Nonthermal Particle Propagation and Radiation in Supernova Remnants

After a supernova (SN) explosion, the expanding ejecta drive a strong shock into the surrounding medium, creating a supernova remnant (SNR). Galactic SNRs are efficient particle accelerators. Nonthermal particles are accelerated in SNRs while propagating through the remnant shocks, and then produce nonthermal radiation. For core collapse supernovae, a magnetized spinning neutron star may be left behind after the SN explosion. The fast relativistic pulsar wind, powered by the spinning neutron star, drives a strong shock into the slow non-relativistic SN ejecta, producing a so-called pulsar wind nebula (PWN) which is characterized by nonthermal synchrotron and Inverse Compton emission. The study of nonthermal particle propagation and radiation in SNRs (including PWNe) is not only important for understanding the evolution of SNRs but also crucial for exploring the nature of cosmic ray (CR) origin. SNRs are believed to be the CR accelerators at least up to the knee of the CR spectrum ( 10^{15} eV).
In this thesis, I present several pieces of work related to nonthermal particle propagation and radiation in SNRs (including PWNe).

In the first part of my thesis, I investigate particle transport in young PWNe such as the Crab Nebula. The classical toroidal magnetohydrodynamic (MHD) model for PWNe treats the shocked pulsar wind as an MHD flow. It successfully explained many observed features close to the central pulsar, including the termination shock and the jet-torus structure, but failed in the outer part of the nebula where more chaotic structure is present. To interpret the observed photon index distribution and shrinking nebular size, I propose a phenomenological advection and diffusion model for particle transport in young PWNe. In this model, advection dominates in the inner part of the nebula with toroidal structure, while diffusion dominates in the outer part of the nebula with chaotic structure. Monte Carlo simulations for the model provide good fits to the observed data. I also derive an analytical solution for particle transport with pure diffusion, which proves to be a good approximation for young PWNe in which toroidal structure is not significant.

Recent observations from both space-based GeV observatories and ground-based TeV observatories have revealed gamma-ray emission consistent with a hadronic origin from several middle-aged SNRs interacting with molecular clouds (MCs) (e.g., SNR IC 443 and W44). To reveal the nature of the observed gamma-ray emission and to identify the CR proton component from these middle-aged SNRs, I studied the interaction between a radiative SNR and MCs along with the associated particle acceleration in slow SNR shocks.
%Identification of the CR proton component in SNRs is a crucial step to establish SNRs as CR accelerators and explore the problem of CR origin.
I developed a 1-dimensional analytical model describing direct interaction between a radiative SNR, with a dense cooling shell, and clumpy MCs, with a moderate density interclump medium and high density molecular clumps. In the model, both the radiative shell and the clump interaction region contribute to the gamma-ray emission, but the clump interaction region dominates the emission due to its higher density. I investigate diffusive shock acceleration (DSA) in the test particle limit, within the framework of re-acceleration of pre-existing CRs in slow SNR shocks, and derive the time-dependent solutions to the problem for both energy-independent diffusion and energy-dependent diffusion. By combining the time-dependent DSA solution and clump interaction model discussed above, the overall shape of the IC 443 and W44 spectra from GeV to TeV energies can be reproduced through pure pion-decay emission with a hadronic origin.