Investigations of Lepton and Baryon Acceleration in Relativistic Astrophysical Shocks

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Title: Investigations of Lepton and Baryon Acceleration in Relativistic Astrophysical Shocks
Author: Double, Glen Paul
Advisors: Donald C. Ellison, Committee Chair
Abstract: Particle acceleration in mildly relativistic shocks, internal to the main blastwave, may explain the early intensity peaks in gamma-ray bursts and the afterglow may be explained by energetic particles accelerated by the main ultrarelativistic blastwave shock as it slows to the mildly relativistic range. To help explain the phenomena, a nonlinear relativistic Monte Carlo model was developed and used to study lepton and baryon acceleration by mildly relativistic modified shocks with the magnetic field parallel to the shock normal. The study showed that for equal densities of leptons and baryons, lepton acceleration is highly sensitive to the shock velocity profile. With the shock fully modified by energetic baryons, the injection efficiency of leptons, relative to baryons, increases with Lorentz factor, and injection efficiency will reach a maximum well below that of baryons at the same momentum. Given the assumptions in this model, if the particles are energized by shock acceleration, leptons will always carry far less energy than baryons when the lepton and baryon densities are of the same order. It was determined that the lepton to baryon number density ratio must be approximately 3 x 10⁵ for both species to equally share the kinetic energy of the shock. This energy equipartition density ratio is independent of shock speed over the range of mildly relativistic Lorentz factors used in the study, but the result may extend to ultrarelativistic speeds. The study was a special case of a larger effort that will include relativistic oblique modified shocks and computer generated gamma-ray spectra when the model is completed. New magnetohydrodynamic conservation laws and relativistic jump conditions were developed for the model, along with a new equation of state and a new method for estimating the adiabatic index in the mildly relativistic range. The present state of the model shows smooth transitions of shock parameters from nonrelativistic to highly relativistic unmodified shocks while allowing oblique magnetic fields and a pressure tensor.
Date: 2003-04-11
Degree: PhD
Discipline: Physics

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