Particle acceleration in magnetized, relativistic outflows of astrophysical sources [electronic resource]

Many powerful and variable gamma-ray sources, including pulsar wind nebulae (particularly the Crab Nebula), active galactic nuclei and gamma-ray bursts, seem capable of accelerating particles to gamma-ray-emitting energies efficiently over very short time scales. These are likely due to rapid dissipation of electromagnetic energy in a highly magnetized, relativistic plasma. We term such a process as "magnetoluminescence". One possible scenario is that in the highly magnetized outflow of the prime mover, an ideal instability causes a tangled, high energy configuration to relax to a lower energy state over light crossing time scales; during the process extended E > B or E · B ≠ 0 regions can be formed and sustained as the particles are accelerated up to the radiation reaction limit, removing the electromagnetic energy in the form of gamma-ray emission. In order to test this conjecture, we devise simple models of magnetized, relativistic plasma configurations, which allow us to study in detail the macroscopic instability that leads to dramatic dissipation of electromagnetic energy. One class of examples are the so-called linear force-free equilibria within confining walls or 3D periodic boxes. Using analytical technique and MHD simulations, we find that many of the short wavelength configurations are unstable to ideal modes; the instability grows on Alfven wave crossing time scales (close to the light crossing time scale when the magnetization is high), and the system eventually relaxes to the longest wavelength state, or lowest energy state, as allowed by a conserved total helicity. We then used one of the lowest order unstable equilibria as a testbed to understand the generic features of particle acceleration and radiation in a relativistic, magnetized plasma, using radiative particle-in-cell (PIC) simulations. We find that the ideal instability forces a dynamic current layer formation and the highest energy particles are first accelerated by the parallel electric field in the current layers; fast variability can be produced by particle bunches ejected from the current layers. Meanwhile, we have been working closely with the observations, particularly the interpretation of multiwavelength data of the inner knot in the Crab Nebula, which was suspected to be the site of the gamma-ray flares. Though no convincing evidence has been found in that respect, we did a careful examination of emission models at the knot, which prompts us to reconsider the main particle acceleration mechanisms responsible for most of the emission (in the Optical/UV/X-ray wavebands) from the nebula. We think all the aforementioned results provide instructive steps along the way to learn about the basic properties of magnetized, relativistic plasmas and extreme particle acceleration. We also propose possible future directions in theoretical analysis, simulations, observations and laboratory astrophysics that may help us better understand these powerful engines in our universe.

Keywords: Physics.
Advisor: Blandford Roger D.
Abel Tom G.
Romani Roger W. (Roger William) advisor. Thesis advisor