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GRB 260310A/SN 2026gfk: Photometric and Spectroscopic Evolution of a Nearby Gamma-ray Burst Supernova and an Exceptionally Bright Afterglow at z = 0.153

  • Brendan O'Connor
  • , Malte Busmann
  • , Xander J. Hall
  • , Kent Taguchi
  • , Masaomi Tanaka
  • , Daniel Gruen
  • , Seiji Toshikage
  • , Ariel J. Amsellem
  • , Ziyuan Zhu
  • , Antonella Palmese
  • , Dylan Green
  • , John Banovetz
  • , Yu-Han Yang
  • , Eleonora Troja
  • , Hendrik Van Eerten
  • , Julius Gassert
  • , Mitra Maleki
  • , Stephen Bailey
  • , Segev BenZvi
  • , Tomás Cabrera
    • Keerthi Kunnumkai, Adam D. Myers, Christoph Ries, David Schlegel, Michael Schmidt, Silona Wilke, Muskan Yadav
  • Carnegie Mellon University
  • Ludwig Maximilian University of Munich
  • Excellence Cluster Origins
  • Kyoto University
  • Tohoku University
  • Lawrence Berkeley National Laboratory
  • Università degli Studi di Roma Tor Vergata
  • University of Rochester
  • University of Wyoming

Research output: Contribution to journal › Article › peer-review

Abstract

The association of broad-lined Type Ic (Ic-BL) supernovae with long-duration gamma-ray bursts (GRBs) has been known for 28 yr. However, only about 70 GRB supernovae (GRB-SNe) have been identified, of which only one-half have spectroscopic classifications. At z = 0.153, GRB 260310A is the twelfth spectroscopically confirmed GRB-SN discovered within 1 Gpc, offering a critical opportunity to follow one of these rare supernovae in detail. We present optical to near-infrared imaging and spectroscopy of GRB 260310A and SN 2026fgk out to 65 days after discovery. The optical afterglow is among the brightest ever observed from a GRB. Spectra obtained more than 2 weeks after the explosion reveal broad absorption features that securely identify SN 2026fgk as a Type Ic-BL supernova (SN). Modeling of the multiwavelength (grizJKs) lightcurve shows that the SN is approximately one-half the luminosity (k98bw = 0.4–0.6) of the canonical GRB-SN 1998bw. We derive a nickel mass of MNi = 0.4–0.5 M⊙with a total ejected mass of Mej ≈ 4–6 M⊙and kinetic energy EK = (3–8) × 1051 erg. The GRB exploded at an extremely large offset of 15 kpc from its host galaxy. Long-slit spectra reveal a “bridge” of nebular emission extending along the galaxy’s disk to the GRB location, which has a subsolar metallicity (∼0.4 Z⊙), compared to a near-solar metallicity for the host galaxy. This indicates that the large offset arises from the galaxy’s extended light profile rather than an isolated environment
Original languageEnglish
Article number210
Number of pages16
JournalThe Astrophysical Journal
Volume1006
Issue number2
Early online date28 Jul 2026
DOIs
Publication statusPublished - 1 Aug 2026

