2023/03/28 by M. E. Ravasio, O. S. Salafia, Ravasio, Maria Edvige +29
Physics and Astronomy · #FOS: Physical sciences #Gamma-ray bursts and supernovae #High Energy Astrophysical Phenomena (astro-ph.HE) #Nuclear Physics and Applications #Particle Detector Development and Performance
paper · pdf · doi:10.48550/arxiv.2303.16223
openalex publication_date 2023/03/28 · openalex created_date 2023/04/05 · openalex updated_date 2026/07/31
The highly variable and energetic pulsed emission of a long gamma-ray burst (GRB) is thought to originate from local, rapid dissipation of kinetic or magnetic energy within an ultra-relativistic jet launched by a newborn compact object, formed during the collapse of a massive star. The spectra of GRB pulses are best modelled by power-law segments, indicating the dominance of non-thermal radiation processes. Spectral lines in the X-ray and soft γ-ray regime for the afterglow have been searched for intensively, but never confirmed. No line features ever been identified in the high energy prompt emission. Here we report the discovery of a highly significant (> 6 σ) narrow emission feature at around 10 MeV in the brightest ever GRB 221009A. By modelling its profile with a Gaussian, we find a roughly constant width σ∼ 1 MeV and temporal evolution both in energy (∼ 12 MeV to ∼ 6 MeV) and luminosity (∼ 1050 erg/s to ∼ 2 × 1049 erg/s) over 80 seconds. We interpret this feature as a blue-shifted annihilation line of relatively cold (kBT≪ mec2) electron-positron pairs, which could have formed within the jet region where the brightest pulses of the GRB were produced. A detailed understanding of the conditions that can give rise to such a feature could shed light on the so far poorly understood GRB jet properties and energy dissipation mechanism.