Statistical Properties of Gamma-Ray Burst Polarization

TL;DR

This paper uses Monte Carlo simulations to analyze how future gamma-ray burst polarimetry missions can distinguish between different emission models based on polarization measurements, helping to understand GRB jet magnetic fields.

Contribution

It introduces a simulation-based framework to predict polarization distributions for various GRB emission models and proposes criteria for model discrimination using upcoming polarimetry data.

Findings

Polarization measurements can differentiate emission models.

The ratio N_m/N_d is a key discriminator.

High polarization degrees (>0.8) favor the Compton drag model.

Abstract

The emission mechanism and the origin and structure of magnetic fields in gamma-ray burst (GRB) jets are among the most important open questions concerning the nature of the central engine of GRBs. In spite of extensive observational efforts, these questions remain to be answered and are difficult or even impossible to infer with the spectral and lightcurve information currently collected. Polarization measurements will lead to unambiguous answers to several of these questions. Recent developments in X-ray and gamma-ray polarimetry techniques have demonstrated a significant increase in sensitivity enabling several new mission concepts, e.g. POET (Polarimeters for Energetic Transients), providing wide field of view and broadband polarimetry measurements. If launched, missions of this kind would finally provide definitive measurements of GRB polarizations. We perform Monte Carlo simulations to derive the distribution of GRB polarizations in three emission models; the synchrotron model with a globally ordered magnetic field (SO model), the synchrotron model with a locally random magnetic field (SR model), and the Compton drag model (CD model). The results show that POET, or other polarimeters with similar capabilities, can constrain the GRB emission models by using the statistical properties of GRB polarizations. In particular, the ratio of the number of GRBs for which the polarization degrees can be measured to the number of GRBs that are detected (N_m/N_d) and the distributions of the polarization degrees (Pi) can be used as the criteria. If N_m/N_d > 30% and Pi is clustered between 0.2 and 0.7, the SO model will be favored. If instead N_m/N_d < 15%, then the SR or CD model will be favored. If several events with Pi > 0.8 are observed, then the CD model will be favored.