Inferred jet velocities close to the speed of
light suggest that jets are formed within a few gravitational radii
of the event horizon of the black hole. Moreover,
very-long-baseline interferometric (VLBI) radio observations reveal
that jets are already collimated at subparsec scales [133, 178]. Current theoretical
models assume that accretion disks are the source of the bipolar
outflows which are further collimated and accelerated via MHD
processes [41, 48
, 190]. There is a large number
of parameters which are potentially important for jet powering: the
black hole mass and spin, the accretion rate and the type of
accretion disk, the properties of the magnetic field and of the
environment [193, 189].
At parsec scales, the jets, observed via their synchrotron and inverse Compton emission at radio frequencies with VLBI imaging, appear to be highly collimated with a bright spot (the core) at one end of the jet and a series of components which separate from the core, sometimes at superluminal speeds [108]. In the standard model [25], these speeds are interpreted as a consequence of relativistic bulk motions in jets propagating at small angles to the line of sight with Lorentz factors up to 20 or more. Moving components in these jets, usually preceded by outbursts in emission at radio wavelengths, are interpreted in terms of traveling shock waves [177].
Finally, the morphology and dynamics of jets at
kiloparsec scales are dominated by the interaction of the jet with
the surrounding extragalactic medium, the jet power being
responsible for dichotomic morphologies [38] (the so called
Fanaroff-Riley I and II classes [90], FR I and
FR II, respectively). While current models [22, 152] interpret FR I
morphologies as the result of a smooth deceleration from
relativistic to non-relativistic, transonic speeds on kiloparsec
scales due to a slower shear layer, flux asymmetries between jets
and counter-jets in the most powerful radio galaxies (FR II)
and quasars indicate that relativistic motion extends up to
kiloparsec scales in these sources, although with smaller values of
the overall bulk speeds [37]. The detection of strong
X-ray emission from jets at large scales (; e.g., PKS0637
752 [51]) by the Chandra
satellite, interpreted as scattered CMB radiation [49], bears additional
support to the hypothesis of relativistic bulk speeds on these
scales.
Although MHD and general relativistic effects seem to be crucial for a successful launch of the jet, purely hydrodynamic, special relativistic simulations are adequate to study the morphology and dynamics of relativistic jets at distances sufficiently far from the central compact object (i.e., at parsec scales and beyond). The development of relativistic hydrodynamic codes based on HRSC techniques (see Sections 3 and 4) has triggered the numerical simulation of relativistic jets at parsec and kiloparsec scales.
|
Highly supersonic models, in which kinematic
relativistic effects due to high beam Lorentz factors dominate,
have extended over-pressured cocoons. These over-pressured cocoons
can help to confine the jets during the early stages of their
evolution [182], and even cause their
deflection when propagating through non-homogeneous
environments [231]. The cocoon
overpressure causes the formation of a series of oblique shocks
within the beam in which the synchrotron emission is enhanced. In
long term simulations (Figure 17
), the evolution is
dominated by a strong deceleration phase during which large lobes
of jet material (like the ones observed in many FR IIs, e.g.,
Cyg A [43]) start to inflate around
the jet’s head. These simulations reproduce some properties
observed in powerful extragalactic radio jets (lobe inflation, hot
spot advance speeds and pressures, deceleration of the beam flow
along the jet) and can help to constrain the values of basic
parameters (such as the particle density and the flow speed) in the
jets of real sources.
The problem of jet composition remains open for
more than two decades. Measurements of circular polarization in
jets [126] favour jets. However, X-ray observations of blazars
associated with OVV quasars impose strong constraints on the
pair content of jets [262]. On the other hand, the
composition of jets is tightly related to their formation
mechanisms [48, 266] and can be on the basis
of the FR I/FR II dichotomy [47]. In Scheck et
al. [255
] the problem of the
jet composition (
versus
) has been
approached in the context of long-term relativistic simulations
(
) searching for signatures of the
composition in the extended morphology of radio jets. Both the
morphology and the dynamic behaviour are almost independent of the
composition assumed for the jets in their 2D simulations (see
Figure 18
and the MPEG movie in
Figure 19
).
|
|
|
Magneto-hydrodynamic simulations of relativistic
jets have been performed in 2D [138, 136
] and 3D [209
, 210
] to study the
implications of ambient magnetic fields in the morphology and
bending properties of relativistic jets. However, despite the
impact of these results on specific problems like, e.g., the
understanding of the misalignment of jets between parsec and
kiloparsec scales, these 3D simulations have not addressed the
effects on the jet structure and dynamics of the third spatial
degree of freedom. This has been the aim of the work of Aloy et
al. [5] and Hughes et
al. [128
]. The latter authors
have also used their three-dimensional code to study the deflection
and precession of relativistic flows when impinging on an oblique
density gradient.
Finally, Koide et al. [140, 141
] have developed a
general relativistic MHD code and applied it to the problem of jet
formation from (Schwarzschild and Kerr) black hole accretion disks
in the context of Blandford and Payne’s mechanism [27]. In the case of jets
from Schwarzschild black holes [139
], jets are formed
with a two-layered shell structure consisting of a fast gas
pressure driven jet (Lorentz factor
) in the inner
part and a slow magnetically driven outflow in the outer part, both
of which are being collimated by the global poloidal magnetic field
penetrating the disk. In the case of counter-rotating disks around
Kerr black holes [137
], a new powerful
magnetically driven jet is formed inside the gas pressure driven
jet. This jet is accelerated by a strong magnetic field created by
frame dragging in the black hole ergosphere. Through this process,
the magnetic field extracts the energy from the black hole
corroborating Blandford and Znajek’s mechanism [28]. The same
authors [142
] have further
explored this second mechanism for jet formation in the case of a
Kerr black hole at maximal rotation immersed in a uniform,
magnetically dominated corona with no disk. The magnetic field
lines across the ergosphere are twisted by frame dragging. The line
twist propagates outwards as a torsional Alfvén wave train carrying
electromagnetic energy and leading to the generation of a Poynting
flux jet. Using a 3D GRMHD code, Nishikawa et al. [211
] have investigated
the dynamics of a freely falling corona and of a Keplerian
accretion disk around a Schwarzschild black hole. The disk and the
corona are threaded by a uniform poloidal magnetic field. The
magnetic field is tightly twisted by the rotation of the disk, and
plasma in the corona is accelerated by the Lorentz force to form
bipolar relativistic jets as in previous simulations assuming
axisymmetry.
Finally, let us note that direct numerical simulations of the Blandford and Znajek mechanism have been undertaken by Komissarov [145], solving the time dependent equations of (force-free, degenerate) electrodynamics in a Kerr black hole magnetosphere. The equations are hyperbolic [146] and are solved by means of a Godunov type method.