Jet Lorentz factor constraint for GRB 221009A based on the optical depth of the TeV photons

TL;DR

This paper constrains the jet Lorentz factor of GRB 221009A by analyzing the optical depth of TeV photons within a two-zone emission model, finding a minimum Lorentz factor of about 300.

Contribution

It introduces a two-zone model considering different emission regions for TeV and MeV photons, refining Lorentz factor constraints for GRB 221009A.

Findings

Minimum Lorentz factor ~300 derived from optical depth calculations.

Two-zone model aligns with afterglow observations.

Proper spectral and light curve evolution considered.

Abstract

The recent detection of tera-electronvolt (TeV) photons from the record-breaking gamma-ray burst GRB 221009A during its prompt phase poses challenges for constraining its Lorentz factor. We re-evaluate the constraints on the jet Lorentz factor considering a two-zone model, wherein the TeV photons originate from the external shock region while the lower energy MeV photons come from the internal prompt emission region. By properly accounting for the evolution of the MeV photon spectrum and light curve, we calculate the optical depth for TeV photons and derive a minimum Lorentz factor about 300. It is consistent with the afterglow modeling for the TeV emission.