Radiation from relativistic particles accelerated at shear layers in relativistic jets

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.