Abstract
Mon. Not. Roy. Astron. Soc. 417 (2011) 1584-1600 Many previous studies have determined that the long lasting emission at
X-ray, optical and radio wavelengths from gamma-ray bursts (GRBs), called the
afterglow, is likely produced by the external forward shock model. In this
model, the GRB jet interacts with the circum-stellar medium and drives a shock
that heats the medium, which radiates via synchrotron emission. In this work,
we carried out a detailed analysis of the late time afterglow data of GRB
090902B using a very careful accounting of the Inverse Compton losses. We find
that in the context of the external forward shock model, the only viable option
to explain the X-ray and optical data of GRB 090920B is to have the electron
energy distribution deviate from a power-law shape and exhibit some slight
curvature immediately downstream of the shock front (we explored other models
that rely on a single power-law assumption, but they all fail to explain the
observations). We find the fraction of the energy of shocked plasma in magnetic
field to be ~10^{-6} using late time afterglow data, which is consistent with
the value obtained using early gamma-ray data. Studies like the present one
might be able to provide a link between GRB afterglow modeling and numerical
simulations of particle acceleration in collisionless shocks. We also provide
detailed calculations for the early (< 10^3 s) high energy (> 100 MeV) emission
and confirm that it is consistent with origin in the external forward shock. We
investigated the possibility that the ~10 keV excess observed in the spectrum
during the prompt phase also has its origin in the external shock and found the
answer to be negative.