369 research outputs found
Anatomy of helical relativistic jets: The case of S5 0836+710
Helical structures are common in extragalactic jets. They are usually
attributed in the literature to periodical phenomena in the source (e.g.,
precession). In this work, we use VLBI data of the radio-jet in the quasar S5
0836+710 and hypothesize that the ridge-line of helical jets like this
corresponds to a pressure maximum in the jet and assume that the helically
twisted pressure maximum is the result of a helical wave pattern. For our
study, we use observations of the jet in S5 0836+710 at different frequencies
and epochs. The results show that the structures observed are physical and not
generated artificially by the observing arrays. Our hypothesis that the
observed intensity ridge-line can correspond to a helically twisted pressure
maximum is confirmed by our observational tests. This interpretation allows us
to explain jet misalignment between parsec and kiloparsec scales when the
viewing angle is small, and also brings us to the conclusion that
high-frequency observations may show only a small region of the jet flow
concentrated around the maximum pressure ridge-line observed at low
frequencies. Our work provides a potential explanation for the apparent
transversal superluminal speeds observed in several extragalactic jets by means
of transversal shift of an apparent core position with time.Comment: Accepted for publication in the Astrophysical Journa
The role of Kelvin-Helmholtz instability in the internal structure of relativistic outflows. The case of the jet in 3C 273
Relativistic outflows represent one of the best-suited tools to probe the
physics of AGN. Numerical modelling of internal structure of the relativistic
outflows on parsec scales provides important clues about the conditions and
dynamics of the material in the immediate vicinity of the central black holes
in AGN. We investigate possible causes of the structural patterns and
regularities observed in the parsec-scale jet of the well-known quasar 3C 273.
We present here the results from a 3D relativistic hydrodynamics numerical
simulation based on the parameters given for the jet by Lobanov & Zensus
(2001), and one in which the effects of jet precession and the injection of
discrete components have been taken into account. We compare the model with the
structures observed in 3C 273 using very long baseline interferometry and
constrain the basic properties of the flow. We find growing perturbation modes
in the simulation with similar wavelengths to those observed, but with a
different set of wave speeds and mode identification. If the observed longest
helical structure is produced by the precession of the flow, longer precession
periods should be expected. Our results show that some of the observed
structures could be explained by growing Kelvin-Helmholtz instabilities in a
slow moving region of the jet. However, we point towards possible errors in the
mode identification that show the need of more complete linear analysis in
order to interpret the observations. We conclude that, with the given viewing
angle, superluminal components and jet precession cannot explain the observed
structures.Comment: Accepted for publication in Astronomy & Astrophysics. 14 pages.
Higher resolution plots available on request to [email protected] and
at http://www.mpifr-bonn.mpg.de/staff/mperuch
Modeling Helical Structures in Relativistic Jets
Many jets exhibit twisted helical structures. Where superluminal motions are
detected, jet orientation and pattern/flow speed are considerably constrained.
