GRB 260808B

General information

GRB 260808B was detected by SVOM/ECLAIRs on 2026-08-08 at 14:20:58 UTC and followed rapidly by SVOM/MXT and SVOM/VT, which identified an X-ray source and optical afterglow. The afterglow position reported by SVOM/VT is RA = 345.233538 deg, Dec = 8.478528 deg with 0.5 arcsec uncertainty.

Wavelength coverage

• Gamma-ray: SVOM/ECLAIRs triggered with SNR 7.52, reported a multiple-peak light curve, T90 = 178 +12/-8 s in 4-120 keV, and fluence = (6.4 ± 0.9) × 10^-7 erg cm^-2.
• X-ray: SVOM/MXT detected an uncatalogued X-ray source 152 s after trigger; EP-FXT detected a consistent source about 1.6 h after trigger with observed flux about 1.6 × 10^-13 erg s^-1 cm^-2.
• Optical: SVOM/VT detected the afterglow from 233 s, followed by detections from OHP/T193, LCO, NOT, Liverpool Telescope, SVOM/COLIBRÍ, MAO/AZT-22, SAO RAS, and GTC/OSIRIS+.
• NIR: OHP/T193 reported Y > 21.8, and WINTER/Palomar reported J > 18.82 AB mag.
• Spectroscopy: GTC/OSIRIS+ obtained spectra covering 3620-7800 Å and detected absorption features. No radio, GeV, TeV, gravitational-wave, or neutrino observations are mentioned.

Lightcurve and spectrum

• SVOM/ECLAIRs reported a multiple-peak prompt light curve with a possible precursor at T0-84.5 s lasting about 2 s.
• SVOM/ECLAIRs measured T90 = 178 +12/-8 s in 4-120 keV.
• The 5-120 keV time-averaged spectrum from T0-45 s to T0+43 s was fit by a power law with alpha = -1.96 +0.14/-0.16.
• SVOM/VT measured VT_B = 20.9 ± 0.2 and VT_R = 20.3 ± 0.1 AB at 233 s, fading to VT_B = 22.8 ± 0.2 and VT_R = 22.0 ± 0.2 AB at 1.51 h.
• SVOM/VT found rapid early fading with temporal decay slope -1.9 in both channels within 400 s after trigger.
• Later optical detections showed rebrightening: SVOM/VT reported VT_B = 21.9 ± 0.2 and VT_R = 21.3 ± 0.2 AB at 2.94 d, SVOM/COLIBRÍ measured r = 21.68 ± 0.04 and z = 21.36 ± 0.11 AB at about 91.5-91.8 h, and multiple groups confirmed the counterpart was brighter around 4.4-4.5 d than earlier epochs.

Redshift

GTC/OSIRIS+ spectra detected Fe II, Mg II, and Mg I absorption features at a common redshift z = 1.295, suggested as the redshift of GRB 260808B.

What’s special vs typical

• The event shows a reported possible gamma-ray precursor before the main prompt emission.
• The optical afterglow did not follow only a simple monotonic decline; several facilities reported significant rebrightening at roughly 3-4.5 days after trigger.
• The dataset is unusually well sampled for a GRB alert, with prompt gamma-ray characterization, X-ray and optical counterparts, late-time optical monitoring, and spectroscopy.