| Gamma-ray Bursts（GRBs）are short-time bursts of gamma-rays that originate at cosmological distances.Since the discovery in 1960s,it has been among the most fas-cinating research areas in astrophysics.Giving the credit to the success of several space missions,people are getting to understand this violent phenomenon better and better.With the accumulation of the observation data,not only we could model a gamma-ray burst and its afterglow in multi-wavelength bands,but also some statistical works of their properties are realized.However,we should keep in mind that there remains sev-eral key issues in this area that still have no unanimous conclusions,such as the central engines,the energy extraction mechanisms,the spectrum of prompt emission,the jet formation and propagation and so on.We need to figure them out in the future.Modern astronomy has entered a multi-messenger era.Except for electromag-netic radiation,we have other approaches to know a certain astrophysical object.Till now,several large experiments have been completed,for instance,the IceCube neu-trino observatory,the Pierre Auger cosmic-ray observatory and the Advanced-LIGO gravitational wave detector.Each of them has published very solid and remarkable observational results.All of these make the frontier of high-energy astrophysics full of vigor,and also put strong constraints on the theoretical models.The research works in this thesis are conducted under this background,and we focus mainly on the high-energy neutrinos.This thesis consists of six chapters.The first chapter is a systematic introduction of gamma-ray burst,high-energy neutrino and cosmic-ray,from both the observational and theoretical points of view.We present the main instruments and some observa-tional highlights at different eras in GRB observation.Then we introduce the detec-tion method,observational results,production mechanisms and oscillation theory of high energy neutrinos,also the observational status,acceleration theory and candidate sources of ultra-high energy cosmic-rays（UHECRs）.Besides,we briefly introduce the collapsar model and derive the dynamics of jet propagation in the envelope of a GRB progenitor.Based on the jet propagation dynamics in the envelope,we calculate the neutrino emission in the jet propagation process of a single gamma-ray burst event in the second chapter,in which we take into account the collimation of the jet for the first time.Our work is much more accurate and reliable than previous works that treated the shape of the jet as a cone.We consider a low-luminosity GRB（LL-GRB）and an ultra-long GRB（UL-GRB）separately,for the reason that they belong to different collimation regimes.We find that PeV neutrinos can be produced in both cases and we obtain their neutrino spectrum and estimate the number of neutrinos in IceCube detector for an individual event,which is 4.2 × 10-3 and 8.3 × 10-2 for LL-GRB/UL-GRB respec-tively.Although it is not easy to detect right now,with the accumulating observation time and the upgrade of IceCube experiment,we can still hope to observe the neutrino precursor of GRBs in the future.Besides,we find that the neutrino spectrum depends on the power-law index of envelope density distribution,making it possible to probe the progenitor envelope with neutrino spectrum in the future.Further in the third chapter,we discuss the issue of neutrino oscillation and fore-cast the flavor ratio of these GRB neutrinos when they reach our Earth.Since very high-energy neutrinos up to PeV can be produced in the jet propagation process of a low-luminosity GRB,their oscillation properties are very different from previous works that concentrated on MeV-TeV neutrinos.For these high-energy neutrinos,the adiabatic conversion approximation is violated and first we need to correct the level crossing effect.We find that the effective mixing angles in the envelope matter are close to zero for PeV neutrinos,thus the mutual transition probabilities are constant and we can expect a constant flavor ratio φνe:φνμ:φντ（?）0.30:0.37:0.33 on Earth.Note that this value may be different since there is some uncertainty in vacuum oscillation parameters and neutrino mass hierarchy.For neutrinos of subPeV or even lower energies,the flavor ratio depends strongly on the envelope density distribution,providing an alternative way to probe the progenitor property in the future.The next two chapters are based on the high-energy neutrino events observed by IceCube,in which we discuss the possible astrophysical origins of them.In the fourth chapter we revisit the star-forming（SFGs）and star-burst galaxies（SBGs）scenario,in which the supernova（SNR）and hypernova remnants（HNR）accelerate CRs and produce high-energy neutrinos.We take into account possible time-dependent effects of the CR acceleration during the SNR evolution for the first time,and find that this will have an impact on the final neutrino spectrum.More importantly,we consider possible contributions of Pop III HNRs up to redshift z（?）10 and show that they are not constrained by the gamma-ray data due to effective absorption of accompanying gamma-rays by the extragalactic background light.At last,we get the two component fitting with low-redshift normal SNRs/HNRs and high-redshift Pop III HNRs,which is capable to explain the diffuse neutrino flux without violating the diffuse gamma-ray background and also making this scenario complete and reasonable.In the fifth chapter,we propose for the first time that white dwarf mergers can serve as sources of IceCube high-energy neutrinos,and calculate the contribution to the diffuse flux in detail.Assuming that the accelerated CR spectrum to be a power-law,the final neutrino spectrum is characterized by two suppression factors.One is the suppression due to other proton cooling mechanisms competing with pp and pγ inter-action,the other suppression is due to pion cooling.Allowing for the uncertainty of the total CR injection power,we consider a nominal case and an optimistic case.If we adopt the optimistic parameters,it is very likely that we can detect the neutrino signal of individual merger event located at a distant of Mpc.Moreover,the contribution of all WD mergers to the diffuse neutrino flux is significant in this case.Note that WD merger events belong to "hidden cosmic-ray accelerators" because the accompanying gamma-rays are absorbed within these sources,consistent with the non-detection of gamma-rays with Fermi-LAT.The sixth chapter is an ending of this thesis.We summarize some key problems remaining in this area and give some perspectives. |
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