Research On The Radiation Of High Energy Photons From Gamma-ray Bursts

Gamma-ray Bursts(GRBs)are extremely violent explosions of gamma-rays in very short time in the universe.Their intensity,randomness and non-repeatability have attracted so many researchers.Since the first case of GRB from outer space was de-tected in 1967,researchers have made great progress in observing their behaviour and developed relevant theories to explain what they have seen.Based on former studies about GRBs of other researchers,a brief review on observations and theoretical descrip-tions of GRBs and the study in high energy photon emission from GRBs are presented in this article.In Chapter 1,an introduction to the feature of spectrum and light curves from GRBs is given.The flux of GRBs decays with different index respectively in low energy band and high energy band.Some GRBs have a break or cutoff in high energy band,which is relevant with the study as follows in chapter 3.It is supposed that there are two phases with the expansion of GRB fire ball,one is prompt phase that produces prompt emission,and the other is afterglow phase which results in complex light curves.It is believed that synchrotron emission from a decelerating relativistic shell that collides with an external medium is the radiation mechanism of afterglow,while the Synchrotron Self-Compton Emission can explain the high energy photons from GRBs.According to the duration of prompt emission,GRBs are generally classified as long GRBs with T90>2 s and short GRBs with T90<2 s.In terms of the origin of GRBs,they can also be sorted into two types,GRBs from the death of massive stars and GRBs from the merge of two compact objects.Chaper 2 is about the background of high energy photon radiation from GRBs.Time delay of arrival of high energy photons,extension of high energy radiation and an extra component in spectrum are main characteristics of GRB high energy emission.Time delay is thought to result from different radiation zone or the change of radiation density with expansion.Extra component and extension of spectrum are supposed to be related with the radiation mechanism.Synchrotron Self-Compton Emission is the possible reason,cascade reaction of particles and acceleration of protons as well.Chapter 3 shows the study of the constraints on the bulk Lorentz factor of gamma-ray bursts with the detection rate by LAT.Some GRBs have breaks or cuttoffs in high energy band of spectrum,which is assumed to be due to the pair-production attenuation.Given the luminosity function,the source evolution function and the properties of the GRB spectrum,the authors simulated a LGRB sample.They extended the Band spec-trum of the sample to high energy band(>100 MeV),taking into account the attenuation effect,calculated the detection rate of high energy photons from simulated sample,then compared it with the real LAT detection rate,found a distribution of GRB bulk Lorentz factor which could make the rate of sample consistent with that of the observed one.So constraints on the bulk Lorentz factor of GRBs are gained.The distribution shows that the bulk Lorentz factor of most GRBs lies between 50 and 250.The constraints are about the GRB group,which is different from the former studies that focused on individual GRBs.Chapter 4 presents the summary of this article and the research prospects of rele-vant fields.