GRB 130310A: very high peak energy and thermal emission

Abstract

Abstract The special GRB 130310A was observed by Fermi Gamma-Ray Burst Monitor and Large Area Telescope, with T 90∼ 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 ∼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 Ep 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 Ep drops to ∼ 1 MeV. The Ep 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 Ep 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 r 0 ∼ 109 cm to allow (1 + σ 15) > 1 in line with the calculation results of the magnetization parameter at ∼1015 cm (σ 15). However, the value of (1 + σ 15) is ≃ 1 in the zone z around 3 for r 0 ∼ 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 γe ∼ 60 and εe ∼ 0.06.