General-Relativistic Model of Magnetically Driven Jet (original) (raw)
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LETTER TO THE EDITOR: General-Relativistic Model of Magnetically Driven Jet
General Relativity and Gravitation, 1997
The general scheme for the construction of the general-relativistic model of the magnetically driven jet is suggested. The method is based on the usage of the 3 + 1 MHD formalism. It is shown that the critical points of the flow and the explicit radial behavior of the physical variables may be derived through the jet “profile function”.
Magnetic acceleration of relativistic jets
2007
This is a brief review of the recent developments in the theory of magnetic acceleration of relativistic jets. We attempt to explain the key results of this complex theory using basic physical arguments and simple calculations. The main focus is on the standard model, which describes steady-state axisymmetric ideal MHD flows. We argue that this model is over-restrictive and discuss various alternatives.
New Astronomy Reviews, 1998
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 (a) 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; (b) 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 and twist in 3-D space. 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.
A class of exact MHD models for astrophysical jets
Monthly Notices of the Royal Astronomical Society, 1999
This paper examines a new class of exact and self-consistent MHD solutions that describe steady and axisymmetric hydromagnetic out¯ows from the atmosphere of a magnetized and rotating central object with possibly an orbiting accretion disc. The plasma is driven against gravity by a thermal pressure gradient, as well as by magnetic rotator and radiative forces. At the Alfve Ânic and fast critical points the appropriate criticality conditions are applied. The out¯ow starts almost radially, but after the Alfve Ân transition and before the fast critical surface is encountered, the magnetic pinching force bends the poloidal streamlines into a cylindrical jet-type shape. The terminal speed, Alfve Ân number and cross-sectional area of the jet, as well as its ®nal pressure and density, obtain uniform values at large distances from the source. The goal of the study is to give an analytical discussion of the two-dimensional interplay of the thermal pressure gradient, gravitational, Lorentz and inertial forces in accelerating and collimating an MHD¯ow. A parametric study of the model is also given, as well as a brief sketch of its applicability to a self-consistent modelling of collimated out¯ows from various astrophysical objects. The analysed model succeeds in giving for the ®rst time an exact MHD solution for jet-type out¯ows extending from the stellar surface to in®nity where the out¯ow can be superfast, in agreement with the MHD causality principle.
Counter-Rotation in Relativistic Magnetohydrodynamic Jets
Astrophysical Journal Letters, 2014
Young stellar object observations suggest that some jets rotate in the opposite direction with respect to their disk. In a recent study, Sauty et al. (2012) have shown that this does not contradict the magnetocentrifugal mechanism that is believed to launch such outflows. Signatures of motions transverse to the jet axis and in opposite directions have recently been measured in M87 (Meyer et al., 2013). One possible interpretation of this motion is the one of counter rotating knots. Here, we extend our previous analytical derivation of counter-rotation to relativistic jets, demonstrating that counter-rotation can indeed take place under rather general conditions. We show that both the magnetic field and a non-negligible enthalpy are necessary at the origin of counter-rotating outflows, and that the effect is associated with a transfer of energy flux from the matter to the electromagnetic field. This can be realized in three cases : if a decreasing enthalpy causes an increase of the Poynting flux, if the flow decelerates, or, if strong gradients of the magnetic field are present. An illustration of the involved mechanism is given by an example of relativistic MHD jet simulation.
Magnetic collimation of relativistic outflows in jets with a high mass flux
Monthly Notices of the Royal Astronomical Society, 2002
Self-collimation of relativistic magnetohydrodynamic (MHD) plasma outflows is inefficient in a single-component model consisting of a wind from a stellar object or an accretion disc, in the sense that the collimated portion of the mass and magnetic fluxes is uncomfortably low. The theory of magnetic collimation is applied here to a two-component model consisting of a relativistic wind-type outflow from a central source and a non-relativistic wind from the surrounding disc. Through a direct numerical simulation of the MHD flow in the nearest zone by the relaxation method and a solution of the steady-state problem in the far zone, it is shown that in this two-component model it is possible to collimate into cylindrical jets, in principle, up to all the mass and magnetic fluxes that are available from the central source. In such a case the non-relativistic disc-wind plays the role of the jet collimator. It is also shown that this collimation is accompanied by the formation of a sequence of shock waves at the interaction interface of the relativistic and non-relativistic outflows.
