A backscattering dominated prompt emission model for the prompt phase of Gamma ray bursts

Abstract

As 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 ${\epsilon }_{peak}\sim few\times 100keV$. We show that this model naturally explains several key observational features including: (1) High energy power law index ${\beta }_1\sim -2\mathrm{to}-5$ with an intermediate thermal spectral region; (2) decay of the prompt emission light curve as $\sim t^{-2}$; (3) Delay of soft photons; (4) peak energy - isotropic energy (the so-called Amati) correlation, ${\epsilon }_{peak}\sim {\epsilon }_{iso}^m$, with $m\sim 0.45$, resulting from different viewing angles. At low luminosities, our model predicts an observable turn off in the Amati relation. (4) An anti-correlation between the spectral full width half maxima (FWHM) and time as $t^{-1}$. (5) Temporal evolution ${\epsilon }_{peak}\sim t^{-1}$, accompanied by an increase of the high energy spectral slope with time. (6) Distribution of peak energies ${\epsilon }_{peak}$ in the observed GRB population. The model is applicable for a single pulse GRB lightcurves and respective spectra. We discuss the consequence of our model in view of the current and future prompt emission observations.

Accepted for publication in ApJ, 20 pages, 17 Figures

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