58 research outputs found
Witnessing the gradual slow-down of powerful extragalactic jets: The X-ray -- optical -- radio connection
A puzzling feature of the {\it Chandra}--detected quasar jets is that their
X-ray emission decreases faster along the jet than their radio emission,
resulting to an outward increasing radio to X-ray ratio. In some sources this
behavior is so extreme that the radio emission peak is located clearly
downstream of that of the X-rays. This is a rather unanticipated behavior given
that the inverse
Compton nature of the X-rays and the synchrotron radio emission are
attributed to roughly the same electrons of the jet's non-thermal electron
distribution. In this note we show that this morphological behavior can result
from the gradual deceleration of a relativistic flow and that the offsets in
peak emission at different wavelengths carry the imprint of this deceleration.
This notion is consistent with another recent finding, namely that the jets
feeding the terminal hot spots of powerful radio galaxies and quasars are still
relativistic with Lorentz factors . The picture of the
kinematics of powerful jets emerging from these considerations is that they
remain relativistic as they gradually decelerate from Kpc scales to the hot
spots, where, in a final collision with the intergalactic medium, they
slow-down rapidly to the subrelativistic velocities of the hot spot advance
speed.Comment: Submitted in ApJ Letters on Jan. 14, 200
Modelling the spectral evolution of classical double radio sources
The spectral evolution of powerful double radio galaxies (FR II's) is thought
to be determined by the acceleration of electrons at the termination shock of
the jet, their transport through the bright head region into the lobes and the
production of the radio emission by synchrotron radiation in the lobes. Models
presented to date incorporate some of these processes in prescribing the
electron distribution which enters the lobes. We have extended these models to
include a description of electron acceleration at the relativistic termination
shock and a selection of transport models for the head region. These are
coupled to the evolution of the electron spectrum in the lobes under the
influence of losses due to adiabatic expansion, by inverse Compton scattering
on the cosmic background radiation and by synchrotron radiation. The
evolutionary tracks predicted by this model are compared to observation using
the power/source-size (P-D) diagram. We find that the simplest scenario, in
which accelerated particles suffer adiabatic losses in the head region which
become more severe as the source expands produces P-D-tracks which conflict
with observation, because the power is predicted to decline too steeply with
increasing size. Agreement with observation can be found by assuming that
adiabatic losses are compensated during transport between the termination shock
and the lobe by a re-acceleration process distributed throughout the head
region.Comment: 14 pages, 6 figures, accepted for publication in Astronomy and
Astrophysic
Acceleration of energetic particles by large-scale compressible magnetohydrodynamic turbulence
Fast particles diffusing along magnetic field lines in a turbulent plasma can
diffuse through and then return to the same eddy many times before the eddy is
randomized in the turbulent flow. This leads to an enhancement of particle
acceleration by large-scale compressible turbulence relative to previous
estimates in which isotropic particle diffusion is assumed.Comment: 13 pages, 3 figures, accepted for publication in Ap
Minimal Stochastic Model for Fermi's Acceleration
We introduce a simple stochastic system able to generate anomalous diffusion
both for position and velocity. The model represents a viable description of
the Fermi's acceleration mechanism and it is amenable to analytical treatment
through a linear Boltzmann equation. The asymptotic probability distribution
functions (PDF) for velocity and position are explicitly derived. The diffusion
process is highly non-Gaussian and the time growth of moments is characterized
by only two exponents and . The diffusion process is anomalous
(non Gaussian) but with a defined scaling properties i.e. and similarly for velocity.Comment: RevTeX4, 4 pages, 2 eps-figures (minor revision
Particle acceleration in rotating and shearing jets from AGN
We model the acceleration of energetic particles due to shear and centrifugal
effects in rotating astrophysical jets. The appropriate equation describing the
diffusive transport of energetic particles in a collisionless, rotating
background flow is derived and analytical steady state solutions are discussed.
In particular, by considering velocity profiles from rigid, over flat to
Keplerian rotation, the effects of centrifugal and shear acceleration of
particles scattered by magnetic inhomogeneities are distinguished. In the case
where shear acceleration dominates, it is confirmed that power law particle
momentum solutions exist, if the mean scattering
time is an increasing function of momentum. We show
that for a more complex interplay between shear and centrifugal acceleration,
the recovered power law momentum spectra might be significantly steeper but
flatten with increasing azimuthal velocity due to the increasing centrifugal
effects. The possible relevance of shear and centrifugal acceleration for the
observed extended emission in AGN is demonstrated for the case of the jet in
the quasar 3C273.Comment: 15 pages (including 8 pages Appendix), 4 figures; accepted for
publication in A&
Spectral evolution of non-thermal electron distributions in intense radiation fields
(abridged) Models of many astrophysical gamma-ray sources assume they contain
a homogeneous distribution of electrons that are injected as a power-law in
energy and evolve by interacting with radiation fields, magnetic fields and
particles in the source and by escaping. This problem is particularly
complicated if the radiation fields have higher energy density than the
magnetic field and are sufficiently energetic that inverse Compton scattering
is not limited to the Thomson regime. We present a simple, time-dependent,
semi-analytical solution of the electron kinetic equation that treats both
continuous and impulsive injection, cooling via synchrotron and inverse Compton
radiation, (taking into account Klein-Nishina effects) and energy dependent
particle escape. The kinetic equation for an arbitrary, time-dependent source
function is solved by the method of Laplace transformations. Using an
approximate expression for the energy loss rate that takes into account
synchrotron and inverse Compton losses including Klein-Nishina effects for
scattering off an isotropic photon field with either a power-law or black-body
distribution, we find explicit expressions for the cooling time and escape
probability of individual electrons. This enables the full, time-dependent
solution to be reduced to a single quadrature. From the electron distribution,
we then construct the time-dependent, multi-wavelength emission spectrum. We
compare our solutions with several limiting cases and discuss the general
appearance and temporal behaviour of spectral features (i.e., cooling breaks,
bumps etc.). As a specific example, we model the broad-band energy spectrum of
the open stellar association Westerlund-2 at different times of its evolution,
and compare it with observations.Comment: 14 pages, 8 figures, acccepted for publication in A&
HST optical spectral index map of the jet of 3C 273
We present HST images at 622 nm and 300 nm of the jet in 3C273 and determine
the run of the optical spectral index at 0.2" along the jet. The smoothness of
spectral index changes shows that the physical conditions are varying smoothly
across the jet. There is no correlation between the optical flux and spectral
index, as would be expected for relativistic electrons suffering strong cooling
due to synchrotron emission. We find no evidence for localized acceleration or
loss sites. This suggests that the spectral shape is not changing much
throughout the jet. We show that relativistic beaming and/or sub-equipartition
magnetic fields cannot remove the discrepancy between light-travel time along
the jet and the lifetime of electrons emitting optical synchrotron radiation.
