A mega–electron volt emission line in the spectrum of a gamma-ray burst
M. E. Ravasio, O. S. Salafia, G. Oganesyan, Alessio Mei, G. Ghirlanda, S. Ascenzi, B. Banerjee, Samanta Macera, M. Branchesi, P. G. Jonker, A. J. Levan, D. Malesani, Katharine Mulrey, A. Giuliani, A. Celotti, G. Ghisellini
Abstract
A long gamma–ray burst (GRB) is observed when the collapse of a massive star produces an ultrarelativistic outflow pointed toward Earth. Gamma-ray spectra of long GRBs are smooth, typically modeled by joint power-law segments describing a continuum, with no detected spectral lines. We report a significant (>6σ) narrow emission feature at ~10 mega–electron volts (MeV) in the spectrum of the bright GRB 221009A. Over 80 seconds, it evolves in energy (~12 to ~6 MeV) and in luminosity (~1.1 to <0.43 × 10 50 erg second −1 ) but has a constant width of ~1 MeV. We interpret this feature as a blueshifted spectral line produced by the annihilation of electron-positron pairs, potentially in the same location responsible for emitting the brightest GRB pulses.