A Backscattering-dominated Prompt Emission Model for the Prompt Phase of Gamma-Ray Bursts

Abstract

Abstract As a gamma-ray burst (GRB) jet drills its way through the collapsing star, it traps a baryonic “cork” ahead of it. Here we explore a prompt emission model for GRBs in which the jet does not cross the cork, but rather photons that are emitted deep in the flow largely by pair annihilation are scattered inside the expanding cork and escape largely from the back end of it as they push it from behind. Due to the relativistic motion of the cork, these photons are easily seen by an observer close to the jet axis peaking at ε peak ∼ few ×100 keV. We show that this model naturally explains several key observational features: (1) a high-energy power-law index β 1 − 2 to − 5 with an intermediate thermal spectral region; (2) decay of the prompt emission light curve as ∼ t −2; (3) delay of soft photons; (4) a peak energy–isotropic energy (the so-called “Amati”) correlation, ε peak ∼ ε iso m , with m ∼ 0.45, resulting from different viewing angles (at low luminosities, our model predicts an observable turnoff in the Amati relation); (5) an anticorrelation between the spectral FWHM and time as t −1; (6) temporal evolution ε peak ∼ t −1; and (7) distribution of peak energies ε peak in the observed GRB population. The model is applicable for single-pulse GRB light curves and their respective spectra. We discuss the consequences of our model in view of current and future prompt emission observations.