Dissertation committee:
Απόστολος Μαστιχιάδης Αναπλ. Καθηγητής ΕΚΠΑ (επιβλέπων), Νεκτάριος Βλαχάκης Επίκ. Καθηγητής, Tsvi Piran Καθηγητής Ινστιτούτο Φυσικής Racah, Πανεπιστήμιο Hebrew
Summary:
The present work focuses on two astrophysically important research subjects:
(i) on the possible radiative instabilities in compact gamma-ray sources and
(ii) on the afterglow emission from the most violent explosions in the
universe, 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 study 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 contains a tangled magnetic field, while
at the same time they may escape from the region. As a second step, we
specified the radiation process responsibe for the gamma-ray emission. In
particular, we assumed that gamma-rays are the synchrotron radiation of
secondary electrons, which are the final decay product 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 are satisfied, 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 their
magnetic 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
lightcurves showing 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 modelling in 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.
Keywords:
Astroparticle physics, Non thermal emission processes, Radiative instabilities, Active galactic nuclei, Gamma-ray bursts
