Abstract
In the past years, based on the observations of Fermi/LAT, indeed, the GeV emission component
exists during Gamma-ray Bursts (GRBs), not only in the prompt emission phase , but also
in the afterglow phase. The observations of the GeV emissions give a new chance for us to
understand the physical origin and the radiation mechanism of GRBs. Besides, the origin of
Ultra-high Energy Cosmic Rays (UHECRs) has always been a great contention of high energy
astrophysics. During the propagation of UHECRs, they would interact with the background
photons through the photonpion process and the subsequent high energy neutrinos, electrons
and photons would be produced. The cascades of the secondary electrons and photons would
effect the detections of them from nearby universe, as well as the diffused GeV photons background.
GRBs have proposed as a possible candidate of the source of UHECRs, and if so, it
is unavoidable to generate the neutrinos during the interactions between the UHECRs and the
keV-MeV emission of GRBs. According to the non/detections of neutrinos corresponding to
GRBs by IceCube, some constrains on the baryon loading and the interaction efficiencies of
hadronic processes in the outflows of GRBs can be given. In this thesis, the origin of the high
energy emission of GRBs and the cascades of secondaries produced during the propagations of
the UHECRs would be studied.
In the first Chapter, the basic properties of the observations of GRBs, including the spectra,
lightcurves, the fireball model and the internal-external forward-reverse shocks model have been
introduced. Besides, the GeV emission during the prompt phases and X-ray flares of GRBs is
discussed as well. The possible sources of UHECRs and the advantages and challenges of
GRBs as the sources of UHECRs are introduced. In last, a summary of some basic radiation
processes in the GRBs or other astrophysical phenomena is given.
In the second Chapter, The extra GeV component observed by Fermi/LAT during the
prompt phases of gamma-ray bursts (GRBs) could arise from the cascades emission of the
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secondaries generated by the hadronic processes, including the photomeson and Bethe-Heitler
processes. However, suggested by many papers, the highest energy of proton given by the Bohm
approximation could not be reached in ultra-relativistic shock waves. So in the limited acceleration
of protons, we reinvestigate the cascades emission inside of the proton-dominated outflows
of GRBs, which could be responsible to the extra GeV component in three classifications, i.e.,
090926A-type, 090902B-type and 080916C-type. Besides, according to the hadronic model,
the expected neutrinos flux is given as well by calibrating the GRB parameters through fitting
the extra GeV spectra. Based on the calibrated parameters, for all LAT GRBs, the hadronic
model could be tested by the upper limit given by IceCube observations in the future.
In the third Chapter, the GeV emission during the X-ray flares of GRB 100728A is studied.
Based on the late internal shock, we investigate the dynamics of the shell-shell collision
and corresponding radiation. The electrons would be accelerated to the very high energy by the
forward-reverse shocks formed during shell-shell collision, and the keV-MeV photons would be
upscattered by these electrons through the inverse Compton scattering. In addition to the synchrotron
self-Compton, the cross inverse Compton sacttering between the electrons and photons
inside of the forward and reverse shocks is also considered. By the analytical and numerical
calculations, the spectrum of the GeV emission of GRB 100728A is well explained.
In the fourth Chapter, the propagation of ultra-high energy (UHE) photons generated during
the interactions between the UHECRs and cosmic background radiation is studied. Taking
all possible interaction processes, we found the effective penetration distance of UHE photons
can be one order larger than the mean free length of UHE photons in the absence of strong
intergalactic magnetic field and radio background. In principle, according to the upper limit of
UHE photons given in nearby universe, the constrains of the propagation environment can be
given. Moreover, muon pairs can be produced in the annihilation of ultrahigh energy (UHE,
E & 1018 eV) photons with low energy cosmic background radiation in the intergalactic space,
giving birth to neutrinos. Although the branching ratio of muon pair production is low, products
of other channels, which are mainly electron/positron pairs, will probably transfer most of
their energies into the new generated UHE photon in the subsequent interaction with the cosmic
background radiation via Compton scattering in deep Klein-Nishina regime. The regeneration
of these new UHE photons then provides a second chance to produce the muon pairs, enhancing
the neutrino flux. Through taking several environments into account, we find that an extra
component of UHE neutrinos will arise from the propagation of UHE cosmic rays due to the
generated UHE photons and electron/positrons. This component is with a flux of at most 10%
of that of the conventional cosmogenic neutrino at a few EeV, in the absence of a strong intergalactic
magnetic field and a strong cosmic radio background. The precise contribution of extra
component depends on several factors, e.g., cosmic radio background, intergalactic magnetic
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field, and the spectrum of proton, which are discussed in this chapter.
In the fifth Chapter, the low energy photon index of prompt phase of GRBs is studied.
The prompt emission of most of gamma-ray bursts (GRBs) typically exhibits a non-thermal
Band component. The synchrotron radiation in the popular internal shock model is generally
put forward to explain such a non-thermal component. However, the low-energy photon index
α ∼ −1.5 predicted by the synchrotron radiation is inconsistent with the observed value α ∼
−1. Here we investigate the evolution of a magnetic field during propagation of internal shocks
within an ultrarelativistic outflow, and revisit the fast cooling of shock-accelerated electrons via
synchrotron and synchrotron self-Compton emission for this evolutional magnetic field. We
find that the magnetic field is first nearly constant and then decays as B′ ∝ t−1, which leads to
a reasonable range of the low-energy photon index, −3/2 < α < −2/3. In addition, if a rising
electron injection rate during a GRB is introduced, we find that α reaches −2/3 more easily.
We thus fit the prompt emission spectra of GRBs 080916c and 080825c. Finally, since there is
only the Band component without a high-energy excess for most of GRBs, we obtain a general
constraint on the ratio of the two energy equipartition factors behind the shocks, ϵe/ϵB . few.
In the last Chapter, a summary and prospect of the high energy emission of GRBs and
UHECRs is given. We hope, in future, more observations can make some questions clear, including
the acceleration mechanism and the baryon loading of GRBs, GRBs-UHECRs scenario,
the anisotropy and the chemical component of UHECRs.
