Naked Forward Shock Seen in the TeV Afterglow Data of GRB 221009A

Abstract

We explore the implications of the light curve of the early TeV gamma-ray afterglow of GRB 221009A reported by the LHAASO Collaboration. We show that the reported offset of the reference time, T _* , allows the determination of the activation time of the main jet component, which occurs approximately 200 s after the Gamma-ray Burst Monitor (GBM) trigger time and closely precedes the moment at which GBM was saturated. We find that while the LHAASO data do not exclude the homogeneous circumburst medium scenario, the progenitor wind scenario looks preferable, finding excellent agreement with the expected size of the stellar bubble. We conclude that the initial growth of the light curve is dominated by processes internal to the jet or by gamma-gamma attenuation on the photons emitted during the prompt phase. Namely, either the activation of the acceleration process or the decrease of internal gamma-gamma absorption can naturally explain the initial rapid flux increase. The subsequent slow flux growth phase observed up to T _* + 18 s can be related to the process of particle accumulation in the production region. The duration of this phase allows an almost parameter-independent determination of the jet’s initial Lorentz factor, Γ _0 ≈ 600, and magnetic field strength, $B^{\prime} \sim 0.3\,{\rm{G}}$ . These values appear to match well those previously revealed through spectral modeling of the GRB emission.

Keywords

• Gamma-rays
• Gamma-ray transient sources
• Gamma-ray bursts
• Gamma-ray sources