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

Brendan O'Connor; Malte Busmann; Xander J. Hall; Kenta 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; Tomas Cabrera; Keerthi Kunnumkai; Adam D. Myers; Christoph Ries; David Schlegel; Michael Schmidt; Silona Wilke; Muskan Yadav

arXiv · 2605.18973

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

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Abstract

The association of broad-lined Type Ic supernovae with long-duration gamma-ray bursts (GRBs) has been known for 28 years. However, only about seventy gamma-ray burst supernovae (GRB-SNe) have been identified, of which only half have spectroscopic classifications. At $z=0.153$, GRB 260310A is the 12th 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 d after discovery. The optical afterglow is among the brightest ever observed from a GRB. Spectra obtained more than two weeks after the explosion reveal broad absorption features that securely identify SN 2026fgk as a Type Ic-BL supernova. Modeling of the multi-wavelength ($grizJK_s$) lightcurve shows that the supernova is approximately half the luminosity ($k_\textrm{98bw}=0.4-0.6$) of the canonical GRB-SN 1998bw. We derive a nickel mass of $M_\textrm{Ni}=0.4-0.5$ $M_\odot$ with a total ejected mass of $M_\textrm{ej}\approx4-6 $ $M_\odot$ and kinetic energy $E_\textrm{K}=(3-8)\times10^{51}$ 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 sub-solar metallicity ($\sim$\,$0.4Z_\odot$), 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.

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Brendan O'Connor, Malte Busmann, Xander J. Hall, Kenta 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, Tomas Cabrera, Keerthi Kunnumkai, Adam D. Myers, Christoph Ries, David Schlegel, Michael Schmidt, Silona Wilke, Muskan Yadav. 2026-05-18. GRB 260310A/SN 2026fgk: Photometric and Spectroscopic Evolution of a Nearby GRB-Supernova and an Exceptionally Bright Afterglow at z=0.153. https://arxiv.org/abs/2605.18973

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