1,206 research outputs found
A new approach to multi-frequency synthesis in radio interferometry
We present a new approach to multi-frequency synthesis in radio astronomy.
Using Bayesian inference techniques, the new technique estimates the sky
brightness and the spectral index simultaneously. In principle, the bandwidth
of a wide-band observation can be fully exploited for sensitivity and
resolution, currently only limited by higher order effects like spectral
curvature. Employing this new approach, we further present a multi-frequency
extension to the imaging algorithm RESOLVE. In simulations, this new algorithm
outperforms current multi-frequency imaging techniques like MS-MF-CLEAN.Comment: 13 pages, 5 fugures, submitted to Astronomy and Astrophysic
Modification of cluster radio halo appearance by the thermal Sunyaev-Zeldovich effect
We discuss the consequences of the spectral and morphological modification of
galaxy cluster radio halos due to the Sunyaev-Zeldovich (SZ) effect for the
interpretation of existing and upcoming high frequency radio observations.
Likely these modifications have affected the interpretation of the existing
Coma cluster radio data. The radio halo emission visible at low (< 5 GHz)
frequencies is at higher (> 10 GHz) frequencies completely over-compensated by
the thermal SZ decrement. Thus, the total radio emission of a galaxy cluster
goes through zero (in comparison to the constant cosmic microwave background
(CMB) emission) at a frequency of several GHz. Since the radio halo brightness
has a narrow radial profile compared to the SZ decrement, a central emission
region is surrounded by a decrement within the intermediate frequency range of
several GHz. The size of this emission regions shrinks with increasing
frequency until the decrement dominates everywhere in the cluster.Comment: accepted by A&A Letters, 4 pages, 3 figurs, typo correcte
Imprints of magnetic power and helicity spectra on radio polarimetry statistics
Statistical properties of turbulent magnetic fields in radio-synchrotron
sources should imprint on the statistics of polarimetric observables. In search
of these imprints, we calculate correlation and cross-correlation functions
from a set of observables containing the total intensity I, the polarized
intensity P and the Faraday depth phi. The correlation functions are evaluated
for all combinations of observables up to fourth order in the magnetic field B.
We derive these as far as possible analytically and from first principles only
using some basic assumptions such as Gaussian statistics of the underlying
magnetic field in the observed region and statistical homogeneity. We further
assume some simplifications to reduce the complexity of the calculations, as
for a start we were interested in a proof of concept. Using this statistical
approach, we show that it is in principle possible to gain information about
the helical part of the magnetic power spectrum, namely via the correlation
functions and . Using this insight, we
construct an easy-to-use test for helicity, called LITMUS (Local Inference Test
for Magnetic fields which Uncovers heliceS). For now, all calculations are
given in a Faraday-free case, but set up in a way so that Faraday rotational
effects could be included later on.Comment: 24 pages, 4 figures; typos corrected; additional explanations in
section 1 and 2; revised and extended derivation in section 5, results
unchange
Reviving Fossil Radio Plasma in Clusters of Galaxies by Adiabatic Compression in Environmental Shock Waves
We give for a plasma with a history of several expansion and contraction
phases an analytical model of the evolution of a contained relativistic
electron population under synchrotron, inverse Compton and adiabatic energy
losses or gains. This is applied to different scenarios for evolution of radio
plasma inside the cocoons of radio galaxies, after the activity of the central
engine has ceased. It is demonstrated that fossil radio plasma with an age of
even up to 2 Gyr can be revived by compression in a shock wave of large-scale
structure formation, caused during the merging events of galaxy clusters, or by
the accretion onto galaxy clusters. We argue, that this is a highly plausible
explanation for the observed cluster radio relics, which are the regions of
diffuse radio emission found in clusters of galaxies, without any likely parent
radio galaxy seen nearby. An implication of this model is the existence of a
population of diffuse, ultra-steep spectrum, very low frequency radio sources
located inside and possibly outside of clusters of galaxies, tracing the
revival of aged fossil radio plasma by the shock waves associated with
large-scale structure formation.Comment: 10 pages, 5 figures, accepted by A&
Cosmic ray confinement in fossil cluster bubbles
Most cool core clusters of galaxies possess active galactic nuclei (AGN) in
their centers. These AGN inflate buoyant bubbles containing non-thermal radio
emitting particles. If such bubbles efficiently confine cosmic rays (CR) then
this could explain ``radio ghosts'' seen far from cluster centers. We simulate
the diffusion of cosmic rays from buoyant bubbles inflated by AGN. Our
