GRB 260708A

General information

GRB 260708A is a likely long GRB detected by Fermi-GBM at 2026-07-08 10:11:16.89 UT and also detected at high energies by Fermi-LAT. The event has independent gamma-ray confirmations, an optical afterglow/counterpart, and a fading X-ray counterpart consistent with the optical source.

Wavelength coverage

• Gamma-ray: Fermi-GBM detected the burst with T90 about 62 s, fluence (2.14 +/- 0.03)E-05 erg/cm^2, and 1-s peak photon flux 19.6 +/- 0.5 ph/s/cm^2; Fermi-LAT detected >100 MeV emission with photon flux (1.9 +/- 0.4)E-5 ph/cm2/s and a highest-energy photon of 2.1 GeV; GECAM-B and Glowbug also detected the burst.
• Optical: MASTER discovered MASTER OT J150237.02-435307.7 / AT 2026stf at unfiltered m~17.6; Kilonova-Catcher, LCO, and GRANDMA reported further detections and fading behavior.
• X-ray: EP/FXT detected an uncatalogued source consistent with the MASTER optical counterpart in two ToO observations, with 0.5-10 keV unabsorbed flux declining from (1.31 +/- 0.13)e-12 to (5.46 +/- 0.68)e-13 erg cm^-2 s^-1. Bands with no observations mentioned: radio, infrared, ultraviolet, neutrino, gravitational wave.

Lightcurve and spectrum

• GBM light curve: sharp peak with T90 about 62 s in 50-300 keV.
• GBM spectrum: time-averaged spectrum from T0+0.5 to T0+49.7 s best fit by a power law with index -1.86 +/- 0.01.
• GECAM-B light curve: multiple pulses with T90 46.5 +5.0/-6.5 s in about 70-6000 keV.
• LAT spectrum: estimated photon index above 100 MeV of -2.5 +/- 0.3.
• Optical lightcurve: fading from MASTER m~17.6 to KNC R=18.93 +/- 0.14 at 1.18 d and LCO g/r/i around 19.2-19.6 AB at about 3.6 d; GRANDMA reports a softer decay than usual for GRBs.
• X-ray spectrum: EP/FXT absorbed power-law fits with photon indices 2.25 +/- 0.32 and 2.36 +/- 0.38 in the two observations.
• X-ray lightcurve: temporal decay index 1.32 +/- 0.24 between 2.62 and 5.08 days.

Redshift

What’s special vs typical

• The burst is LAT-detected, including a 2.1 GeV photon, which makes it more notable than a routine GBM-only long GRB.
• Multiple independent gamma-ray instruments detected it, and both optical and X-ray counterparts were reported.
• The optical and X-ray emission are described as slowly evolving, with GRANDMA noting a softer decay than usual for GRBs and EP/FXT encouraging spectroscopy to determine the classification.