Thermal Components in Gamma-ray Bursts. II. Constraining the Hybrid Jet Model

TL;DR

This study applies the hybrid jet model to a sample of gamma-ray bursts, revealing that most bursts involve both hot fireball and cold Poynting-flux components, and magnetic reconnection may power their nonthermal emission.

Contribution

It extends the hybrid jet model analysis to a larger GRB sample, providing new insights into jet composition and emission mechanisms.

Findings

Most bursts have high dimensionless entropy, indicating hot fireball dominance.

Several bursts show magnetization parameters suggesting magnetic reconnection as the emission mechanism.

The hybrid jet model can explain the observed properties of the majority of GRBs.

Abstract

In explaining the physical origin of the jet composition of gamma-ray bursts (GRBs), a more general picture, i.e. the hybrid jet model (which introduced another magnetization parameter $\sigma_{0}$ on the basis of the traditional fireball model), has been well studied in Gao \& Zhang. However, it still has not yet been applied to a large GRB sample. Here, we first employ the "top-down" approach of Gao \& Zhang to diagnose the photosphere properties at the central engine to see how the hybrid model can account for the observed data as well, through applying a {\it Fermi} GRB sample (eight bursts) with the detected photosphere component, as presented in Li (our Paper I). We infer all physical parameters of a hybrid problem with three typical values of the radius of the jet base ($r_{0}$ = 10$^{7}$, 10$^{8}$, and 10$^{9}$ cm). We find that the dimensionless entropy for all the bursts shows $\eta\gg$ 1 while the derived (1+$\sigma_{0}$) for five bursts (GRB 081224, GRB 110721A, GRB 090719, GRB 100707, and GRB 100724) is larger than unity, indicating that in addition to a hot fireball component, another cold Poynting-flux component may also play an important role. Our analysis also shows that in a few time bins for all $r_{0}$ in GRB 081224 and GRB 110721A, the magnetization parameter at $\sim$ 10$^{15}$cm (1+$\sigma_{\rm r15}$) is greater than unity, which implies that internal-collision-induced magnetic reconnection and turbulence may be the mechanism to power the nonthermal emission, rather than internal shocks. We conclude that the majority of bursts (probably all) can be well explained by the hybrid jet problem.