1,588 research outputs found
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
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
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
Constraining the population of cosmic ray protons in cooling flow clusters with gamma-ray and radio observations: Are radio mini-halos of hadronic origin?
We wish to constrain the cosmic-ray proton (CRp) population in galaxy
clusters. By hadronic interactions with the thermal gas of the intra-cluster
medium (ICM), the CRp produce gamma-rays for which we develop an analytic
formalism to deduce their spectral distribution. Assuming the CRp-to-thermal
energy density ratio X_CRp and the CRp spectral index to be spatially constant,
we derive an analytic relation between the gamma-ray and bolometric X-ray
fluxes, F_gamma and F_X. Based on our relation, we compile a sample of suitable
clusters which are promising candidates for future detection of gamma-rays
resulting from hadronic CRp interactions. Comparing to EGRET upper limits, we
constrain the CRp population in the cooling flow clusters Perseus and Virgo to
X_CRp < 20%. Assuming a plausible value for the CRp diffusion coefficient
kappa, we find the central CRp injection luminosity of M 87 to be limited to
10^43 erg s^-1 kappa/(10^29 cm^2 s^-1). The synchrotron emission from secondary
electrons generated in CRp hadronic interactions allows even tighter limits to
be placed on the CRp population using radio observations. We obtain excellent
agreement between the observed and theoretical radio brightness profiles for
Perseus, but not for Coma without a radially increasing CRp-to-thermal energy
density profile. Since the CRp and magnetic energy densities necessary to
reproduce the observed radio flux are very plausible, we propose synchrotron
emission from secondary electrons as an attractive explanation of the radio
mini-halos found in cooling flow clusters. This model can be tested with future
sensitive gamma-ray observations of the accompanying pi0-decays. We identify
Perseus (A 426), Virgo, Ophiuchus, and Coma (A 1656) as the most promising
candidate clusters for such observations.Comment: 20 pages, 8 figures. Corrected Figure 3 to match the erratum accepted
by A&
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
Low Frequency VLA Observations of Abell 754: Evidence for a Cluster Radio Halo and Possible Radio Relics
We present 74 MHz and 330 MHz VLA observations of Abell 754. Diffuse,
halo-like emission is detected from the center of the cluster at both
frequencies. At 330 MHz the resolution of 90'' distinguishes this extended
emission from previously known point sources. In addition to the halo and at a
much lower level, outlying steep-spectrum emission regions straddle the cluster
center and are seen only at 74 MHz. The location, morphology and spectrum of
this emission are all highly suggestive of at least one, and possibly two
cluster radio relics. Easily obtained higher resolution, higher sensitivity VLA
observations at both frequencies are required to confirm the extended nature of
the halo-like emission and the 74 MHz relic detections. However, since there is
prior evidence that this cluster is or has recently been in the process of a
major merger event, the possible discovery of relics in this system is of great
interest in light of recent observational and theoretical evidence in favor of
a merger-relic connection. We discuss the possible role the merger shock waves,
which are seen in the X-ray emission, may have played in the formation of the
halo and radio relics in A754.Comment: 15 pages including 4 figures. Accepted for publication by Ap
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