Monthly Notices of the Royal Astronomical Society, 2017
The paradigm in which magnetic fields play a crucial role in launching/collimating outflows in many astrophysical objects continues to gain support. However, semi-analytical models including the effect of magnetic fields on the dynamics and morphology of jets are still missing due to the intrinsic difficulties in integrating the equations describing a collimated, relativistic flow in the presence of gravity. Only few solutions have been found so far, due to the highly non-linear character of the equations together with the need to blindly search for singularities. These numerical problems prevented a full exploration of the parameter space. We present a new integration scheme to solve r-self-similar, stationary, axisymmetric magnetohydrodynamic (MHD) equations describing collimated, relativistic outflows crossing smoothly all the singular points (Alfvén point and modified slow/fast points). For the first time, we are able to integrate from the disc mid-plane to downstream of the modified fast point. We discuss an ensemble of jet solutions, emphasizing trends and features that can be compared to observables. We present, for the first time with a semi-analytical MHD model, solutions showing counter-rotation of the jet for a substantial fraction of its extent. We find diverse jet configurations with bulk Lorentz factors up to 10 and potential sites for recollimation between 10 3 and 10 7 gravitational radii. Such extended coverage of the intervals of quantities, such as magnetic-to-thermal energy ratios at the base or the heights/widths of the recollimation region, makes our solutions suitable for application to many different systems where jets are launched.
Observations and Simulations of Relativistic Jets
Lecture Notes in Physics, 2002
The recent improvement in VLBI arrays is providing information of the emission and magnetic field structure of relativistic jets, both extragalactic and galactic (microquasars), with unprecedented spatial and temporal resolution. These observations are revealing the importance of the hydrodynamical processes that govern the jet evolution, which can be studied by the recently developed time-dependent relativistic hydrodynamical models. Computation of the non-thermal emission from these hydrodynamical models, and its comparison with actual sources, is proving as one of the most powerful tools in the understanding of the physical processes taking place in these jets. This paper reviews some of the recent observational results, as well as the numerical models used to interpret them.
The paradigm in which magnetic fields play a crucial role in launching/collimating outflows in many astrophysical objects continues to gain support. However, semi-analytical models including the effect of magnetic fields on the dynamics and morphology of jets are still missing due to the intrinsic difficulties in integrating the equations describing a collimated, relativistic flow in the presence of gravity. Only few solutions have been found so far, due to the highly nonlinear character of the equations together with the need to blindly search for singularities. These numerical problems prevented a full exploration of the parameter space. We present a new integration scheme to solve r-self-similar, stationary, axisymmetric magnetohydrodynamics equations describing collimated, relativistic outflows crossing smoothly all the singular points (Alfvén point and modified slow/fast points). For the first time, we are able to integrate from the disk mid-plane to downstream of the modified fast point. We discuss an ensemble of jet solutions, emphasising trends and features that can be compared to observables. We present, for the first time with a semi-analytical MHD model, solutions showing counter-rotation of the jet for a substantial fraction of its extent. We find diverse jet configurations with bulk Lorentz factors up to 10 and potential sites for recollimation between 10 3 − 10 7 gravitational radii. Such extended coverage of the intervals of quantities , such as magnetic-to-thermal energy ratios at the base or the heights/widths of the recollimation region, makes our solutions suitable for application to many different systems where jets are launched.
General relativistic magnetohydrodynamical simulations of the jet in M 87
Astronomy & Astrophysics, 2016
Context. The connection between black hole, accretion disk, and radio jet can be constrained best by fitting models to observations of nearby low-luminosity galactic nuclei, in particular the well-studied sources Sgr A* and M 87. There has been considerable progress in modeling the central engine of active galactic nuclei by an accreting supermassive black hole coupled to a relativistic plasma jet. However, can a single model be applied to a range of black hole masses and accretion rates? Aims. Here we want to compare the latest three-dimensional numerical model, originally developed for Sgr A* in the center of the Milky Way, to radio observations of the much more powerful and more massive black hole in M 87. Methods. We postprocess three-dimensional GRMHD models of a jet-producing radiatively inefficient accretion flow around a spinning black hole using relativistic radiative transfer and ray-tracing to produce model spectra and images. As a key new ingredient in these models, we allow the proton-electron coupling in these simulations depend on the magnetic properties of the plasma. Results. We find that the radio emission in M 87 is described well by a combination of a two-temperature accretion flow and a hot single-temperature jet. Most of the radio emission in our simulations comes from the jet sheath. The model fits the basic observed characteristics of the M 87 radio core: it is "edge-brightened", starts subluminally, has a flat spectrum, and increases in size with wavelength. The best fit model has a mass-accretion rate ofṀ ∼ 9 × 10 −3 M yr −1 and a total jet power of P j ∼ 10 43 erg s −1. Emission at λ = 1.3 mm is produced by the counter-jet close to the event horizon. Its characteristic crescent shape surrounding the black hole shadow could be resolved by future millimeter-wave VLBI experiments. Conclusions. The model was successfully derived from one for the supermassive black hole in the center of the Milky Way by appropriately scaling mass and accretion rate. This suggests the possibility that this model could also apply to a wider range of low-luminosity black holes.