Observation of spectral lines in the exceptional GRB 221009A
arxiv(2024)
摘要
As the brightest gamma-ray burst ever observed, GRB 221009A provided a
precious opportunity to explore spectral line features. In this paper, we
performed a comprehensive spectroscopy analysis of GRB 221009A jointly with
GECAM-C and Fermi/GBM data to search for emission and absorption lines. For the
first time we investigated the line feature throughout this GRB including the
most bright part where many instruments suffered problems, and identified
prominent emission lines in multiple time intervals. The central energy of the
Gaussian emission line evolves from about 37 MeV to 6 MeV, with a nearly
constant ratio (about 10%) between the line width and central energy.
Particularly, we find that both the central energy and the energy flux of the
emission line evolve with time as a power law decay with power law index of -1
and -2 respectively. We suggest that the observed emission lines most likely
originate from the blue-shifted electron positron pair annihilation 511 keV
line. We find that a standard high latitude emission scenario cannot fully
interpret the observation, thus we propose that the emission line comes from
some dense clumps with electron positron pairs traveling together with the jet.
In this scenario, we can use the emission line to directly, for the first time,
measure the bulk Lorentz factor of the jet (Γ) and reveal its time
evolution (i.e. Γ∼ t^-1) during the prompt emission.
Interestingly, we find that the flux of the annihilation line in the co-moving
frame keeps constant. These discoveries of the spectral line features shed new
and important lights on the physics of GRB and relativistic jet.
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