Relation Between The GRB Rate And The Cosmic Star Formation Rate

Gamma-ray bursts (GRBs) are the most luminous explosions ever detected in the Universe, which are short intense flashes of gamma-rays from cosmological distances. GRBs has become one of the major focuses of research ever since they were first detected in1967. Although great progress has been made over the last forty years, especially after the launch of Swift and Fermi satellite, there is still many problems remain poorly understood, including the nature of the central engine of GRBs. The main goals of this thesis focus on analyzing the relation between the GRB rate and the star formation rate to gain some insight on the nature of the progenitors of GRBs, based on the latest observational data of GRBs.We first briefly review the history of the study of GRBs in Chapter1. We summarize the main observational characteristics of GRBs and their host galax-ies in Chapter2, and theoretical models are summarized in Chapter3. Then two investigations on the relation between the star formation rate and the GRB rate are presented in Chapter4and Chapter5, for long GRBs and short GRBs, respectively.The first investigation is about the metallicity of the progenitor of long GRBs. Based on an extensive sample of162long GRBs, we analyze various models of star formation rate and the possible effect of the evolution of cosmic metallicity under the assumption that LGRBs tend to occur in low-metallicity galaxies. The models of star formation rate tested in this work include empirical fits from obser-vational data as well as a self-consistent model calculated in the framework of the hierarchical structure formation. Comparing with the observational data, we find a relatively higher metallicity cut of Zth>0.6Z⊙, in contrast to previous work which suggest a cut of～0.1-0.3Z⊙. Especially when considering a self-consistent star formation model, the calculated expectations show strong consistency with the observational data, requiring no metallicity cut at all. These results imply that there is no strong metallicity preference for the host galaxy of LGRBs, in contrast to previous work. Our results also show that the inferred dependencies of LGRBs on their host galaxy properties are strongly related to the specific models of star formation rate one adopts. Furthermore, a significant fraction of LGRBs occur in small dark matter halos down to3×108M⊙can provide an alterna-tive explanation for the discrepancy between the star formation rate history and LGRB rate history.The second investigation is about delay time distribution of progenitors of short GRBs. We constrain the luminosity function of SGRBs using an extended sample of30Swift SGRBs with known redshift and fluence. Based on this best-fit luminosity function, the rate of SGRBs under various theoretical delay time distributions is calculated and compared to the observational data. We find that both of the models of a lognormal delay distribution with T*=3Gyr and a power law (T-1) delay distribution are consistent with the observational data, in contrast to previous studies. These results strongly support the "primordial" NS-NS systems as the SGRB progenitors. However, it does not rule out the other systems with long time delays, as we find that there is little difference between the predictions of the model of the power law (T-1) delay distribution and that of the model of a narrow lognormal delay distribution with (T*=3Gyr). Overall, our result is significantly smaller than previous estimates.Conclusions and open issues are presented in Chapter6.The cosmological parameters used are Ωm=0.266,Ω,=0.734, Ωb=0.0449, h=0.71and σ8=0.801.