## Abstract

The present work focuses on two astrophysically important research sub-jects: (i) on the possible radiative instabilities in compact gamma-ray sourcesand (ii) on the afterglow emission from the most violent explosions in theuniverse, i.e. from Gamma-Ray Bursts (GRBs). The first subject, although of more theoretical interest, has important implications for astrophysical sources, such as Active Galactic Nuclei (AGN). Instead of studying the applications of radiative instabilities that were discovered in the past, we focused on one newly discovered radiative instability called ‘automatic γ-ray quenching’. First, we specified the conditions which enable the growth of the instability. For this, we studied using analytical methods the stability of the steady-state solutions that describe the following physical system: gamma-rays, that are emitted by some non-thermal radiation process, are being constantly injected into a spherical region that containsa tangled magnetic field, while at the sa ...

The present work focuses on two astrophysically important research sub-jects: (i) on the possible radiative instabilities in compact gamma-ray sourcesand (ii) on the afterglow emission from the most violent explosions in theuniverse, i.e. from Gamma-Ray Bursts (GRBs). The first subject, although of more theoretical interest, has important implications for astrophysical sources, such as Active Galactic Nuclei (AGN). Instead of studying the applications of radiative instabilities that were discovered in the past, we focused on one newly discovered radiative instability called ‘automatic γ-ray quenching’. First, we specified the conditions which enable the growth of the instability. For this, we studied using analytical methods the stability of the steady-state solutions that describe the following physical system: gamma-rays, that are emitted by some non-thermal radiation process, are being constantly injected into a spherical region that containsa tangled magnetic field, while at the same time they may escape from theregion. As a second step, we specified the radiation process responsibe forthe gamma-ray emission. In particular, we assumed that gamma-rays arethe synchrotron radiation of secondary electrons, which are the final decayproduct of charged pions. The latter, are produced through photopion interactions of relativistic protons with some ambient photon field. In other words, we studied the properties of ‘automatic γ-ray quenching’ when this is embedded in a leptohadronic magnetized plasma, i.e. a magnetized plasma that initially consists of relativistic protons, electrons, and, in some cases photons. Using an eigenvector/eigenvalue analysis of the linearized system of differential equations describing the leptohadronic processes, we derived the criteria for the growth of the instability and showed that, if these aresatisfied, the dynamics of the proton-electron-photon system resemble that of a prey-predator one. In some cases, we showed analytically that the photon lightcurve and the energy density of protons oscillate periodically, while we tested our results against those obtained from a fully numerical treatment of the problem. As a final step, we applied the ideas of ‘automatic γ-ray quenching’ to a subclass of AGN, i.e. to γ-ray emitting blazars, for constraining some of their properties, such as their Doppler factor and theirmagnetic field strength; in particular, we chose the blazar 3C 279 which is a prototype of this subclass.The second research subject is relevant to the long-lasting multiwavelength emission that follows the Gamma-Ray Burst itself, the so-called afterglow emission. The theoretical understanding of the GRB afterglow physics changed radically after the first X-ray observations of the Swift satellite, which revealed a whole new class of X-ray afterglow lightcurves. One of the new features is that the X-ray emission does not, in general, decay as a power-law with time but it consists of several power-law segments. In the second part of the present work, we attempted to give an explanation of the newly discovered X-ray afterglow phenomenology. In particular , we showed that different X-ray lightcurve morphologies can be obtained within the standard afterglow model by varying only the maximum Lorentz factor of the electron distribution, which is responsible for the non-thermal multiwavelength afterglow emission. For example, we showed that lightcurvesshowing a shallow decay phase may be obtained if the maximum energy of the distribution is a few times larger than the minimum one. Since the maximum energy of radiating electrons emerged as an important parameter in our analysis, we attempted as a second step, to derive it self-consistently instead of treating it as a free-parameter. For this, we applied the ideas of the ‘box’-model acceleration to the GRB afterglow phase. By modellingin an approximate manner the acceleration timescales and by numerically solving the kinetic equation of electrons including both synchrotron and synchrotron-self Compton cooling, we derived time-dependent solutions of the electron and photon distributions. These solutions are relevant to the GRB afterglow phenomenology only if electron acceleration is mediated by Fermi-type shock acceleration and the escape of electrons from the acceleration zone is fast.

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