AGILE gamma-ray detection of the exceptional GRB 221009A

TL;DR

The AGILE satellite detected and analyzed the exceptional long-duration gamma-ray burst GRB 221009A, revealing complex spectral features and providing crucial insights into high-energy emission mechanisms during its prompt and afterglow phases.

Contribution

This study presents the first detailed gamma-ray spectral analysis of GRB 221009A across MeV to GeV energies, highlighting simultaneous MeV and GeV emissions with different origins.

Findings

Detection of intense GeV emission up to 10,000 seconds after trigger

Observation of simultaneous MeV and GeV emissions with different spectral characteristics

Identification of physical mechanisms in different locations producing MeV and GeV emissions

Abstract

Gamma-ray emission in the MeV-GeV range from explosive cosmic events is of invaluable relevance to understanding physical processes related to the formation of neutron stars and black holes. Here we report on the detection by the AGILE satellite in the MeV-GeV energy range of the remarkable long-duration gamma-ray burst GRB 221009A. The AGILE onboard detectors have good exposure to GRB 221009A during its initial crucial phases. Hard X-ray/MeV emission in the prompt phase lasted hundreds of seconds, with the brightest radiation being emitted between 200 and 300 seconds after the initial trigger. Very intense GeV gamma-ray emission is detected by AGILE in the prompt and early afterglow phase up to 10,000 seconds. Time-resolved spectral analysis shows time-variable MeV-peaked emission simultaneous with intense power-law GeV radiation that persists in the afterglow phase. The coexistence during the prompt phase of very intense MeV emission together with highly nonthermal and hardening GeV radiation is a remarkable feature of GRB 221009A. During the prompt phase, the event shows spectrally different MeV and GeV emissions that are most likely generated by physical mechanisms occurring in different locations. AGILE observations provide crucial flux and spectral gamma-ray information regarding the early phases of GRB 221009A during which emission in the TeV range was reported.