Modeling the broadband emission of Fermi/LAT GRB 090902B

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.

💡 Research Summary

Gamma‑ray burst GRB 090902B was simultaneously detected by Fermi’s Large Area Telescope (LAT) and Gamma‑ray Burst Monitor (GBM) on 2 September 2009. The LAT recorded photons above 100 MeV for up to 10³ seconds, with a temporal decay following a power‑law of t⁻¹·⁵. Follow‑up observations with Swift X‑ray Telescope, UV/Optical Telescope, and several ground‑based facilities provided afterglow data in the X‑ray, optical, and radio bands. An early optical flash was observed around 5 × 10³ seconds after trigger, decaying faster than t⁻¹·⁶, which led to the hypothesis that it originated from a reverse shock.

The authors first examined the reverse‑shock scenario. By adjusting the electron‑energy fraction (ε_e), magnetic‑field fraction (ε_B), and external density (n), they could reproduce the steep optical decay, but the model inevitably over‑predicted the radio flux by a factor of three to five, contradicting the measured radio light curve. This inconsistency indicates that a simple reverse‑shock origin cannot simultaneously account for both the early optical and the later radio emission.

Next, a partially radiative blast‑wave model was considered. In this framework, only a fraction of the shock‑generated internal energy is radiated away, leading to a rapid decline of the electron population and thus a steep optical decay. While the model can generate the required optical slope, it fails to reproduce the high‑energy LAT emission, especially the early LAT light curve before ~50 s, and cannot match the broadband spectral energy distribution (SED) that spans radio to X‑ray frequencies.

The authors then introduced a two‑component jet model, consisting of a narrow, energetic core (opening angle ≈ 0.05 rad, isotropic‑equivalent energy ≈ 3 × 10⁵⁴ erg) surrounded by a wider, less energetic sheath (opening angle ≈ 0.2 rad, isotropic‑equivalent energy ≈ 5 × 10⁵³ erg). The core jet dominates the early high‑energy LAT and X‑ray afterglow, while the sheath accounts for the long‑lasting radio and optical emission. Electron acceleration is assumed to proceed at the Bohm diffusion limit, allowing electrons to reach Lorentz factors γ_max ≈ 10⁸. Under these optimistic conditions, the 33 GeV photon detected at 80 s can be marginally produced by synchrotron radiation in the forward shock, consistent with the observed t⁻¹·⁵ decay. The model parameters (ε_e, ε_B, external density, electron spectral index p) are tuned to simultaneously satisfy the observed spectral index (β ≈ −1.1) and temporal decay indices across all bands.

The study concludes that neither the reverse‑shock nor the partially radiative blast‑wave models can fully explain the multi‑wavelength data of GRB 090902B. In contrast, the two‑component jet scenario provides a coherent picture: the early LAT photons (including the sub‑MeV prompt component) are likely of internal origin, while the afterglow from ~50 s onward is dominated by external forward‑shock emission from a structured jet. This work highlights the importance of jet stratification in interpreting the broadband behavior of energetic GRBs and suggests that similar multi‑component configurations may be required for other bursts with extended high‑energy emission.