Study On The Jet Composition And Energy Spectrum Characteristics Of Fermi Multi-pulse Gamma-ray Burst

In gamma-ray bursts（GRB）,the composition of jets remains an open question.Several previous studies have suggested that GRB jets are more likely to originate from a hybrid system consisting of a quasi-thermal（hot fireball）component and a non-thermal（Poynting-flux）component at the central engine.In this paper,we first use a top-down approach to confine the multi-pulse Fermi GRBs that detect photospheric components,and then perform statistical analysis on the central engine characteristics and energy spectrum characteristics,to study the jets of GRBs.Composition and radiation mechanism provide clues.We obtained 8 bright multi-pulse gamma-ray bursts with“quiet zones”as samples for analysis.To obtain the observed parameters,we use Bayesian analysis and MCMC methods to fit our samples.Find samples with thermal components according to the Bayesian Information Criterion（BIC）,then infer the central engine parameters:Dimensionless entropyη,central engine magnetization factor1+σ₀,photosphere radius r_ph,photosphere Lorentz factor（?）_ph,photosphere magnetization factor 1+σ_ph,and magnetization factor1+σ_{1 5} at radius r=10¹⁵cm.Through statistical analysis of the central engine characteristics and energy spectrum characteristics in the sample,it is found that the value of dimensionless entropyηis greater than the magnetization factor1+σ₀,and most of the values of1+σ₀ are greater than 1,indicating that in the thermal component-dominated jet,Poynting-flux also played a key role.1+σ_ph and1+σ_{1 5} are equal to 1 in most time slices,indicating that the dissipation mechanism of non-thermal radiation is shock wave（IS）in most time slices in the study sample.The evolution of the low energy spectrum indexαis mainly based on the flux“tracking mode”;while the evolution of the peak energyE_p is more complex,there are“hard to soft”,flux“tracking mode”and“hard to soft”and then in flux“tracking mode”three.an evolutionary behavior.In this paper,detailed time-resolved spectroscopic analysis of GRB 180720B（with a very bright multi-pulse gamma-ray burst）reveals that the thermal components are concentrated at the onset and peak of the pulse,thus supporting the jet flow of the gamma-ray burst.The composition is a transition from matter-dominated to magnetic-dominant.Our results also suggest that the Band function and the synchrotron radiation model may have the same physical origin in the GBM energy range.In addition,GRB 210610B was analyzed in this paper.By thermal component analysis,thermal components are present in about 76.47%of the time-resolved spectra.At the same time,we also use the multicolor black body（mBB）and synchrotron radiation model to fit the spectrum,and find that the spectrum can be well fitted,and the burst can also be well confined by the mixed jet model.Our analytical results also suggest that the empirical model CPL may be interpreted as mBB.