GRB 130310A: very high peak energy and thermal emission

The special GRB 130310A was observed by Fermi Gamma-Ray Burst Monitor and Large Area Telescope, with T (90)similar to 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 T (0) + [4.03 - 4.14] s) spectrum with BB temperature (kT) being similar to 7 similar to 5 keV, which can be interpreted as photosphere emission. The precursor of GRB 130310A can be fitted well with a BB component with kT similar to 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 similar to 8.5 similar to 5.2 MeV and similar to 11.1 similar to 7.4 MeV, respectively. Afterwards, the E-p drops to similar 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 similar to 0.1 similar to 8, the radius of the jet base r (0) similar to 10(9) cm to allow (1 + sigma (15)) > 1 in line with the calculation results of the magnetization parameter at similar to 10(15) cm (sigma (15)). However, the value of (1 + sigma (15)) is similar or equal to 1 in the zone z around 3 for r (0) similar to 10(9) 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 gamma(e) similar to 60 and epsilon(e) similar to 0.06.