82 research outputs found
Simulations of cosmic ray propagation
We review numerical methods for simulations of cosmic ray (CR) propagation on
galactic and larger scales. We present the development of algorithms designed
for phenomenological and self-consistent models of CR propagation in kinetic
description based on numerical solutions of the Fokker-Planck equation. The
phenomenological models assume a stationary structure of the galactic
interstellar medium and incorporate diffusion of particles in physical and
momentum space together with advection, spallation, production of secondaries
and various radiation mechanisms. The self-consistent propagation models of CRs
include the dynamical coupling of the CR population to the thermal plasma. The
CR transport equation is discretized and solved numerically together with the
set of magneto-hydrodynamic (MHD) equations in various approaches treating the
CR population as a separate relativistic fluid within the two-fluid approach or
as a spectrally resolved population of particles evolving in physical and
momentum space. The relevant processes incorporated in self-consistent models
include advection, diffusion and streaming well as adiabatic compression and
several radiative loss mechanisms.
We discuss applications of the numerical models for the interpretation of CR
data collected by various instruments. We present example models of
astrophysical processes influencing galactic evolution such as galactic winds,
the amplification of large-scale magnetic fields and instabilities of the
interstellar medium.Comment: 99 pages, 13 figures, to be published in the Living Reviews of
Computational Astrophysic
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
Coaxial Jets and Sheaths in Wide-Angle-Tail Radio Galaxies
We add 20, 6 and 3.6 cm wavelength VLA observations of two WATs, 1231+674 and
1433+553, to existing VLA data at 6 and 20 cm, in order to study the variations
of spectral index as a function of position. We apply the spectral tomography
process that we introduced in our analysis of 3C67, 3C190 and 3C449. Both
spectral tomography and polarization maps indicate that there are two distinct
extended components in each source. As in the case of 3C449, we find that each
source has a flat spectrum jet surrounded by a steeper spectrum sheath. The
steep components tend to be more highly polarized than the flat components. We
discuss a number of possibilities for the dynamics of the jet/sheath systems,
and the evolution of their relativistic electron populations. While the exact
nature of these two coaxial components is still uncertain, their existence
requires new models of jets in FR I sources and may also have implications for
the dichotomy between FR Is and FR IIs.Comment: 29 text pages plus 13 figures. Scheduled for publication in May 10,
1999 Ap
Testing Galactic Magnetic Field Models using Near-Infrared Polarimetry
This work combines new observations of NIR starlight linear polarimetry with
previously simulated observations in order to constrain dynamo models of the
Galactic magnetic field. Polarimetric observations were obtained with the Mimir
instrument on the Perkins Telescope in Flagstaff, AZ, along a line of constant
Galactic longitude (\ell = 150\circ) with 17 pointings of the 10' \times 10'
field of view between -75\circ < b < 10\circ, with more frequent pointings
towards the Galactic midplane. A total of 10,962 stars were photometrically
measured and 1,116 had usable polarizations. The observed distribution of
polarization position angles with Galactic latitude and the cumulative
distribution function of the measured polarizations are compared to predicted
values. While the predictions lack the effects of turbulence and are therefore
idealized, this comparison allows significant rejection of A0-type magnetic
field models. S0 and disk-even halo-odd magnetic field geometries are also
rejected by the observations, but at lower significance. New predictions of
spiral-type, axisymmetric magnetic fields, when combined with these new NIR
observations, constrain the Galactic magnetic field spiral pitch angle to
-6\circ \pm 2\circ.Comment: 11 pages, 10 figures, Accepted for publication in Ap
A Sample of Low Redshift BL Lacs. I. The Radio Data
We present a new sample of 30 nearby (z<0.2) BL Lacs, selected to study the
nuclear as well as the large scale properties of low power radio sources. In
this first paper, we show and discuss new radio data taken with the VLA (19
objects at 1.4 GHz, either in A or C configuration, or both) as well as with
the VLBA (15 sources at 5 GHz). On the kiloparsec scale, all objects exhibit a
compact core and a variety of radio morphologies (jets, halos, secondary
compact components). On the parsec scale, we find weak cores and a few short,
one-sided, jets. From the jet/counter-jet ratio, core dominance, and
synchrotron self Compton model we estimate the intrinsic orientation and
velocity of the jets. The resulting properties of BL Lacs are similar to those
of a parent population composed of FR I radio galaxies.Comment: 46 pages, 12 figures, accepted for publication in Ap
A Radio and Optical Polarization Study of the Magnetic Field in the Small Magellanic Cloud
We present a study of the magnetic field of the Small Magellanic Cloud (SMC),
carried out using radio Faraday rotation and optical starlight polarization
data. Consistent negative rotation measures (RMs) across the SMC indicate that
the line-of-sight magnetic field is directed uniformly away from us with a
strength 0.19 +/- 0.06 microGauss. Applying the Chandrasekhar-Fermi method to
starlight polarization data yields an ordered magnetic field in the plane of
the sky of strength 1.6 +/- 0.4 microGauss oriented at a position angle 4 +/-
12 degs, measured counter-clockwise from the great circle on the sky joining
the SMC to the Large Magellanic Cloud (LMC). We construct a three-dimensional
magnetic field model of the SMC, under the assumption that the RMs and
starlight polarization probe the same underlying large-scale field. The vector
defining the overall orientation of the SMC magnetic field shows a potential
alignment with the vector joining the center of the SMC to the center of the
