| dc.description.abstract | Relativistic jets, highly collimated and high-velocity outflows of particles and electromagnetic
radiation, are a common phenomenon associated with various astrophysical objects
including stellar-mass compact objects such as white dwarfs, neutron stars, and black
holes, as well as supermassive black holes residing in the centers of active galaxies. Relativistic
jets have been observed to propagate over immense distances, ranging from parsecs
to kiloparsecs, while maintaining their momentum and kinetic energy. Despite extensive
ongoing research, several unsolved problems persist in the study of relativistic jets. These
include comprehending the processes of jet formation, collimation, particle acceleration,
long-range stability, interactions with the surrounding environment, and complex radiative
mechanisms. Mechanisms such as Fermi-type acceleration, magnetic reconnection,
and plasma instabilities have been proposed to explain particle acceleration within relativistic
jets. These mechanisms involve the interaction of particles with magnetic fields,
resulting in the transfer of energy and acceleration to relativistic speeds. Blazars, a type of
Active Galactic Nuclei (AGNs), exhibit distinctive Spectral Energy Distributions (SEDs)
consisting of two broad, non-thermal components. The lower energy bump originates from
synchrotron emission by relativistic leptons and extends from radio to optical/UV or Xrays
in the case of High-frequency-peaked BL Lacs (HBLs). In leptonic models, the higher
energy bump is attributed to inverse Compton scattering, where the same leptons scatter
synchrotron or external photons.
Empirical observations and theoretical analyses, incorporating Magneto-hydrodynamic
(MHD) simulations, substantiate the presence of radial stratification within jets emitted
from AGNs. This stratification manifests as an inner spine, characterized by high velocities,
encompassed by an outer sheath with comparably slower motion. The interface
between these distinct regions engenders SBLs, resulting from the velocity shear and disparate
hydrodynamic characteristics observed between the spine and sheath. Those SBLs
within jets from AGNs and GRBs hold promising prospects as sites for relativistic particle
acceleration.
This thesis centers on investigating the acceleration mechanism and radiation output from
relativistic particles that are accelerated within SBLs present in relativistic jets originating
from AGNs and GRBs. Particle-in-Cell (PiC) simulations were employed to investigate
the self-generation of electric and magnetic fields, as well as particle acceleration within
the SBLs of relativistic jets. The influence of inverse Compton cooling on relativistic parv
ticles accelerated in SBLs is examined, incorporating the self-consistent calculation of the
radiation spectrum resulting from inverse Compton scattering of relativistic electrons with
an isotropic external soft photon field. Notably, the Compton emission produced exhibits
high anisotropy, displaying stronger beaming along the direction of the jet compared to
the anticipated 1/Γ pattern that arises from intrinsically isotropic emission within the
comoving frame of an emission region moving along the jet with a bulk Lorentz factor. These findings offer a potential resolution to the long-standing problem known as the
Doppler Factor Crisis. | en_US |
