Non-thermal emission in astrophysical environments: From pulsars to supernova remnants
Abstract
The study of electromagnetic radiation from distant astrophysical objects provides essential data in understanding physics of these sources. In particular, non-thermal radiation provides great insight into the properties of local environments, particle populations, and emission mechanisms, knowledge which otherwise would remain untapped. Throughout the projects conducted for this dissertation, we modeled certain aspects of observed non-thermal emission from three classes of sources: radio pulsars, pulsar wind nebulae, and supernova remnants. Orbital variation in the double pulsar system PSR J0737-3039A/B can be used to probe the details of the magnetospheric structure of pulsar B. Strongly magnetized wind from pulsar A distorts the magnetosphere of pulsar B in a way similar to the solar wind's distortion of the Earth's magnetosphere. Using the two complimentary models of pulsar B's magnetosphere, adapted from the Earth's magnetosphere models by Dungey and Tsyganenko, we determine the precise location of the coherent radio emission generation region in pulsar B's magnetosphere. This analysis is complemented by modeling the observed evolution of the pulse profiles of B due to geodetic precession. The emission region is located at about 3750 stellar radii and has a horseshoe-like shape centered on the polar magnetic field lines. The best fit angular parameters of the emission region indicate that radio emission is generated on the field lines which, according to the theoretical models, originate close to the poles and carry the maximum current. When considered together, not only do the results of the two models converge, they can explain why the modulation of B's radio emission at A's period is observed only within a certain orbital phase region. We discuss the implications of these results for pulsar magnetospheric models and mechanisms of coherent radio emission generation. We also developed a spatially-resolved, analytic model for the high-energy non-thermal emission from pulsar wind nebulae (PWNe). Theoretically, synchrotron cooling should cause a gradual change in particle spectrum downstream. This effect is indeed observed in the X-ray spectra of The Crab Nebula , 3C 58, and G21.5.0.9. However, current theoretical models of PWNe that only account for the bulk motion in the pulsar outflow overestimate the steepening of the resulted emission spectrum. This implies that there is an additional mechanism of particle transport which would supply energetic particles to the outer layers of the PWN. Our model solves the lack of high-energy electrons in the outer regions of the nebula by taking the diffusion of particles into account. The resulting multi-wavelength spectra exhibits multiple breaks, which is in agreement with observations. Thin non-thermal X-ray filaments are often seen near shock fronts in young supernova remnants (SNRs), often spatially coincident with the high energy gamma-ray emission. The formation of such discrete features is likely influenced by the combined effects of radiative cooling, advection, and diffusion. Spatially-resolved spectral studies of the filaments may, therefore, provide significant insights into the relative importance of main physical processes involved in young SNRs. Using 1 Ms Chandra observation of Cassiopeia A, we perform advection-diffusion modeling of synchrotron emission of filaments to measure the magnetic field, shock obliquity, the diffusion strength and the plasma turbulence level.
Degree
Ph.D.
Advisors
Lyutikov, Purdue University.
Subject Area
Astrophysics
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