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 (>6σ) 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 (kB​T≪me​c2)
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.Comment: Submitte