GRB 260604C

General information

GRB 260604C triggered Fermi/GBM at 2026-06-04 20:18:44 UT and was also detected by SVOM/ECLAIRs, GECAM-B, AstroSat/CZTI, and Konus-Wind. The refined SVOM/ECLAIRs localization was RA=224.396 deg, Dec=28.742 deg with a 90% confidence radius of 8.6 arcmin, consistent with the MASTER optical afterglow.

Wavelength coverage

• Gamma-ray / hard X-ray: Fermi/GBM, SVOM/ECLAIRs, GECAM-B, AstroSat/CZTI, and Konus-Wind detected the prompt emission; Fermi/GBM reported T90 about 6.4 s, GECAM-B T90=7±0.5 s, AstroSat/CZTI T90 about 2.7-3.3 s, and Konus-Wind duration about 8.5 s.
• Optical / near-infrared: MASTER discovered the optical transient; LAST, Jinshan, COLIBRÍ, Kilonova-Catcher, GOTO, Liverpool Telescope, LCO, GRANDMA, SAO RAS, OPD/LNA, Simeiz, Mondy, SVOM/C-GFT, and CFHT/MegaCam reported detections or limits in clear, R/Rc, r, g, i, z, and y bands.
• Radio: VLA C-band observations at 6 GHz, 38 h after trigger, detected no radio source at the optical transient position to about 24 uJy/beam, 3 sigma. Bands with no observations mentioned: neutrino, gravitational-wave, submillimeter, ultraviolet.

Lightcurve and spectrum

• MASTER measured the optical transient at unfiltered magnitude 17.5 on 2026-06-04.86020 UT, with no minor planet at the position and no prior source in a 2020 reference image to limiting magnitude 18.1.
• LAST measured rapid optical fading: AB 16.42±0.02, 16.63±0.02, 16.85±0.02, and 17.12±0.02 at T0+0.33, 0.45, 0.57, and 0.75 h.
• Jinshan detected the counterpart at R=16.1±0.1 about 13 min after the burst.
• COLIBRÍ detected the counterpart at 7.34-7.52 h with g=19.69±0.04, r=19.56±0.03, i=19.38±0.04, z=19.21±0.06, and y=19.26±0.08 AB, and estimated a temporal decay index of about -1.2 compared with LAST.
• GOTO measured GOTO-L AB magnitudes 18.45±0.04, 18.70±0.08, and 19.06±0.12 at T0+3.46, 4.59, and 5.70 h.
• Kilonova-Catcher measured Rc=17.43±0.03 at T0+1.47 h, r=18.41±0.05 at T0+3.17 h, and Rc=18.43±0.06 at T0+4.01 h.
• Later optical detections included Rc=21.65±0.07 at T0+1.11195 d, R=21.35±0.14 at T0+1.07291 d, R=21.53±0.19 at T0+2.03341 d, R=21.18±0.06 at T0+2.86396 d, and Rc=23.08±0.18 at T0+4.01102 d.
• Mondy and GRANDMA reported possible optical rebrightening; COLIBRÍ confirmed a late rebrightening that began approximately 30 h after trigger and lasted about 30 h.
• COLIBRÍ measured color evolution from g-r=0.13±0.07 and g-z=0.48±0.10 in the first epoch to g-r=0.47±0.06 and g-z=0.86±0.09 at 3.43-3.54 d, indicating significant reddening.
• Fermi/GBM prompt spectroscopy was fit by a cutoff power law with index -1.03±0.01 and Epeak=551±2 keV, with 10-1000 keV fluence (2.55±0.05)e-5 erg/cm2.
• GECAM-B measured a cutoff power-law spectrum with photon index -1.41(+0.12/-0.09), Epeak=759(+342/-210) keV, and 10-1000 keV fluence (2.88+0.086/-0.11)e-5 erg/cm2.
• Konus-Wind measured emission up to about 4 MeV, total 20 keV-10 MeV fluence (3.60±0.26)e-5 erg/cm2, and a Band-function time-averaged spectrum with alpha=-0.92, beta=-2.33, and Ep=421 keV.

Redshift

What’s special vs typical

• The event is better covered than a routine GRB alert, with prompt detections from multiple high-energy missions and extensive optical follow-up over several days.
• The late optical rebrightening and significant reddening reported by COLIBRÍ and GRANDMA/Mondy are notable features beyond a simple monotonic afterglow decay.
• Despite the rich follow-up, no redshift measurement or secure host association is reported in the provided circulars.