciences under Contract No. AST-0950945 to the NSF’s National Optical Infrared Astronomy Research Laboratory; the Science and Technology Facilities Council of the United Kingdom; the Gordon and Betty Moore Foundation; the Heising–Simons Foundation; the French Alternative Energies and Atomic Energy Commission (CEA); the National Council of Humanities, Science and Technology of Mexico (CONAHCYT); the Ministry of Science, Innovation and Universities of Spain (MICIU/AEI/10.13039/ 501100011033), and by the DESI Member Institutions: https:// www.desi.lbl.gov/collaborating-institutions. Additional Rubin Observatory funding comes from private donations, grants to 18 The Astrophysical Journal Letters, 1006:L13 (26pp), 2026 July 20 O’Connor et al. universities, and in-kind support from LSST-DA Institutional Members. This work acknowledges support from the National Science Foundation through AST award No. 2432476 (PI: Kasliwal; co-PI: Coughlin) and leverages experience from the ZTF (co-PIs: Graham and Kasliwal).
Keywords
- Core-collapse supernovae (304)
- Time domain astronomy (2109)
- Type Ic supernovae (1730)
- X-ray transient sources (1852)
ASJC Scopus subject areas
- Astronomy and Astrophysics
- Space and Planetary Science
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