Xiang-Yu Wang - LHAASO observations of GRB 221009A

Friday 12 May 2023, 10:30 → 12:01 Europe/Berlin

Remote

Decaying afterglows have been observed from a few gamma-ray bursts (GRBs) in the TeV band, but the onset of the TeV afterglow remains undetected. Here we report the detection of the very early TeV afterglow of the brightest-of-all-time GRB 221009A by the Large High Altitude Air Shower Observatory (LHAASO). More than 64,000 photons above 0.2 TeV were observed by LHAASO-WCDA within the first 3000 seconds. The observation reveals a rapid flux rise to a peak after an initial quiescent period during the main burst, followed by a decay phase. The rising phase represents the onset of a TeV afterglow, produced by an external shock before being significantly decelerated. A clear steepening is seen at  650 s after the peak, which is consistent with a jet break with a half-opening angle of only ~0.8 degree. This implies that the observers happened to see the brightest core of a structured jet, explaining the extremely large isotropic energy of this GRB.

DESY.pdf