Statistical Study Of The Spectral Properties Of Gamma-Ray Bursts Main And Post Bursts

The jet composition in gamma-ray bursts(GRBs)is still an unsolved issue.Previous studies have shown that the jet composition of GRBs varies during different outbreak periods,which means that there may be different central engines in each period.In this paper,we systematically investigate the evolution of the instantaneous radiation energy spectrum composition and the constraints on the fireball model by performing detailed energy spectral analyses of GRB 160509 A and GRB 130427 A,which possess both main and post-bursts.These findings not only provide clues to determine whether the main and post-bursts have the same origin,but also deepen our understanding of the nature of GRBs.First,we carried out an energy-spectral composition analysis of GRB160509 A and GRB 130427 A to obtain clues about the composition of the gamma burst jets.Using Bayesian time-resolved spectral analysis,we compared the energy-spectral components and spectral properties of the main and post-bursts.It is found that the thermal component is present in both bursts,but the presence of the thermal component is more pronounced in the main burst,while the post-burst is mainly composed of non-thermal components.Furthermore,we find that in the main burst,some time slices have low-energy spectral indices that exceed the so-called synchrotron dead line,while in the post-burst,only four time slices of GRB 160509 A exceed this dead line,while none of GRB 130427 A exceed the dead line.We then constrain the outflow properties of the two bursts and find the main burst is consistent with the typical properties of photospheric radiation.Thus,our results support a shift in the composition of the jet from fireball-dominated to Poynting flux-dominated.Finally,we analysed the correlation and parametric evolution of the energy spectral parameters of the main and post-bursts and found that the correlation of the energy spectral parameters between the main and postbursts exhibits different behaviours.In addition,the trends of the parameter evolution of the main and postbursts also show consistent and inconsistent behaviour,so we cannot be sure whether the main and postbursts come from the same origin.Next,we sifted through to find a sample of 12 GRBs containing both main and post-bursts.Using time-resolved spectral analysis,we found that the main bursts of these bursts all have a significant thermal component,while the post-bursts have a weaker thermal component.The typical values of the low-energy spectral index a(a =-1)are similar in the main and post-bursts,with more time slices of a above the synchronous dead line in the main burst.Furthermore,when the BB component is added,the avalues for the main and post-bursts do not change much.We then constrained the outflow properties of these bursts and found that the main bursts are consistent with the typical properties of photospheric radiation.Thus,we can see a trend towards a diminishing thermal component from the main burst to the post-burst.Our results support a shift in jet composition from fireball-dominated to Poynting flux-dominated.In addition,we have analysed the correlation and parameter evolution of the energy spectral parameters of the main and postbursts and found that the energy spectral parameters of these bursts show a similar behaviour in the main and postbursts.In addition,the parameter evolution trends of the main and postbursts also show a similar behaviour.Finally,the analysis of the characteristic radius of the photospheric radiation parameters leads us to the trend of the transition from the main burst to the post burst.Therefore,we can conclude that there is a greater possibility that the main and postbursts come from the same origin.