Curvature Effects in Gamma Ray Burst Colliding Shells

Abstract

An elementary kinematic model for emission produced by relativistic spherical colliding shells is studied. The case of a uniform blast-wave shell with jet opening angle $θ_j \gg 1/Γ$ is considered, where $Γ$ is the Lorentz factor of the emitting shell. The shell, with comoving width $Δr^\prime$, is assumed to be illuminated for a comoving time $Δt^\prime$ and to radiate a broken power-law $νL_ν$ spectrum peaking at comoving photon energy $\e_{pk,0}^{\prime}$. Synthetic GRB pulses are calculated, and the relation between energy flux and internal comoving energy density is quantified. Curvature effects dictate that the measured $νF_ν$ flux at the measured peak photon energy $\e_{pk}$ is proportional to $\e^3_{pk}$ in the declining phase of a GRB pulse. Possible reasons for discrepancy with observations are discussed, including adiabatic and radiative cooling processes that extend the decay timescale, a nonuniform jet, or the formation of pulses by external shock processes. A prediction of a correlation between prompt emission properties and times of the optical afterglow beaming breaks is made for a cooling model, which can be tested with Swift.