Magnetized relativistic outflows in astrophysics
Abstract
Relativistic outflows are commonly thought to be a magnetic phenomenon in that they are launched, accelerated, and collimated in regions where the outflow's Poynting flux dominates over its particle (or matter) flux. In the case of AGN jets, this scenario leads to a helical large-scale magnetic field structure, which I argue leaves a signature in the form of observed asymmetries in a jet's transverse direction. That is, the jet may be intrinsically cylindrically symmetric, but nonetheless display asymmetrical profiles of observable quantities like intensity, linear fractional polarization, Faraday rotation, and spectral index. Importantly, these asymmetries will correlate with one another in a way specified by the handedness of the helical magnetic field, the jet's bulk Lorentz factor (Γ), and the jet viewing angle (&thetas; ob). A theoretical unknown in the magnetic model of outflows concerns the stability and dissipation of magnetic fields. If the outflow becomes unstable, then the magnetic field may become sufficiently distorted for magnetic reconnection to occur. In magnetic reconnection, magnetic free energy is converted into random particle energy and bulk kinetic energy by the topological rearrangement of magnetic field lines. This may explain fast flaring behavior in outflows, since magnetic reconnection produces both non-thermal particles and the bulk relativistic motion necessary for Doppler beaming. I have found that observed gamma-ray flares in the Crab Nebula display evidence of both bulk relativistic motion and a distribution of accelerated particles described by a hard power-law, possibly with a pile-up, as predicted in many reconnection models. Assuming reconnection events in the Crab Nebula follow Poisson statistics and are Doppler boosted, I have also created an analytical statistical model of the nebula gamma-ray light curve that predicts all of its statistical moments (e.g. time average, variance, etc.). This model can then be compared against forthcoming observations. Thus, it is possible that magnetic reconnection may be an important, if not dominant, mechanism of particle acceleration within pulsar wind nebulae. In addition, this reconnection model may be applicable to variability in AGN jets. One way for reconnection to occur in Poynting dominated flows is through the m = 1 kink mode. When the field rapidly becomes toroidally dominated, this mode becomes unstable and distorts the large-scale cylindrical (or conical) structure of the outflow by displacing the symmetry axis into a helical structure. This process may produce observable effects in outflows, both direct and indirect. Indirectly, it may cause reconnection as discussed above. Directly, I speculate that it may induce long term ([special characters omitted] light crossing time) variability such as certain observed variations in polarization and emission in AGN jets.
Degree
Ph.D.
Advisors
Lyutikov, Purdue University.
Subject Area
Astrophysics
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