Acknowledgements

The authors thank the referee for providing useful comments and suggestions. B.O. thanks the Gemini Director Time Review Board (GDTRB), including Aleksandar Cikota, Hwihyun Kim, Bryan Miller, Siyi Xu, and Elena Sabbi, for approving the DDT request. B.O. is grateful for the excellent support of Atsuko Nitta, Teo Mocnik, Jen Andrews, Sunny Stewart, Denise Hung, Kristin Chiboucas, Hyewon Suh, and the entire GeminiNorth staff in rapidly scheduling and obtaining these observations. B.O. is supported by the McWilliams Postdoctoral Fellowship in the McWilliams Center for Cosmology and Astrophysics at Carnegie Mellon University. This paper contains data obtained at the Wendelstein Observatory of the Ludwig-Maximilians University Munich. Funded by the Deutsche Forschungsgemeinschaft (DFG; German Research Foundation) under Germany’s Excellence Strategy–EXC-2094/2–390783311. The authors acknowledge the IT Support Team of the Faculty of Physics at LMU for maintaining and supporting the high-performance computing (HPC), storage, and computational infrastructure used in this work. Figure 10. BPT diagram for long GRB host galaxies (squares) and explosion sites (circles) compared to a background population of Sloan Digital Sky Survey galaxies (small gray circles; D. G. York et al. 2000; H. Aihara et al. 2011; V. Stampoulis et al. 2019). Both the explosion site (red star) and integrated host galaxy (red square) of GRB 260310A are shown. The long GRB data are compiled from L. Christensen et al. (2008), X. H. Han et al. (2010), P. L. Kelly et al. (2013), S. Schulze et al. (2014), E. R. Stanway et al. (2015), T. Krühler et al. (2015), L. Izzo et al. (2017), Z. Cano et al. (2017b), K. E. Heintz et al. (2018), A. de Ugarte Postigo et al. (2018, 2020), A. Melandri et al. (2019), and C. C. Thöne et al. (2024). 11 The Astrophysical Journal, 1006:210 (16pp), 2026 August 1 O’Connor et al. This material is based upon work supported by the U.S. 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. Additional support for DESI was provided by the U.S. National Science Foundation (NSF), Division of Astronomical Sciences under Contract No. AST-0950945 to the NSF’s National OpticalInfrared 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/collaboratinginstitutions. 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 U. S. National Science Foundation, the U. S. Department of Energy, or any of the listed funding agencies. Based on observations obtained at the international Gemini Observatory, a program of NSF’s OIR Lab, which is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation on behalf of the Gemini Observatory partnership: the National Science Foundation (United States), National Research Council (Canada), Agencia Nacional de Investigación y Desarrollo (Chile), Ministerio de Ciencia, Tecnología e Innovación (Argentina), Ministério da Ciência, Tecnologia, Inovações e Comunicações (Brazil), and Korea Astronomy and Space Science Institute (Republic of Korea). The data were acquired through the Gemini Observatory Archive at NSF NOIRLab and processed using Data Reduction for Astronomy from Gemini Observatory North and South (DRAGONS). The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. This paper contains data from observations obtained with the Hobby-Eberly Telescope (HET), which is a joint project of the University of Texas at Austin, the Pennsylvania State University, Ludwig-Maximillians-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 high-performance computing, visualization, and storage resources that have contributed to the results reported within this paper. The Low Resolution Spectrograph 2 (LRS2) was developed and funded by the University of Texas at Austin, McDonald Observatory, Department of Astronomy, and Pennsylvania State University. We thank the LeibnizInstitut für Astrophysik Potsdam (AIP) and the Institut für Astrophysik Göttingen (IAG) for their contributions to the construction of the integral-field units. The authors thank the TriCCS developer team (which has been supported by the JSPS KAKENHI grant Nos. JP18H05223, JP20H00174, and JP20H04736, and by NAOJ Joint Development Research). This research has made use of the GRBSN webtool (G. Finneran et al. 2025), available at https://grbsn. watchertelescope.ie. Facilities: WO:2m, Seimei, Gemini:Gillett, Mayall, HET. Software: Astropy (Astropy Collaboration et al. 2013, 2018, 2022), SFFT (L. Hu et al. 2022), AstrOmatic (E. Bertin & S. Arnouts 1996; E. Bertin 2006, 2010; E. Bertin et al. 2002), DRAGONS (K. Labrie et al. 2019, 2023), pPXF (M. Cappellari & E. Emsellem 2004; M. Cappellari 2012, 2017), redback (N. Sarin et al. 2024), Bilby (G. Ashton et al. 2019), dynesty (J. S. Speagle 2020), sncosmo (K. Barbary et al. 2016).

Funding

This paper contains data obtained at the Wendelstein Observatory of the Ludwig-Maximilians University Munich. Funded by the Deutsche Forschungsgemeinschaft (DFG; German Research Foundation) under Germany’s Excellence Strategy–EXC-2094/2–390783311

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