Study On The Origin Of X-ray Flare And Thermal Components In Gamma-ray Bursts

Both the Fireball photosphere model and the Cocoon model predict that there should be thermal component in the spectrum of GRBs.However,the GRBs with distinct thermal components are rarely detected,especially the cases with thermal components throughout the whole prompt phase.Observation shows that thermal components have been identified in some X-ray flares,and its physical origin has not been clearly explained.In this work,GRB 170519A(z=0.818)and GRB 190109A are taken as examples to study the origin of thermal components and X-ray flare in GRBs.GRB 170519A is characterized by bright X-ray flare observed by Swift/XRT,which also have observed optical data with The Katzman Automatic Imaging Telescope(KAIT)and other telescopes.We use the multi-wave lightcurves data to do the analysis and get the results are as follow(1)Based on the joint spectra analysis used the BAT,XRT and Optical data,the first X-ray falre(Flare 1)is well described by PL function and the second X-ray falre(Flare 2)is well described by PL function plus a thermal components(BB);(2)The relation between isotropic luminosity LBB and temperature of blackbody is LBB,(170-240s)∝kT 3.8±1.0,which is consistent with the prediction of phothsphere model.We suggest that the thermal components is originate from the fireball photosphere The thermal components in the X-ray flare indicates that it is related to the central engine reactivation Both the Flare 2 and soft y-ray pulse are the same components,which can well described by PL function and be produced by later activities of the central engine.(3)Assume the thermal components originate from the photosphere of the jet,we estimate the photosphere parameters,and derive the photosphere radius,rph～(1.52～6.01)×1012 cm,and the bulk Lorentz factor of the photosphere,Γph～20-26.(4)The multi-wave lightcurves of later afterglow show achromatic light-curve behavior,consistent with the predictions of the external shock model.The best-fitting parameters of the standard external forward shock model are Ek,iso～9.0×1053 erg,Γ0～60,εe～0.014,εB～1.1×10-4,n～15,θj～0.08 rad and p～2.39.Considering the εB evolves with time in the early phase,εB,t<550 s ∝ t-3.66±0.11,εB,550<t<2000 ∝ t-0.47±0.11,the lightcurves is consistent with the predicted lightcurve of external shock model.GRB 190109A is a special long duration burst.Through the systematic study of the joint spectrum of prompt phase and afterglow analysis,we obtain the main results are as follow:(1)The time-integrated and time-resolved spectra analysis results show that the prompt emission is dominated by thermal components and also throughout four pulse.The BB temperature,kT,varies from 24.7 to 8.2 keV for whole prompt phase.(2)We also obtain the relation of F∝kT-0.40 for the early Phase(Pulse I)and Foc kT3.33±0.76 for the late Phase(Pulse Ⅱ-Ⅳ),respectively.The significant deviation of the kT-F relation in the early epoches from that in the late epochs likely suggests it may be the transition from cocoon surroundings by a jet to the photosphere of matter-dominated jet(3)We derive the photosphere parameters are rph～1×1011-2×1012 cm,the bulk Lorentz factor of the photosphere Γph～50—500 and the value of dimensionless is range from 200 to 5000Based on the systematic analysis of GRB 170519A and GRB 190109A,we find that the characteristic of thermal components dominated at least in some GRBs.The thermal components,at different time phase,may correspond to different physical processes of jet or central engine.The physical processes that produce thermal radiation are cocoon or photosphere model in the jet.These two kinds of radiation mechanisms may be dominated by different mechanisms in different periods.