Time-dependent photospheric radiative transfer in structured GRB jets: spectral evolution and polarization diagnostics

TL;DR

This study models the time-dependent radiative transfer in structured GRB jets, linking jet structure, dissipation, and polarization to observable spectral and polarization signatures, providing testable predictions for future polarimetry.

Contribution

It introduces a coupled SRHD and Monte Carlo radiative transfer framework that accounts for jet structure, dissipation, and polarization evolution in GRB prompt emission.

Findings

Jet structure and optical depth geometry influence spectral and polarization evolution.

Dissipation depth and pair loading affect spectral stability and high-energy tails.

Model predictions can be tested with upcoming high-energy polarimeters.

Abstract

Photospheric emission from relativistic gamma-ray burst (GRB) jets is a promising mechanism for producing the Band-like spectra observed in the prompt phase, yet the connections between jet structure, dissipation location, and polarization signatures remain unclear. We investigate time-dependent photospheric radiation transfer in structured relativistic jets by coupling two-dimensional axisymmetric special relativistic hydrodynamic (SRHD) simulations with Monte Carlo photon propagation. Photon escape and subphotospheric dissipation are characterized using the residual line-of-sight optical depth tau_out evaluated along each photon trajectory, allowing a direction-dependent treatment of photon decoupling in structured jets. The radiative transfer includes Klein-Nishina Compton scattering and polarization evolution using the Mueller matrix formalism. We perform a systematic parameter study exploring the effects of viewing angle, electron-positron pair loading (Z_pm), and the optical-depth window of subphotospheric dissipation. The model produces time-resolved spectra, peak-energy evolution E_pk(t), Band parameters, polarization degree Pi(E,t), and last-scattering statistics. We find that jet angular structure and the geometry of the line-of-sight optical depth strongly regulate spectral evolution and polarization signatures. The dissipation depth and pair loading jointly control the stability of E_pk, the formation of high-energy spectral tails, and the energy dependence of polarization. These results provide quantitative predictions for GRB prompt-emission spectra and polarization that can be tested with current and upcoming high-energy polarimeters.