Sub-photospheric turbulence as a heating mechanism in gamma-ray bursts

TL;DR

This paper investigates how sub-photospheric turbulence in gamma-ray burst jets can dissipate energy through different regimes, potentially powering nonthermal radiation, and revises existing models of turbulent Comptonization.

Contribution

It identifies two regimes of turbulence dissipation in GRB jets and analyzes their impact on energy transfer and radiation emission, offering new insights into jet dynamics.

Findings

Dissipation regime switches from viscous to collisionless during jet expansion.

Turbulent dissipation can energize plasma particles, leading to nonthermal emission.

Revisions to turbulent Comptonization models in black hole accretion disks are proposed.

Abstract

We examine the possible role of turbulence in feeding the emission of gamma-ray bursts (GRBs). Turbulence may develop in a GRB jet as the result of hydrodynamic or current-driven instabilities. The jet carries dense radiation and the turbulence cascade can be damped by Compton drag, passing kinetic fluid energy to photons through scattering. We identify two regimes of turbulence dissipation: (1) "Viscous" - the turbulence cascade is Compton damped on a scale $\ell_{\rm damp}$ greater than the photon mean free path $\ell_\star$. Then turbulence energy is passed to photons via bulk Comptonization by smooth shear flows on scale $\ell_\star<\ell_{\rm damp}$. (2) "Collisionless" - the cascade avoids Compton damping and extends to microscopic plasma scales much smaller than $\ell_\star$. The collisionless dissipation energizes plasma particles, which radiate the received energy; how the dissipated power is partitioned between particles needs further investigation with kinetic simulations. We show that the dissipation regime switches from viscous to collisionless during the jet expansion, at a critical value of the jet optical depth which depends on the amplitude of turbulence. Turbulent GRB jets are expected to emit nonthermal photospheric radiation. Our analysis also suggests revisions of turbulent Comptonization in black hole accretion disks discussed in previous works.