Speaker
Description
On 2026 February 26 at 10:37:55 UTC, the Fermi Gamma-ray Burst Monitor (GBM) triggered on the exceptionally bright, long-duration GRB 260226A. Owing to its extreme intensity, the burst also triggered the Large Area Telescope (LAT) onboard approximately 20 s later, marking only the second time during the Fermi mission that an autonomous onboard trigger was initiated from a LAT seed localization. The burst was observed by several missions in the hard X-ray/gamma-ray band, however, no optical or radio counterpart was identified. Ground-based imaging atmospheric Cherenkov telescopes were unable to observe the event because of Moon constraints, while other space-based follow-up opportunities were limited by operational or Sun-angle constraints, leaving the redshift unknown.
The LAT observations reveal one of the brightest bursts detected by the instrument, with a Test Statistic exceeding 3000 over the first 1500 s after trigger and more than 600 significant photons (above 100 MeV) detected. Remarkably, despite the burst brightness, the highest-energy LAT photon was only 1.1 GeV, arriving at T0 + 420 s. GRB 260226A also establishes a new record in LAT Low Energy (LLE) data, reaching a significance of 75.3σ. No evidence of detector saturation or pile-up is found in either GBM or LAT data.
A pronounced dip is observed in both the GBM light curve (around T0 + 25 s) and the LLE emission (around T0 + 23 s), potentially signaling the onset of afterglow emission while prompt activity was still ongoing. Joint time-integrated and time-resolved spectral analysis of GBM, LLE, and LAT data further reveals a persistent spectral cutoff at energies of tens of MeV throughout the main emission episode, placing constraints on the bulk Lorentz factor and opacity of the outflow. We present the temporal and spectral properties of this extraordinary event and discuss their implications for prompt and early afterglow emission mechanisms.
