This project is the first study of important three-dimensional effects in relativistic strongly-magnetized outflows. It will lay a firm basis for the realistic modeling of jets from black hole binary star systems, known as microquasars, and from active galactic nuclei, and of pulsar wind outflows and gamma-ray bursts. It will add substantially to our understanding of relativistic magnetohydrodynamic processes. Numerical simulations and theoretical development will provide an understanding of the role of strong magnetic fields in relativistic jets and shocks. The results will be used with observations of jets in very different environments to make predictions about the acceleration and collimation process and accompanying magnetization, and means of energy transport, in different astrophysical systems and at different spatial scales. This research directly addresses the relationship between jet dynamics and magnetic processes, adding insight into the physics of jet acceleration, collimation, propagation and particle acceleration.
This study also enhances the partnership between scientists at two campuses of the University of Alabama, at the National Space Science and Technology Center (Huntsville, Alabama), at Ben Gurion University (Israel), and at other national and international facilities. Student training in mathematical and computational techniques, their involvement in forefront research, and their participation in scientific meetings, will help prepare them for a range of science-based careers. The researchers are severally involved in public outreach at a variety of levels.
Observation and Theory of Jets from Active Galactic Nuclei: M 87 is a giant elliptical galaxy whose center contains one of the most massive black holes known. A highly collimated jet of relativistically moving material emanates from the black hole accretion disk system. Radio telescope observation of the jet and black hole system in collaboration with Dr. Craig Walker (NRAO) and others using the VLBA has found that a very high energy gamma-ray flare was associated with an ejection event from the black hole accretion disk system. Ongoing work reveals details of the jet structure, acceleration and rotation near the black hole not previously observed. At larger distances (100s to 1000s of parsecs) the jet contains twisted filaments produced by jet interaction with the galaxy's interstellar medium. Theoretical modeling work in collaboration with Dr. Jean Eilek (NMT & NRAO) found that the filament twisting indicates that the jet slows and heats and the result is decollimation of the jet flow into a turbulent plume at a distance of a few 1000 parsecs from the central black hole engine. Jets like the one in M 87 do not typically propagate very far from the central black hole engine before they decollimate. Decollimation appears to involve a shock and/or entrainment of external material into the jet flow. Since the jet is large many mass losing stars can add material to the jet that slow and heat the jet. An international collaboration with Dr. Manolo Perucho (U. Valencia, Spain) and others have found that only very low power jets like the one in M 87 can be slowed and decollimated by entrainment of material from the mass losing stars within the jet. More powerful jets appear to be decollimated by shocks and/or other means of entraining mass into the jet flow. Magnetohydrodynamic Simulations of Relativistic Jet Instability: The highly relativistic jet flows from Active Galactic Nuclei (AGN) should be unstable to helical twisting either as a result of velocity shear between the jet and the external medium or as a result of strong helical magnetic fields contained in the jet. Such instability should result in entrainment of external material and decollimation of jet flow and in fact this is likely to be the entrainment mechanism that slows, heats, shocks and decollimates medium power jets. However, high power jets remain collimated and apparently stable over vast distances (100s of thousands of parsecs) and even medium power jets propagate farther than might naively be expected. Magnetohydrodynamic simulation work carried out with Dr. Ken-Ichi Nishikawa (UAH) and Dr. Yosuke Mizuno (NTHU, Taiwan) and others, has been used to find particularly stable jet configurations. We find that certain transverse density, helical magnetic field and jet flow velocity configurations will be sufficiently stable. Conditions such as a tenuous high speed jet spine containing a transversely decreasing helical magnetic field surrounded by a denser and slower more weakly magnetized sheath along with some transverse expansion of the jet flow should be sufficiently stable. In fact such a configuration is indicated by jet black hole accretion disk generation simulations and is suggested but not proved by observations. Particle-in-Cell Simulations of Shocks, Velocity Shears and Emission: The ionized plasmas in relativistic jets are collisionless at the "microscopic" individual particle level and only behave like "fluids" on the "macroscopic" level because embedded magnetic fields tie the charged particles together on large observable scales. In order to understand how ultra-high energy electrons are produced and generate the observed emission we must study processes on the microscopic level. Particle-in-cell simulation and collisionless plasma theory work carried out with Dr. Ken-Ichi Nishikawa (UAH) is being used to investigate the microscopic processes in the collisionless shocks and velocity shears that are likely responsible for particle acceleration, magnetic field generation and emission. Our simulations involving relativistic plasma jet propagation and including emission indicate that the power law particle energy distribution and generated magnetic fields produced in relativistic collisionless shocks can lead to the emission observed in AGN and gamma-ray bursts (GRBs). Our recent simulation and theoretical work has shown magnetic field generation in velocity shears and how the longitudinal and transverse structure depend on plasma composition and Lorentz factor. Ultimately further collisionless shock and shear work will provide important constraints on the otherwise unobservable plasma compositions and magnetizations in AGN jets and GRBs through the emission spectrum.
