Magnetization Factors of Gamma-Ray Burst Jets Revealed by a Systematic Analysis of the Fermi Sample

The composition of gamma-ray burst (GRB) jets remained a mystery until recently. In practice, we usually characterize the magnetization of the GRB jets (σ0) through the ratio between the Poynting flux and matter (baryonic) flux. With the increasing value of σ0, magnetic energy gradually takes on a dominant role in the acceleration and energy dissipation of the jet, causing the proportion of thermal component in the prompt-emission spectrum of GRBs to gradually decrease or even be completely suppressed. In this work, we conducted an extensive analysis of the time-resolved spectrum for all Fermi GRBs with known redshift, and we diagnose σ0 for each time bin by contrasting the thermal and nonthermal radiation components. Our results suggest that most GRB jets should contain a significant magnetic energy component, likely with magnetization factors σ0 ≥ 10. The value of σ0 seems to vary significantly within the same GRB. Future studies with more samples, especially those with lower-energy spectral information coverage, will further verify our results.