LMC, suggesting the possibility of a "pan-Magellanic'' magnetic field. A
cosmic-ray driven dynamo is the most viable explanation of the observed field
geometry, but has difficulties accounting for the observed uni-directional
field lines. A study of Faraday rotation through the Magellanic Bridge is
needed to further test the pan-Magellanic field hypothesis.Comment: 28 pages, 6 figures, accepted for publication in Ap
Radiation from the Relativistic Jet: a Role of the Shear Boundary Layer
Recent radio and optical large scale jets' observations suggest a
two-component jet morphology, consisting of a fast central spine surrounded
with a boundary layer with a velocity shear. We study radiation of electrons
accelerated at such boundary layers as an option for standard approaches
involving internal shocks in jets. The acceleration process in the boundary
layer yields in a natural way a two component electron distribution: a
power-law continuum with a bump at the energy, where energy gains equal
radiation losses, followed by a cut-off. For such distributions we derive the
observed spectra of synchrotron and inverse-Compton radiation, including
comptonization of synchrotron and CMB photons. Under simple assumptions of
energy equipartition between the relativistic particles and the magnetic field,
the relativistic jet velocity at large scales and a turbulent character of the
shear layer, the considered radiation can substantially contribute to the jet
radiative output. In the considered conditions the synchrotron emission is
characterized by a spectral index of the radio-to-optical continuum being
approximately constant along the jet. A characteristic feature of the obtained
broad-band synchrotron spectrum is an excess at X-ray frequencies, similar to
the one observed in some objects by Chandra. As compared to the uniform jet
models, the velocity shear across the radiating boundary region leads to
decrease and frequency dependence of the observed jet-counterjet radio
brightness asymmetry. We conclude that a careful investigation of the
observational data looking for the derived effects can allow to evaluate the
role of the boundary layer acceleration processes and/or impose constraints for
the physical parameters of such layers in large scale jets.Comment: 30 pages, 4 figures included. Modified version, accepted for
publication in Astrophysical Journa
Rotation-Measures across Parsec-scale Jets of FRI radio galaxies
We present the results of a parsec-scale polarization study of three FRI
radio galaxies - 3C66B, 3C78 and 3C264 - obtained with the Very Long Baseline
Array at 5, 8 and 15 GHz. Parsec-scale polarization has been detected in a
large number of beamed radio-loud active galactic nuclei, but in only a handful
of the relatively unbeamed radio galaxies. We report here the detection of
parsec-scale polarization at one or more frequencies in all three FRI galaxies
studied. We detect Faraday rotation measures of the order of a few hundred
rad/m^2 in the nuclear jet regions of 3C78 and 3C264. In 3C66B polarization was
detected at 8 GHz only. A transverse rotation measure gradient is observed
across the jet of 3C78. The inner-jet magnetic field, corrected for Faraday
rotation, is found to be aligned along the jet in both 3C78 and 3C264, although
the field becomes orthogonal further from the core in 3C78. The RM values in
3C78 and 3C264 are similar to those previously observed in nearby radio
galaxies. The transverse RM gradient in 3C78, the increase in the degree of
polarization at the jet edge, the large rotation in the polarization angles due
to Faraday rotation and the low depolarization between frequencies, suggests
that a layer surrounding the jet with a sufficient number of thermal electrons
and threaded by a toroidal or helical magnetic field is a good candidate for
the Faraday rotating medium. This suggestion is tentatively supported by Hubble
Space Telescope optical polarimetry but needs to be examined in a greater
number of sources.Comment: Accepted for publication in The Astrophysical Journal, March 2009 -
20 v694 issu
3D Hydrodynamic Simulations of Relativistic Extragalactic Jets
We describe a new numerical 3D relativistic hydrodynamical code, the results
of validation tests, and a comparison with earlier, 2D studies. The 3D code has
been used to study the deflection and precession of relativistic flows. We find
that even quite fast jets (gamma~10) can be significantly influenced by
impinging on an oblique density gradient, exhibiting a rotation of the Mach
disk in the jet's head. The flow is bent via a potentially strong, oblique
internal shock that arises due to asymmetric perturbation of the flow by its
cocoon. In extreme cases this cocoon can form a marginally relativistic flow
orthogonal to the jet, leading to large scale dynamics quite unlike that
normally associated with astrophysical jets. Exploration of a gamma=5 flow
subject to a large amplitude precession (semi-angle 11.25dg) shows that it
retains its integrity, with modest reduction in Lorentz factor and momentum
flux, for almost 50 jet-radii, but thereafter, the collimated flow is
disrupted. The flow is approximately ballistic, with velocity vectors not
aligned with the local jet `wall'. We consider simple estimators of the flow
emissivity in each case and conclude that a) while the oblique internal shocks
which mediate a small change in the direction of the deflected flows have
little impact on the global dynamics, significantly enhanced flow emission (by
a factor of 2-3) may be associated with such regions; and b) the convolution of
rest frame emissivity and Doppler boost in the case of the precessed jet
invariably leads to a core-jet-like structure, but that intensity fluctuations
in the jet cannot be uniquely associated with either change in internal
conditions or Doppler boost alone, but in general are a combination of both
factors.Comment: 41 pages, including 15 figures. Submitted to ApJ. Version with
complete abstract. and full resolution, color figures available from
http://www.astro.lsa.umich.edu/users/hughes/icon_dir/cfd.htm
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