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

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 ( grizJK s ) lightcurve shows that the SN is approximately one-half the luminosity ( k 98bw = 0.4–0.6) of the canonical GRB-SN 1998bw. We derive a nickel mass of M Ni = 0.4–0.5 M ⊙ with a total ejected mass of M ej ≈ 4–6 M ⊙ and kinetic energy E K = (3–8) × 10 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 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.