GRB 260411B

General information

GRB 260411B triggered Fermi/GBM at 2026-04-11 08:04:35 UT and is reported as a likely long GRB, with prompt detections by multiple gamma-ray instruments and an associated optical transient (GOTO26dam) consistent with the burst region.

Wavelength coverage

• Gamma-rays/Hard X-rays: Detected by Fermi/GBM, SVOM/GRM, GECAM-B, Konus-Wind, Glowbug (ISS), TERI (ISS), and Mars-Odyssey/HEND; IPN triangulation reported.
• High-energy gamma-rays (>100 MeV): Fermi-LAT reported no significant excess in standard likelihood analysis; LAT LLE showed a coincident count excess likely dominated by <100 MeV photons.
• Optical: GOTO discovery and multi-epoch detections by DOT, Ondrejov D50, and COLIBRÍ (g,r,i,z); ATLAS forced photometry provided a pre-burst non-detection constraint. Bands with no observations mentioned: X-ray, ultraviolet, infrared, radio.

Lightcurve and spectrum

• GBM: two spikes in a single emission episode; T90 ~33 s (50–300 keV). Time-averaged cutoff power-law fit with index -0.62±0.02 and Epeak=348±4 keV; Band alternative with Epeak=293±3 keV, alpha=-0.54±0.05, beta=-2.3±0.1; fluence (10–1000 keV) (2.31±0.05)×10^-5 erg/cm^2.
• SVOM/GRM: two emission episodes; T90=16.2 (+2.1/-1.4) s (15–5000 keV). Time-averaged cutoff power-law index -0.81±0.09, Epeak=385 (+44/-37) keV; fluence (10–1000 keV) (2.37±0.09)×10^-5 erg/cm^2.
• Konus-Wind: multipeaked emission with total duration ~55.6 s; emission detected up to ~10 MeV. Time-averaged Band fit alpha=-0.62, beta=-2.12, Ep=290 keV; fluence (20 keV–10 MeV) 4.50×10^-5 erg/cm^2.
• Optical: GOTO26dam detected at L=17.55±0.02 AB at t0+0.89 h and reported as rapidly fading with decay index alpha ~ -2.1±0.4; later detections include r=19.74±0.01 at ~12.41 h and g/r/i/z measurements at 18.86–20.68 h.

Redshift

What’s special vs typical

• The optical transient GOTO26dam lies within the IPN triangulation box, which was explicitly noted as strengthening the GRB–optical association.
• A relatively bright early optical detection (~17.6 mag at <1 h) with rapid fading and multi-epoch confirmation provides an unusually complete early-to-late optical follow-up chain for a GBM-triggered event.