GRB 260610B

General information

GRB 260610B was detected by Fermi/GBM at 2026-06-10 23:46:14.25 UT as a likely long GRB, with an optical counterpart AT2026owq/GOTO26fua rapidly identified inside the GBM localization. The counterpart position is consistent across optical, millimeter, and radio detections, with VLA reporting RA=14:32:38.25, Dec=+27:00:17.64.

Wavelength coverage

• Gamma-ray: Fermi/GBM detected the burst with T90 about 62 s, three peaks, Epeak about 46.8 keV, and 10-1000 keV fluence of 1.30e-5 erg/cm2; SVOM/GRM also detected it with T90 = 65.0 +/- 8.0 s and fluence about 9.48e-6 erg/cm2.
• Optical/NIR: GOTO, ATLAS, DDOTI, TRT, COLIBRÍ, NOT, IAC-80, OASDG, SVOM/VT, Liverpool Telescope, SAO RAS, GRANDMA, Pan-STARRS, Fraunhofer/Wendelstein, DFOT, VLT/FORS2, CrAO, and INASAN reported detections or monitoring of AT2026owq.
• Millimeter: ALMA detected a roughly 0.3 mJy source at 43 GHz at 2.62 days after burst, positionally consistent with the optical counterpart.
• Radio: VLA detected the counterpart at 9 GHz with flux density about 0.4 mJy at 2.89 days after burst. Bands with no observations mentioned: X-ray, neutrino, gravitational-wave, and FRB/radio-burst searches.

Lightcurve and spectrum

• Optical onset was tightly constrained by ATLAS: non-detection 12.5 minutes before the Fermi/GBM trigger to >20.35 AB mag and detection 7.9 minutes after trigger at w=16.26 +/- 0.02 AB mag.
• GOTO measured L=16.66 +/- 0.01 AB mag at +0.21 h and L=17.83 +/- 0.03 AB mag at +1.4 h, consistent with a t^-0.56 decay.
• DDOTI detected the source at w=18.82 +/- 0.04 AB mag at 5.1-5.9 h and found continued fading with decay index approximately -0.6 relative to the early GOTO point.
• COLIBRÍ reported no strong break across g, r, i, z, and y at 7.08-8.45 h, suggesting a low-redshift event.
• Fermi/GBM prompt spectrum was fit by a cutoff power law with index -1.50 +/- 0.01 and Epeak = 46.8 +/- 0.4 keV; a Band fit gave Epeak = 45.9 +/- 0.5 keV, alpha = -1.4800 +/- 0.0009, and beta = -3.17 +/- 0.01.
• SVOM/GRM prompt spectrum was fit by a cutoff power law with index -1.59 +0.18/-0.22 and Epeak = 34 +6/-8 keV.
• Multiple optical data sets reported flattening or rebrightening beginning around 80 hours to 5 days after trigger, seen in griz and R-band monitoring.
• VLT/FORS2 spectroscopy at 6.04 days showed a power-law continuum during the late optical flare peak, interpreted as likely due to late jet activity, and also noted faint undulations consistent with an early, still faint supernova but not conclusive.
• Later R-band monitoring reported relatively fast brightening starting about 5 days after trigger followed by slow fading until about day 15.

Redshift

NOT/ALFOSC spectroscopy identified Mg II and Ca II absorption at z=0.473, suggested as the redshift of GRB 260610B. VLT/FORS2 later detected Ca II absorption and [O II] emission and measured z=0.474, confirming the previous redshift; at z=0.473, SVOM/GRM estimated Eiso about 8.8e51 erg and Pan-STARRS estimated a rest-frame g absolute magnitude about -21.4 at 2.9 rest-frame days, while later monitoring estimated peak M_R about -20.8.

What’s special vs typical

• The event has an unusually dense observational record for a long GRB, spanning gamma-ray prompt emission, rapid optical discovery, optical/NIR monitoring, spectroscopy, millimeter detection, and radio detection.
• The optical counterpart was detected within minutes of trigger and had a well-documented early decay followed by late flattening/rebrightening across multiple filters.
• The late optical behavior is notable: circulars discuss achromatic rebrightening, possible late jet activity, and a possible emerging supernova component, though the supernova was not confirmed in the provided reports.