Observation of spectral lines in the exceptional GRB 221009A

Abstract

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 ($\Gamma$) and reveal its time evolution (i.e. $\Gamma \sim t^{-1}$) 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.

Accepted by SCIENCE CHINA Physics, Mechanics & Astronomy (SCPMA)

References in corpus (15)

• Gravitational Waves and Gamma-rays from a Binary Neutron Star Merger: GW170817 and GRB 170817A
• The Supernova -- Gamma-Ray Burst Connection
• A tera-electronvolt afterglow from a narrow jet in an extremely bright gamma-ray burst 221009A
• GRB 221009A, The BOAT
• Fermi-GBM Discovery of GRB 221009A: An Extraordinarily Bright GRB from Onset to Afterglow
• Properties of the extremely energetic GRB~221009A from Konus-WIND and SRG/ART-XC observations
• GRB 221009A: Discovery of an Exceptionally Rare Nearby and Energetic Gamma-Ray Burst
• Ground-based calibration and characterization of the Fermi Gamma-Ray Burst Monitor Detectors
• Structure of Gamma-Ray Burst jets: intrinsic versus apparent properties
• Limits on Neutrino Emission from GRB 221009A from MeV to PeV using the IceCube Neutrino Observatory
• The first JWST spectrum of a GRB afterglow: No bright supernova in observations of the brightest GRB of all time, GRB 221009A
• The design and performance of GRD onboard the GECAM satellite
• The First GECAM Observation Results on Terrestrial Gamma-ray Flashes and Terrestrial Electron Beams
• GECAM detection of a bright type-I X-ray burst from 4U 0614+09: confirmation its spin frequency at 413 Hz
• Non-detection of the Gamma-ray Burst X-ray Emission Line: The Down-Comptonization Effect

Cited by in corpus (10)

• Relation between the keV-MeV and TeV emission of GRB 221009A and its implications
• The GECAM Ground Search System for Gamma-ray Transients
• Implication of Jet Physics from MeV Line Emission of GRB 221009A
• A Novel Model for the MeV Emission Line in GRB 221009A
• Prospects for Time-Domain and Multi-Messenger Science with eXTP
• Probable evidence for a transient mega-electron volt emission line in the GRB 221023A
• Extending the short gamma-ray burst population from sub-threshold triggers in Fermi/GBM and GECAM data and its implications
• Emission and Absorption Lines in Photospheric Radius Expansion Bursts of 4U 1820$-$30
• Design and preliminary performance study of the broad-band spectrometer detector for POLAR-2
• SSC Radiation in the ICMART Model: Spectral Simulations and Application to the Record-Breaking GRB 221009A