Modeling the broadband emission of Fermi/LAT GRB 090902B

TL;DR

This paper analyzes the broadband afterglow emission of GRB 090902B, finding that a two-component jet model best explains the observed high-energy and low-energy data, while other models are inconsistent with observations.

Contribution

It demonstrates that a two-component jet model can successfully explain the broadband afterglow of GRB 090902B, improving understanding of jet structure in gamma-ray bursts.

Findings

Two-component jet model fits broadband data including LAT high-energy emission.

Reverse shock model overpredicts radio flux and is inconsistent with observations.

Early high-energy emission likely has an internal origin.

Abstract

GRB 090902B, detected by Fermi Large Array Telescope (Fermi/LAT), shows extend high-energy emission (>100 MeV) up to 10^3 s after the burst, which decays with time in a power-law as t^{-1.5}. It has been also observed by several follow-up low-energy instruments, including an early optical detection around 5000 s after the burst. The optical emission at early time decays faster than t^{-1.6}, which has been suspected to originate from the reverse shock. We here explore the models that can possibly explain the the broadband afterglow emission of GRB 090902B. We find that the reverse shock model for the early optical emission would overpredict the radio afterglow flux that is inconsistent with observations. A partially radiative blast wave model, which though is able to produce a sufficiently steep decay slope, can not explain the broadband data of GRB 090902B. The two-component jet model, which consists of a narrow and bright jet component in the core and a surrounding wider and less energetic jet component, is shown to be able to explain the broadband afterglow data, including the LAT high-energy data after ~50 s and low-energy (radio, optical and X-ray) afterglow data. The early-time high-energy emission detected by LAT before ~50 s is likely due to internal origin as that of the sub-MeV emission. The highest energy (33 GeV) photon of GRB090902B detected at 80 s can be marginally accommodated within the forward shock emission under the optimistic condition that electrons are accelerated by the Bohm diffusive shock.