In this case modeling efforts can place strong limits on conditions in the jet
and in the external environment. This can be done by modeling the spatial
development of helical structures which are sensitively dependent on these
conditions. Along an expanding jet this sensitivity manifests itself in
predictable changes in pattern speed and observed wavelength. In general,
twists of low frequency relative to the local resonant frequency are advected
along the expanding jet into a region in which the twist frequency is high
relative to the local resonant frequency. The wave speed can be very different
in these two frequency regimes. Potential effects include helical twists with a
nearly constant apparent wavelength, an apparent wavelength scaling
approximately with the jet radius for up to two orders of magnitude of jet
expansion, or multiple twist wavelengths with vastly different intrinsic scale
and vastly different wave speeds that give rise to similar observed twist
wavelengths but with very different observed motion. In this paper I illustrate
the basic intrinsic and observed behavior of these structures and show how to
place constraints on jet conditions in superluminal jets using the apparent
structures and motions in the inner 3C 120 jet.Comment: 18 pages, 7 figure
The Stability of Radiatively Cooling Jets I. Linear Analysis
The results of a spatial stability analysis of a two-dimensional slab jet, in which optically thin radiative cooling is dynamically important, are presented. We study both magnetized and unmagnetized jets at external Mach numbers of 5 and 20. We model the cooling rate by using two different cooling curves: one appropriate to interstellar gas, and the other to photoionized gas of reduced metallicity. Thus, our results will be applicable to both protostellar (Herbig-Haro) jets and optical jets from active galactic nuclei. We present analytical solutions to the dispersion relations in useful limits and solve the dispersion relations numerically over a broad range of perturbation frequencies. We find that the growth rates and wavelengths of the unstable Kelvin-Helmholtz (K-H) modes are significantly different from the adiabatic limit, and that the form of the cooling function strongly affects the results. In particular, if the cooling curve is a steep function of temperature in the neighborhood of the equilibrium state, then the growth of K-H modes is reduced relative to the adiabatic jet. On the other hand, if the cooling curve is a shallow function of temperature, then the growth of K-H modes can be enhanced relative to the adiabatic jet by the increase in cooling relative to heating in overdense regions. Inclusion of a dynamically important magnetic field does not strongly modify the important differences between an adiabatic jet and a cooling jet, provided the jet is highly supermagnetosonic and not magnetic pressure-dominated. In the latter case, the unstable modes behave more like the transmagnetosonic magnetic pressure-dominated adiabatic limit. We also plot fluid displacement surfaces associated with the various waves in a cooling jet in order to predict the structures that might arise in the nonlinear regime. This analysis predicts that low-frequency surface waves and the lowest order body modes will be the most effective at producing observable features in the jet
A Comparison of the Morphology and Stability of Relativistic and Nonrelativistic Jets
We compare results from a relativistic and a nonrelativistic set of 2D
axisymmetric jet simulations. For a set of five relativistic simulations that
either increase the Lorentz factor or decrease the adiabatic index we compute
nonrelativistic simulations with equal useful power or thrust. We examine these
simulations for morphological and dynamical differences, focusing on the
velocity field, the width of the cocoon, the age of the jets, and the internal
structure of the jet itself. The primary result of these comparisons is that
the velocity field of nonrelativistic jet simulations cannot be scaled up to
give the spatial distribution of Lorentz factors seen in relativistic
simulations. Since the local Lorentz factor plays a major role in determining
the total intensity for parsec scale extragalactic jets, this suggests that a
nonrelativistic simulation cannot yield the proper intensity distribution for a
relativistic jet. Another general result is that each relativistic jet and its
nonrelativistic equivalents have similar ages (in dynamical time units, =
R/a_a, where R is the initial radius of a cylindrical jet and a_a is the sound
speed in the ambient medium). In addition to these comparisons, we have
completed four new relativistic simulations to investigate the effect of
varying thermal pressure on relativistic jets. The simulations generally
confirm that faster (larger Lorentz factor) and colder jets are more stable,
with smaller amplitude and longer wavelength internal variations. The apparent
stability of these jets does not follow from linear normal mode analysis, which
suggests that there are available growing Kelvin-Helmholtz modes. (Abridged.)Comment: 32 pages, AASTEX, to appear in May 10, 1999 issue of ApJ, better
versions of Figures 1 and 6 are available at