We consider this further evidence in favour of a distributed electron
acceleration process.Comment: Accepted for publication by Astronomy and Astrophysics (13 pages, 8
figures
Canonical Particle Acceleration in FRI Radio Galaxies
Matched resolution multi-frequency VLA observations of four radio galaxies
are used to derive the asymptotic low energy slope of the relativistic electron
distribution. Where available, low energy slopes are also determined for other
sources in the literature. They provide information on the acceleration physics
independent of radiative and other losses, which confuse measurements of the
synchrotron spectra in most radio, optical and X-ray studies. We find a narrow
range of inferred low energy electron energy slopes, n(E)=const*E^-2.1 for the
currently small sample of lower luminosity sources classified as FRI (not
classical doubles). This distribution is close to, but apparently inconsistent
with, the test particle limit of n(E)=const*E^-2.0 expected from strong
diffusive shock acceleration in the non-relativistic limit. Relativistic shocks
or those modified by the back-pressure of efficiently accelerated cosmic rays
are two alternatives to produce somewhat steeper spectra. We note for further
study the possiblity of acceleration through shocks, turbulence or shear in the
flaring/brightening regions in FRI jets as they move away from the nucleus.
Jets on pc scales and the collimated jets and hot spots of FRII (classical
double) sources would be governed by different acceleration sites and
mechanisms; they appear to show a much wider range of spectra than for FRI
sources.Comment: 16 figures, including 5 color. Accepted to Astrophysical Journa
On the Momentum Diffusion of Radiating Ultrarelativistic Electrons in a Turbulent Magnetic Field
Here we investigate some aspects of stochastic acceleration of
ultrarelativistic electrons by magnetic turbulence. In particular, we discuss
the steady-state energy spectra of particles undergoing momentum diffusion due
to resonant interactions with turbulent MHD modes, taking rigorously into
account direct energy losses connected with different radiative cooling
processes. For the magnetic turbulence we assume a given power spectrum of the
type . In contrast to the previous approaches, however, we
assume a finite range of turbulent wavevectors , consider a variety of
turbulence spectral indexes , and concentrate on the case of a
very inefficient particle escape from the acceleration site. We find that for
different cooling and injection conditions, stochastic acceleration processes
tend to establish a modified ultrarelativistic Maxwellian distribution of
radiating particles, with the high-energy exponential cut-off shaped by the
interplay between cooling and acceleration rates. For example, if the timescale
for the dominant radiative process scales with the electron momentum as
, the resulting electron energy distribution is of the form
, where , and
is the equilibrium momentum defined by the balance between stochastic
acceleration and energy losses timescales. We also discuss in more detail the
synchrotron and inverse-Compton emission spectra produced by such an electron
energy distribution, taking into account Klein-Nishina effects. We point out
that the curvature of the high frequency segments of these spectra, even though
being produced by the same population of electrons, may be substantially
different between the synchrotron and inverse-Compton components.Comment: 42 pages, 14 figures included. Slightly modified version, accepted
for publication in Ap
A method of estimation of the dynamical age of FR II-type radio sources from multifrequency data
We propose a new approach in determining ages of FR II type radio sources. We
apply the assumed dynamical model of Kaiser et al. (1997) to a number of FR II
type radio galaxies observed at different radio frequencies, and fit - for each
frequency separately - the model free parameters to the observed sources'
quantities. Such a procedure, using enlarged in fact a number of observables,
enables us to determine relatively precise ages and other crucial
characteristics of the analyzed sources. The resulting age estimates agree very
well with those obtained by means of `classical' spectral ageing method for
objects not older than 10 Myr, for which good-quality spectral data are
available. The presented method is however also applicable in the case of the
sources older than this, and/or the ones for which the only available
low-resolution radio data do not allow for detailed spectral ageing studies.
Our analysis indicates that the main factor precluding precise age
determination for FR II type radio galaxies regards the poorly known shape of
the initial electron energy distribution injected by the jet terminal shocks to
the expanding lobes/cocoons. We briefly consider this issue, and conclude that
the broad-band single power-law form assumed here may be accurate enough for
the presented estimates, although most likely it does not strictly correspond
to some well-defined realistic particle acceleration process. Instead, it
should be considered as a simplest model approximation of the initial electron
continuum, averaged over a very broad energy range and over the age of the
source, with the effective spectral index which may be different for different
sources.Comment: 15 pages, 9 figures included. Accepted for publication in A&
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