574 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
Dynamics and Structure of Three-Dimensional Poloidally Magnetized Supermagnetosonic Jets
A set of 3D MHD simulations of magnetized jets has been performed. The jets
contain an equipartition primarily poloidal magnetic field and the effect of
jet density on jet dynamics and structure is evaluated. The jet is precessed at
the origin to excite Kelvin-Helmholtz unstable helical modes. We extensively
compare the structure in these simulations with linear stability theory. The
jet that is dense with respect to the external medium develops a high speed
core surrounded by a less dense sheath consisting of slower moving jet fluid.
These simulations suggest that extended extragalactic jets propagate to such
large distances because they are surrounded by a lobe or cocoon whose density
is less than the jet density. (Abridged abstract.)Comment: 30 pages, AASTeX, to appear in ApJ, much better versions of Figures
2-5 are available at http://crux.astr.ua.edu/~rosen/hcr/hcr.htm
The Effect of Expansion on Mass Entrainment and Stability of Super-Alfv\'enic Jets
We extend investigations of mass entrainment by jets, which previously have
focused on cylindrical supermagnetosonic jets and expanding trans-Alfv\'enic
jets, to a set of expanding supermagnetosonic jets. We precess these jets at
the origin to excite the helical mode of the Kelvin-Helmholtz (or KH)
instability, in order to compare the results with predictions from linear
stability analysis. We analyze this simulation set for the spatial development
of magnetized mass, which we interpret as jet plus entrained, initially
unmagnetized external mass. As with the previous simulation sets, we find that
the growth of magnetized mass is associated with the growth of the KH
instability through linear, nonlinear, and saturated stages and with the
expansion of magnetized material in simulated observations of the jet. From
comparison of measured wavelengths and wave speeds with the predictions from
linear stability analysis, we see evidence that the KH instability is the
primary cause for mass entrainment in these simulations, and that the expansion
reduces the rate of mass entrainment. This reduced rate can be observed as a
somewhat greater distance between the two transition points separating the
three stages of expansion.Comment: 18 pages, 6 figures, AASTeX, to appear in Nov 1 issue of ApJ (vol
543), postscript versions of Figures 3 and 5 are available at
http://crux.astr.ua.edu/~rosen/supcon/rh.htm
Physical properties of the jet in 0836+710 revealed by its transversal structure
Studying the internal structure of extragalactic jets is crucial for
understanding their physics. The Japanese-led space VLBI project VSOP has
presented an opportunity for such studies, by reaching baseline lengths of up
to 36,000 km and resolving structures down to an angular size of
mas at 5 GHz. VSOP observations of the jet in 0836+710 at 1.6 and 5 GHz have
enabled tracing of the radial structure of the flow on scales from 2 mas to 200
mas along the jet and determination of the wavelengths of individual
oscillatory modes responsible for the formation of the structure observed. We
apply linear stability analysis to identify the oscillatory modes with modes of
Kelvin-Helmholtz instability that match the wavelengths of the structures
observed. We find that the jet structure in 0836+710 can be reproduced by the
helical surface mode and a combination of the helical and elliptic body modes
of Kelvin-Helmholtz instability. Our results indicate that the jet is
substantially stratified and different modes of the instability grow inside the
jet at different distances to the jet axis. The helical surface mode can be
driven externally, and we discuss the implications of the driving frequency on
the physics of the active nucleus in 0836+710.Comment: Accepted for publication in Astronomy & Astrophysics Letter
S5 0836+710: An FRII jet disrupted by the growth of a helical instability?
The remarkable stability of extragalactic jets is surprising, given the
reasonable possibility of the growth of instabilities. In addition, much work
in the literature has invoked this possibility in order to explain observed jet
structures and obtain information from these structures. For example, it was
recently shown that the observed helical structures in the jet in S5 0836+710
could be associated with helical pressure waves generated by Kelvin-Helmholtz
instability. Our aim is to resolve the arc-second structure of the jet in the
quasar S5 0836+710 and confirm the lack of a hot-spot (reverse jet-shock) found
by present observing arrays, as this lack implies a loss of jet collimation
before interaction with the intergalactic medium. In this work, we use an
observation performed in 2008 using EVN and MERLIN. The combined data reduction
has provided a complete image of the object at arc-second scales. The lack of a
hot-spot in the arc-second radio structure is taken as evidence that the jet
losses its collimation between the VLBI region and the region of interaction
with the ambient medium. This result, together with the previous identification
of the helical structures in the jet with helical pressure waves that grow in
amplitude with distance, allow us to conclude that the jet is probably
disrupted by the growth of Kelvin-Helmholtz instability. This observational
evidence confirms that the physical parameters of jets can be extracted using
the assumption that instability is present in jets and can be the reason for
many observed structures. Interestingly, the observed jet is classified as a
FRII object in terms of its luminosity, but its large-scale morphology does not
correspond to this classification. The implications of this fact are discussed.Comment: Accepted for publication in Astronomy & Astrophysic
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