2021RAA....21...72Q

The special GRB 130310A was observed by Fermi Gamma-Ray Burst Monitor and Large Area Telescope, with T₉₀ ∼ 2.4 s. With a combination of a Band function and a blackbody (BB) function, the time-resolved spectral analysis of GRB 130310A confirmed that there is a sub-dominate thermal component in the early period (e.g., slice T₀ + [4.03 - 4.14] s) spectrum with BB temperature (kT) being ∼ 7 ∼ 5 keV, which can be interpreted as photosphere emission. The precursor of GRB 130310A can be fitted well with a BB component with kT ∼ 45 keV, which is higher than that of the main burst. It suggests that the radiation of GRB 130310A is in transition from thermal to non-thermal. Such a transition is an indication of the change in jet composition from a fireball to a Poynting-flux-dominated jet. A very high peak energy is obtained in the first time bin, with the peak energy E_p of the Band component for Band+BB and Band model being ∼ 8.5 ∼ 5.2 MeV and ∼ 11.1 ∼ 7.4 MeV, respectively. Afterwards, the E_p drops to ∼ 1 MeV. The E_p evolution patterns with respect to the pulses in the GRB 130310A light curves show a hard-to-soft evolution. The interpretation of the high peak energy E_p within the photosphere and internal shock model is difficult. It also suggests that at least for some bursts, the Band component must invoke a non-thermal origin in the optically thin region of a GRB outflow. Assuming the redshift is z ∼ 0.1 ∼ 8, the radius of the jet base r₀ ∼ 10⁹ cm to allow (1 + σ₁₅ )> 1 in line with the calculation results of the magnetization parameter at ∼ 10¹⁵ cm (σ₁₅). However, the value of (1 + σ₁₅) is ≃ 1 in the zone z around 3 for r₀ ∼ 10⁹ cm, suggesting the non-excluded possibility that the origin is from ICMART with a low value. The photosphere-internal shock seems capable of interpreting the high peak energy, which requires electron Lorentz factor γ_e ∼ 60 and ε_e ∼ 0.06.