Helene Sol

Three-dimensional Magnetohydrodynamic Simulations of Relativistic Jets Injected along a Magnetic Field

We present the first numerical simulations of moderately hot, supersonic jets propagating initially along the field lines of a denser magnetized background medium with Lorentz factor W = 4.56 and evolving in a four-dimensional spacetime. Compared with previous simulations in two spatial dimensions, the resulting structure and kinematics differ noticeably: the density of the Mach disk is lower, and the head speed is smaller. This is because the impacted ambient fluid and its embedded magnetic field make efficient use of the third spatial dimension as they are deflected circularly off of the head of the jet. As a result, a significant magnetic field component normal to the jet is created near the head. If the field is strong, backflow and field reversals are strongly suppressed; upstream, the field closes back on the surface of the beam and assists the collimation of the jet. If the field is weak, backflow and field reversals are more pronounced, although still not as extended as in the corresponding plane-parallel case. In all studied cases, the high-pressure region is localized near the jet head irrespective of the presence/strength of the magnetic field, and the head decelerates efficiently by transferring momentum to the background fluid that recedes along a thin bow shock in all directions. Furthermore, two oppositely directed currents circle near the surface of the cylindrical beam, and a third current circles on the bow shock. These preliminary results underline the importance of performing fully three-dimensional simulations to investigate the morphology and propagation of relativistic extragalactic jets.

Radiation drag effects on magnetically dominated outflows around compact objects

The effects of radiation drag force on the structure of relativistic electron–positron and electron–proton outflows are considered within the one-fluid approximation for a quasi-monopole cold outflow. It is shown that for a Poynting-dominated case, the drag force does not change the particle energy inside a fast magnetosonic surface. In this region, the action of the drag results in a diminution of the Poynting flux, not the particle flux. Outside the fast magnetosonic surface, for intermediate photon density, the drag force may result in additional acceleration of the plasma. This acceleration is the result of the disturbance of magnetic surfaces under the action of the drag. At even larger distances, particles are not frozen into the magnetic field and the drag force decelerates them efficiently. n n n nIn the case of extreme photon densities, the disturbance of magnetic surfaces becomes large and the drag force changes the total energy flux significantly, the particles becoming non-relativistic. n n n nWe find that for active galactic nuclei, the photon density is too low to disturb the parameters of an ideal magnetohydrodynamic outflow. The drag action may result in additional acceleration of outgoing plasma only for central engines with very high luminosities. For cosmological gamma-ray bursts, the drag force can strongly affect the process of formation of a Poynting-dominated outflow.

Three-dimensional Magnetohydrodynamic Simulations of Relativistic Jets Injected into an Oblique Magnetic Field

We discuss the structure and relativistic kinematics that develop in three spatial dimensions when a moderately hot, supersonic jet propagates into a denser background medium and encounters resistance from an oblique magnetic field. Our simulations incorporate relativistic MHD in a four-dimensional spacetime and clearly show that (1) relatively weak, oblique fields (at 1/16 of the equipartition value) have only a negligible influence on the propagating jet and they are passively pushed away by the relativistically moving head; (2) oblique fields in equipartition with the ambient plasma provide more resistance and cause bending at the jet head but the magnitude of this deflection and the associated backflow are small compared to those identified by previous studies. The new results are understood as follows: Relativistic simulations have consistently shown that these jets are effectively heavy, and so they do not suffer substantial momentum losses and are not decelerated as efficiently as their nonrelativistic counterparts. In addition, the ambient magnetic field, however strong, can be pushed aside with relative ease by the beam, provided that the degrees of freedom associated with all three spatial dimensions are followed self-consistently during the simulations. The effect is analogous to pushing Japanese noren or vertical venetian blinds out of the way while the slats are allowed to bend in three-dimensional space rather than as a two-dimensional slab structure. Applied to relativistic extragalactic jets from blazars, the new results are encouraging, since superluminal outflows exhibit bending near their sources and their environments are profoundly magnetized?but observations do not provide support for irregular kinematics such as large-scale vortical motions and pronounced reverse flows near the points of origin.

Microscopic Processes in Global Relativistic Jets Containing Helical Magnetic Fields

In the study of relativistic jets one of the key open questions is their interaction with the environment on the microscopic level. Here, we study the initial evolution of both electron–proton ( e − – p + ) and electron–positron ( e ± ) relativistic jets containing helical magnetic fields, focusing on their interaction with an ambient plasma. We have performed simulations of “global” jets containing helical magnetic fields in order to examine how helical magnetic fields affect kinetic instabilities such as the Weibel instability, the kinetic Kelvin-Helmholtz instability (kKHI) and the Mushroom instability (MI). In our initial simulation study these kinetic instabilities are suppressed and new types of instabilities can grow. In the e − – p + jet simulation a recollimation-like instability occurs and jet electrons are strongly perturbed. In the e ± jet simulation a recollimation-like instability occurs at early times followed by a kinetic instability and the general structure is similar to a simulation without helical magnetic field. Simulations using much larger systems are required in order to thoroughly follow the evolution of global jets containing helical magnetic fields.

Particle-in-cell Simulations of Global Relativistic Jets with Helical Magnetic Fields

We study the interaction of relativistic jets with their environment, using 3-dimensional relativistic particle-in-cell simulations for two cases of jet composition: (i) electron-proton (

Microscopic Processes in Global Relativistic Jets Containing Helical Magnetic Fields: Dependence on Jet Radius

e^{-}-p^{+}

Dynamics of relativistic jets

) and (ii) electron-positron (

Two-flow model for extragalactic radio jets

e^{pm}

A non-linear radio pulsar emission mechanism

) plasmas containing helical magnetic fields. We have performed simulations of global jets containing helical magnetic fields in order to examine how helical magnetic fields affect kinetic instabilities such as the Weibel instability, the kinetic Kelvin-Helmholtz instability and the Mushroom instability. We have found that these kinetic instabilities are suppressed and new types of instabilities can grow. For the

Extragalatic magnetic fields

e^{-}-p^{+}