The Production Mechanism of the High-energy Emission Line in the Brightest Cosmic Burst

Abstract As a characteristic feature of the spectrum, the emission line carries critical information on the underlying physics of the radiation. After extensive efforts in decades, the first high-significant detection of a series of emission lines evolving from 37 MeV to 6 MeV has been detected in the ever-bright gamma-ray burst GRB 221009A. However, the physical mechanism of the entire evolutionary trend of the lines remains elusive. To provide a self-consistent interpretation, we propose a novel scenario in which the photons of the line undergo a radiation transfer process called down-Comptonization after generation by the electron–positron pair annihilation. By incorporating the gamma-ray burst dynamical evolution, we systematically reproduce the observed evolution of the central energy, width, and flux of the emission line and further impose stringent constraints on the production of high-energy emission lines in general. Our study provides a new direction to the research of extreme cosmic bursts.