The Connection Between Thermal and Non-Thermal Emission in Gamma-ray Bursts: General Considerations and GRB090902B as a Case Study

TL;DR

This paper presents a comprehensive model linking thermal and non-thermal emissions in gamma-ray bursts, demonstrated through analysis of GRB090902B, revealing insights into the physical conditions and energy dissipation mechanisms.

Contribution

It introduces a self-consistent analysis method connecting thermal and non-thermal spectra in GRBs, applied to GRB090902B, challenging the internal shock model for energy dissipation.

Findings

Lorentz factor estimated between 920 and 1070

Photospheric radius around 7.2-8.4 x 10^{11} cm

High energy dissipation fraction (~85-95%)

Abstract

Photospheric (thermal) emission is inherent to the gamma-ray burst (GRB) "fireball" model. We show here, that inclusion of this component in the analysis of the GRB prompt emission phase naturally explains some of the prompt GRB spectra seen by the Fermi satellite over its entire energy band. The sub-MeV peak is explained as multi-color black body emission, and the high energy tail, extending up to the GeV band, results from roughly similar contributions of synchrotron emission, synchrotron self Compton(SSC) and Comptonization of the thermal photons by energetic electrons originating after dissipation of the kinetic energy above the photosphere. We show how this analysis method results in a complete, self consistent picture of the physical conditions at both emission sites of the thermal and non-thermal radiation. We study the connection between the thermal and non-thermal parts of the spectrum, and show how the values of the free model parameters are deduced from the data. We demonstrate our analysis method on GRB090902B: We deduce a Lorentz factor in the range 920 <= \eta <= 1070, photospheric radius r_{ph} ~ 7.2 - 8.4 * 10^{11} cm and dissipation radius r_\gamma >= 3.5 - 4.1 * 10^{15} cm. By comparison to afterglow data, we deduce that a large fraction, epsilon_d ~85% - 95% of the kinetic energy is dissipated, and that large fraction, ~equipartition of this energy is carried by the electrons and the magnetic field. This high value of epsilon_d questions the "internal shock" scenario as the main energy dissipation mechanism for this GRB.