simulations include the effects of the anisotropic particle diffusion
introduced by magnetic fields. Our models are consistent with the X-ray
morphology of AGN bubbles, with disruption being suppressed by the magnetic
draping effect. We conclude that for such magnetic field topologies, a
substantial fraction of cosmic rays can be confined inside the bubbles on
buoyant rise timescales even when the parallel diffusivity coefficient is very
large. For isotropic diffusion at a comparable level, cosmic rays would leak
out of the bubbles too rapidly to be consistent with radio observations. Thus,
the long confinement times associated with the magnetic suppression of CR
diffusion can explain the presence of radio ghosts. We show that the partial
escape of cosmic rays is mostly confined to the wake of the rising bubbles, and
speculate that this effect could: (1) account for the excitation of the
H filaments trailing behind the bubbles in the Perseus cluster, (2)
inject entropy into the metal enriched material being lifted by the bubbles
and, thus, help to displace it permanently from the cluster center and (3)
produce observable -rays via the interaction of the diffusing cosmic
rays with the thermal intracluster medium (ICM).Comment: submitte
On the escape of cosmic rays from radio galaxy cocoons
(Abridged) A model for the escape of CR particles from radio galaxy cocoons
is presented here. It is assumed that the radio cocoon is poorly magnetically
connected to the environment. An extreme case of this kind is an insulating
boundary layer of magnetic fields, which can efficiently suppress particle
escape. More likely, magnetic field lines are less organised and allow the
transport of CR particles from the source interior to the surface region. For
such a scenario two transport regimes are analysed: diffusion of particles
along inter-phase magnetic flux tubes (leaving the cocoon) and cross field
transport of particles in flux tubes touching the cocoon surface. The cross
field diffusion is likely the dominate escape path, unless a significant
fraction of the surface is magnetically connected to the environment. Major
cluster merger should strongly enhance the particle escape by two complementary
mechanisms. i) The merger shock waves shred radio cocoons into filamentary
structures, allowing the CRs to easily reach the radio cocoon boundary due to
the changed morphology. ii) Also efficient particle losses can be expected for
radio cocoons not compressed in shock waves. There, for a short period after
the sudden injection of large scale turbulence, the (anomalous) cross field
diffusion can be enhanced by several orders of magnitude. This lasts until the
turbulent energy cascade has reached the microscopic scales, which determine
the value of the microscopic diffusion coefficients.Comment: A&A in press, 12 pages, 5 figures, minor language improvement
Analytical Study on the Sunyaev-Zeldovich Effect for Clusters of Galaxies. II. comparison of covariant formalisms
We study a covariant formalism for the Sunyaev-Zeldovich effects developed in
the previous papers by the present authors, and derive analytic expressions for
the redistribution functions in the Thomson approximation. We also explore
another covariant formalism recently developed by Poutanen and Vurm. We show
that the two formalisms are mathematically equivalent in the Thomson
approximation which is fully valid for the cosmic microwave background photon
energies. The present finding will establish a theoretical foundation for the
analysis of the Sunyaev-Zeldovich effects for the clusters of galaxies.Comment: Accepted version, 7 pages, 1 figure, accepted by Physical Review D
for publicatio
Magnetic power spectra from Faraday rotation maps - REALMAF and its use on Hydra A
We develop a novel maximum-a-posteriori method to measure magnetic power
spectra from Faraday rotation data and implement it in the REALMAF code. A
sophisticated model for the magnetic autocorrelation in real space permits us
to alleviate previously required simplifying assumptions in the processing. We
also introduce a way to treat the divergence relation of the magnetic field
with a multiplicative factor in Fourier space, with which we can model the
magnetic autocorrelation as a spherically symmetric function. Applied to the
dataset of Hydra A north, we find a power law power spectrum on spatial scales
between 0.3 kpc and 8 kpc, with no visible turnover at large scales within this
range and a spectral index consistent with a Kolmogorov-like power law regime.
The magnetic field strength profile seems to follow the electron density
profile with an index alpha=1. A variation of alpha from 0.5 to 1.5 would lead
to a spectral index between 1.55 and 2.05. The extrapolated magnetic field
strength in the cluster centre highly depends on the assumed projection angle
of the jet. For an angle of 45 degree we derive extrapolated 36 muG in the
centre and directly probed 16 muG at 50 kpc radius.Comment: 13 pages, 13 figures, version accepted by A&A with restructured
introduction and language improvement
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