Implication of Jet Physics from MeV Line Emission of GRB 221009A

TL;DR

The paper uses the MeV line evolution in GRB 221009A to constrain jet physics, revealing a large emission radius, short cooling timescales, and a magnetically dominated jet if the Lorentz factor exceeds 400.

Contribution

It provides the first detailed analysis linking MeV line evolution to jet properties, constraining emission region size, cooling timescales, and jet magnetization in GRBs.

Findings

Emission region radius > 10^16 cm

Line width implies short cooling timescales

Jet likely magnetically dominated if Lorentz factor > 400

Abstract

Ultrarelativistic jets are believed to play an important role in producing prompt emission and afterglow of gamma-ray bursts (GRBs), but the nature of the jet is poorly known owing to the lack of decisive features observed in the prompt emission. The discovery of an emission line evolving from about 37 to 6 MeV in the brightest-of-all-time GRB 221009A provides an unprecedented opportunity to probe GRB jet physics. The time evolution of the central energy of the line with power-law index $-1$ is naturally explained by the high-latitude curvature effect. Under the assumption that the line emission is generated in the prompt emission by $e^\pm$ pair production, cooling, and annihilation in the jet, we can strictly constrain jet physics with observed line emission properties. We find that the radius of the emission region is $r\gtrsim10^{16}$ cm. The narrow line width of $\sim10\%$ requires that the line emission occurs within $\sim10\%$ of the dynamical time, which further implies short timescales of pair cooling to the nonrelativistic state and pair annihilation, as well as a slightly clumpy emission region. If the jet's Lorentz factor is $\Gamma\gtrsim400$, the fast cooling requirement needs an energy density of magnetic field in the jet much larger than that of prompt gamma rays, i.e., a magnetically dominated jet. The temporal behavior of line flux suggests some angle dependence of line emission. We also discuss the difficulties of other scenarios for the observed emission line.