http://crux.astr.ua.edu/~rosen/rel/rhdh.htm
Resonant Kelvin-Helmholtz modes in sheared relativistic flows
Qualitatively new aspects of the (linear and non-linear) stability of sheared
relativistic (slab) jets are analyzed. The linear problem has been solved for a
wide range of jet models well inside the ultrarelativistic domain (flow Lorentz
factors up to 20; specific internal energies ). As a distinct
feature of our work, we have combined the analytical linear approach with
high-resolution relativistic hydrodynamical simulations, which has allowed us
i) to identify, in the linear regime, resonant modes specific to the
relativistic shear layer ii) to confirm the result of the linear analysis with
numerical simulations and, iii) more interestingly, to follow the instability
development through the non-linear regime. We find that very high-order
reflection modes with dominant growth rates can modify the global, long-term
stability of the relativistic flow. We discuss the dependence of these resonant
modes on the jet flow Lorentz factor and specific internal energy, and on the
shear layer thickness. The results could have potential applications in the
field of extragalactic relativistic jets.Comment: Accepted for publication in Physical Review E. For better quality
images, please check
http://www.mpifr-bonn.mpg.de/staff/mperucho/Research.htm
Magnetic Field Generation in Core-Sheath Jets via the Kinetic Kelvin-Helmholtz Instability
We have investigated magnetic field generation in velocity shears via the
kinetic Kelvin-Helmholtz instability (kKHI) using a relativistic plasma jet
core and stationary plasma sheath. Our three-dimensional particle-in-cell
simulations consider plasma jet cores with Lorentz factors of 1.5, 5, and 15
for both electron-proton and electron-positron plasmas. For electron-proton
plasmas we find generation of strong large-scale DC currents and magnetic
fields which extend over the entire shear-surface and reach thicknesses of a
few tens of electron skin depths. For electron-positron plasmas we find
generation of alternating currents and magnetic fields. Jet and sheath plasmas
are accelerated across the shear surface in the strong magnetic fields
generated by the kKHI. The mixing of jet and sheath plasmas generates
transverse structure similar to that produced by the Weibel instability.Comment: 28 pages, 12 figures, in press, ApJ, September 10, 201
Jet stability and the generation of superluminal and stationary components
We present a numerical simulation of the response of an expanding
relativistic jet to the ejection of a superluminal component. The simulation
has been performed with a relativistic time-dependent hydrodynamical code from
which simulated radio maps are computed by integrating the transfer equations
for synchrotron radiation. The interaction of the superluminal component with
the underlying jet results in the formation of multiple conical shocks behind
the main perturbation. These trailing components can be easily distinguished
because they appear to be released from the primary superluminal component,
instead of being ejected from the core. Their oblique nature should also result
in distinct polarization properties. Those appearing closer to the core show
small apparent motions and a very slow secular decrease in brightness, and
could be identified as stationary components. Those appearing farther
downstream are weaker and can reach superluminal apparent motions. The
existence of these trailing components indicates that not all observed
components necessarily represent major perturbations at the jet inlet; rather,
multiple emission components can be generated by a single disturbance in the
jet. While the superluminal component associated with the primary perturbation
exhibits a rather stable pattern speed, trailing components have velocities
that increase with distance from the core but move at less than the jet speed.
The trailing components exhibit motion and structure consistent with the
triggering of pinch modes by the superluminal component.Comment: Accepted by ApJ Letters. LaTeX, 19 pages, 4 PostScript figure
Stability Properties of Strongly Magnetized Spine Sheath Relativistic Jets
The linearized relativistic magnetohydrodynamic (RMHD) equations describing a
uniform axially magnetized cylindrical relativistic jet spine embedded in a
uniform axially magnetized relativistically moving sheath are derived. The
displacement current is retained in the equations so that effects associated
with Alfven wave propagation near light speed can be studied. A dispersion
relation for the normal modes is obtained. Analytical solutions for the normal
modes in the low and high frequency limits are found and a general stability
condition is determined. A trans-Alfvenic and even a super-Alfvenic
relativistic jet spine can be stable to velocity shear driven Kelvin-Helmholtz
modes. The resonance condition for maximum growth of the normal modes is
obtained in the kinetically and magnetically dominated regimes. Numerical
solution of the dispersion relation verifies the analytical solutions and is
used to study the regime of high sound and Alfven speeds.Comment: 42 pages includes 7 figures, to appear in Ap
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