GRB 260310A

General information

GRB 260310A triggered Fermi/GBM at 2026-03-10 04:57:10 UT and was classified as a likely long (type II) GRB. A multi-wavelength counterpart (AT2026fgk) was identified and followed extensively, with spectroscopy establishing z=0.153 and later spectra confirming an associated broad-lined Type Ic supernova.

Wavelength coverage

• Gamma-ray: Fermi/GBM prompt properties reported (duration, spectrum, fluence); AstroSat/CZTI detection in 100–500 keV veto channel.
• X-ray: Einstein Probe FXT detected and characterized an uncatalogued source consistent with the optical position; later EP monitoring found a decay steepening; NuSTAR detected 3–79 keV emission with a power-law spectrum.
• Optical: Extensive multi-epoch photometry (multiple facilities) and spectroscopy; late-time spectra show SN-like undulations.
• Near-IR: Multiple J/H/K detections reported (e.g., WINTER, SYSU IR, TNG/NICS, LBT/LUCI, MDM).
• Radio (cm): AMI-LA and VLA detections; VLA multi-frequency spectrum at ~17 d shows an SED peak near ~15 GHz.
• mm/submm: NOEMA (100 GHz) and SMA (230 GHz) detections. No observations mentioned in: GeV/TeV high-energy gamma-ray detections, neutrinos, gravitational waves.

Lightcurve and spectrum

• Prompt emission: T90 reported 60 s (GBM) and 30 s (CZTI veto); a later GBM reanalysis reported multiple episodes with T90=86 s. GBM time-averaged spectrum reported as cutoff power law (index -0.15±0.01, Epeak=172±5 keV) and alternatively Band with Ep175 keV in reanalysis.
• Optical temporal evolution: early decay described as ~-0.6 for the first ~7 days, steepening around ~7–8 days to ~-1.35 (P60); CAHA reported a shallow decay between two R-band epochs; COLIBRÍ reported a rebrightening near ~10 days while other datasets reported no rebrightening.
• Optical/NIR SED notes: an optical/NIR spectral index beta ≈ -1.1 was reported; colors evolved at ~17 days in a way interpreted as emergence of a thermal (SN) component.
• Optical polarimetry: no significant linear polarization; 3σ upper limit P < 1.5%.
• X-ray spectra: EP FXT reported Γ=1.55±0.09 with unabsorbed 0.5–10 keV flux (5.51±0.22)×10^-12 erg s^-1 cm^-2; NuSTAR reported Γ=1.76±0.22 with unabsorbed 2–10 keV flux (1.16±0.15)×10^-12 erg cm^-2 s^-1.
• Radio/mm: AMI-LA detected 15.2 GHz emission; VLA detections at multiple frequencies (3–45 GHz) with the spectral peak near ~15 GHz at ~17 d; NOEMA and SMA reported mm/submm flux densities.

Redshift

Spectroscopy reported emission lines consistent with z=0.153 (including identification of H-alpha at 7566 Å and multiple nebular lines), with emission extending to the transient position, supporting association of AT2026fgk with the z=0.153 host environment.

What’s special vs typical

The event is nearby (z=0.153) for a long GRB and shows a well-documented GRB–supernova connection: late-time spectra are consistent with a broad-lined Type Ic SN near maximum light, and contemporaneous color evolution was interpreted as emergence of a thermal SN component.