2011ApJ...730....1L

GRB 090902B, detected by the Fermi Large Array Telescope (Fermi/LAT), shows extended high-energy emission (>100 MeV) up to 10³ s after the burst, and decays with time in a power law as t^–1.5. It has also been 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, and has been suspected to originate from the reverse shock. We here explore the models that can possibly explain 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 able to produce a sufficiently steep decay slope, cannot 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 an internal origin similar to that of the sub-MeV emission. The highest energy (33 GeV) photon of GRB 090902B detected at 80 s can marginally be accommodated within the forward-shock emission under the optimistic condition that electrons are accelerated by the Bohm diffusive shock.