What absorbs the early TeV photons of GRB 221009A?

Abstract

The tera-electronvolt (TeV) light curve of gamma-ray burst (GRB) 221009A exhibits an unprecedentedly rapid rise at its start. This phenomenon could be due to the strong absorption of photons and electrons within the emitting region. As the external shock expands outward and the radius increases, the volume of matter also grows, resulting in a gradual reduction of the optical depth for TeV photons. We investigate several potential explanations for the early TeV light curves. We calculate the optical depth for TeV photons, considering both annihilation with lower-energy photons in the external shock and their scattering by electrons generated through the cascading of the TeV emission. Even under the favorable assumptions, we have determined that the optical depths for these processes are orders of magnitude too small to explain the observed light curve. Additional sources of absorbers, including electrons in the ejecta or external shock, also do not result in sufficient optical depths. Hence, the cause behind the early peculiar TeV light curve remains unclear.