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

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 epsilon(peak) similar to few x100 keV. We show that this model naturally explains several key observational features: (1) a high-energy power-law index beta(1) 2 to 5 with an intermediate thermal spectral region; (2) decay of the prompt emission light curve as similar to t(-2); (3) delay of soft photons; (4) a peak energy-isotropic energy (the so-called Amati) correlation, epsilon(peak) similar to e(iso)(m), with m similar to 0.45, resulting from different viewing angles (at low luminosities, our model predicts an observable turnoff in the Amati relation); (4) an anticorrelation between the spectral FWHM and time as t(-1); (6) temporal evolution epsilon(peak) similar to t(-1), accompanied by an increase of the high-energy spectral slope with time; and (7) distribution of peak energies epeak 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.

Automated summary

In this study, a prompt emission model for gamma-ray bursts (GRBs) is proposed where the jet does not cross a baryonic cork, but rather photons emitted deep in the flow are scattered inside and escape from the expanding cork. This model explains key observational features such as high-energy power-law index, prompt emission light curve decay, delay of soft photons, and peak energy-isotropic energy correlation.