Study on Energy Spectrum Characteristics of E_P and α “Double Tracking” Gamma-ray Bursts

GRB 131231A is a special Gamma-Ray Burst (GRB) with E_P and α possessing the “dual tracking” behavior. To further analyze the energy spectrum characteristics of the burst, we use the Bayesian method to perform a detailed time-resolved spectral analysis and find that 16 of the 24 resolved spectra have significant thermal components, which are mainly distributed in the early and peak stages of the pulse. Near the peak, the transition from matter-dominated to magnetic-dominated GRB jets is supported. After adding thermal components, parameters E_P and α become hard, while the peak flux F_p is almost unchanged. We calculate the characteristic parameters of the relativistic flow using the composition of the photosphere, and find that the initial radius r₀ of the flow characteristic parameter, the radius of the saturation layer r_s, the radius of the photosphere layer r_ph, and the Lorentz factor Γ during the coast phase of the flow all evolve significantly. The evolution of temperature over time can also be fitted by the inflection power-law model. The power-law index of its rising and falling stages are a=7.65 ± 3.48 and b=-2.10 ± 3.48, respectively. But b has a large deviation from the index value close to -2/3 obtained by previous study of other thermal pulses.