2020ApJ...894..100L

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 σ₀ 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 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 (r₀ = 10⁷, 10⁸, and 10⁹ cm). We find that the dimensionless entropy for all the bursts shows η ≫ 1 while the derived (1+σ₀) 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 r₀ in GRB 081224 and GRB 110721A, the magnetization parameter at ∼10¹⁵ cm (1+σ_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.