GRB 260618B

General information

GRB 260618B is reported as a likely long GRB triggered by Fermi-GBM at 16:55:04.71 UT on 2026-06-18, with high-energy detection by Fermi-LAT and independent detection by Konus-Wind. A corrected SVOM/VT optical candidate position is R.A. = 354.333559 deg, Dec. = -44.735518 deg with 0.5 arcsec uncertainty.

Wavelength coverage

• Gamma-ray: Fermi-GBM detected a likely long GRB; Fermi-LAT detected significant high-energy emission above 100 MeV; Konus-Wind detected long-duration emission up to about 4 MeV.
• Optical: SVOM/VT detected an uncatalogued candidate in VT_B and VT_R at 10.53 h, LCO detected the counterpart in r at 31.25 h, and SVOM/VT later measured fainter, redder emission at 21.73 d. No radio, X-ray, neutrino, or gravitational-wave observations are mentioned.

Lightcurve and spectrum

• GBM reports a single emission episode with T90 about 20 s in 50-300 keV.
• GBM time-averaged spectrum from T0 to T0+10 s is fit by a cutoff power law with index -0.42 +/- 0.01 and Epeak = 1030 +/- 20 keV; a Band model fits equally well with Epeak = 1000 +/- 20 keV, alpha = -0.41 +/- 0.02, beta = -3.2 +/- 0.2.
• GBM fluence is (9.4 +/- 0.2)E-06 erg/cm^2 in 10-1000 keV, and the 1-s peak photon flux is 4.3 +/- 0.2 ph/s/cm^2.
• LAT reports photon flux above 100 MeV of (1.99 +/- 0.56)E-5 ph/cm2/s over 0-100 s, photon index -1.82 +/- 0.21, and a highest-energy photon of 6.7 GeV at 1.8 s.
• Konus-Wind reports a multi-peaked light curve starting about T0-1.6 s with total duration about 61.2 s and emission up to about 4 MeV.
• Konus-Wind reports total fluence 2.26(-0.34,+0.43)x10^-5 erg/cm2 and 64-ms peak flux 4.90(-0.98,+1.01)x10^-6 erg/cm2/s in 20 keV-10 MeV.
• SVOM/VT measured VT_B = 20.8 +/- 0.1 AB and VT_R = 20.9 +/- 0.1 AB at 10.53 h.
• LCO measured r = 20.49 +/- 0.06 AB at 31.25 h and z > 20.9 AB at 32.35 h.
• SVOM/VT measured VT_B = 22.3 +/- 0.2 AB and VT_R = 21.6 +/- 0.1 AB at 21.73 d, indicating fading and reddening.

Redshift

What’s special vs typical

• The event has high-energy LAT detection with a 6.7 GeV photon, independent Konus-Wind confirmation, and an optical counterpart followed to 21.73 d.
• The late SVOM/VT circular explicitly reports shallow decay and reddening of the optical candidate relative to earlier observations.