2,226 research outputs found
New radio halos and relics in clusters of galaxies
We present here new images of relics and halo sources in rich cluster of
galaxies and the correlation between the halo radio surface brightness versus
the cluster bolometric X-ray luminosity.Comment: 2 pages, 4 figures. To appear in the Proceedings of the International
Conference: "The Origin and Evolution of Cosmic Magnetism"; Bologna 29 August
- 2 September 2005; eds R. Beck, G. Brunetti, L. Feretti, and B. Gaensler
(Astronomische Nachrichten, 2006
Structure of the magnetoionic medium around the FR Class I radio galaxy 3C 449
The goal of this work is to constrain the strength and structure of the
magnetic field associated with the environment of the radio source 3C 449,
using observations of Faraday rotation, which we model with a structure
function technique and by comparison with numerical simulations. We assume that
the magnetic field is a Gaussian, isotropic random variable and that it is
embedded in the hot intra-group plasma surrounding the radio source. For this
purpose, we present detailed rotation measure images for the polarized radio
source 3C 449, previously observed with the Very Large Array at seven
frequencies between 1.365 and 8.385 GHz. We quantify the statistics of the
magnetic-field fluctuations by deriving rotation measure structure functions,
which we fit using models derived from theoretical power spectra. We quantify
the errors due to sampling by making multiple two-dimensional realizations of
the best-fitting power spectrum.We also use depolarization measurements to
estimate the minimum scale of the field variations. We then make
three-dimensional models with a gas density distribution derived from X-ray
observations and a random magnetic field with this power spectrum. Under these
assumptions we find that both rotation measure and depolarization data are
consistent with a broken power-law magnetic-field power spectrum, with a break
at about 11 kpc and slopes of 2.98 and 2.07 at smaller and larger scales
respectively. The maximum and minimum scales of the fluctuations are around 65
and 0.2 kpc, respectively. The average magnetic field strength at the cluster
centre is 3.5 +/-1.2 micro-G, decreasing linearly with the gas density within
about 16 kpc of the nucleus.Comment: 19 pages; 14 figures; accepted for publication on A&A. For a high
quality version use ftp://ftp.eso.org/pub/general/guidetti
Supporting Early Career Researchers through Gender Action Plans. A Design and Methodological Toolkit
The spectral index image of the radio halo in the cluster Abell 520 hosting a famous bow shock
Synchrotron radio emission is being detected from an increasing number of
galaxy clusters. Spectral index images are a powerful tool to investigate the
origin, nature, and connection of these sources with the dynamical state of the
cluster. The aim of this work is to investigate the spectral index distribution
of the radio halo in the galaxy cluster A520, a complex system from an optical,
radio, and X-ray point of view. We present deep Very Large Array observations
in total intensity at 325 and 1400 MHz. We produced and analyzed spectral index
images of the radio halo in this frequency range at a resolution of 39" and 60"
and looked for possible correlations with the thermal properties of the
cluster. We find an integrated radio halo spectral index alpha(325-1400) ~
1.12. No strong radial steepening is present and the spectral index
distribution is intrinsically complex with fluctuations only partially due to
measurement errors. The radio halo integrated spectral index and the cluster
temperature follow the global trend observed in other galaxy clusters although
a strong point-to-point correlation between the spectral index and the thermal
gas temperature has not been observed. The complex morphology in the spectral
index image of the radio halo in A520 is in agreement with the primary models
for radio halo formation. The flatness of the radial profile suggests that the
merger is still ongoing and is uniformly and continuously (re-) accelerating
the population of relativistic electrons responsible of the radio emission even
at large (~ 1 Mpc) distances from the cluster center.Comment: 12 pages, 10 figures, A&A accepte
Magnetic Fields and Faraday Rotation in Clusters of Galaxies
We present a numerical approach to investigate the relationship between
magnetic fields and Faraday rotation effects in clusters of galaxies. We can
infer the structure and strength of intra-cluster magnetic fields by comparing
our simulations with the observed polarization properties of extended cluster
radio sources such as radio galaxies and halos. We find the observations
require a magnetic field which fluctuates over a wide range of spatial scales
(at least one order of magnitude). If several polarized radio sources are
located at different projected positions in a galaxy cluster, as is the case
for A119, detailed Faraday rotation images allow us to constrain both the
magnetic field strength and the slope of the power spectrum. Our results show
that the standard analytic expressions applied in the literature overestimate
the cluster magnetic field strengths by a factor of about 2. We investigate the
possible effects of our models on beam depolarization of radio sources whose
radiation traverses the magnetized intracluster medium. Finally, we point out
that radio halos may provide important information about the spatial power
spectrum of the magnetic field fluctuations on large scales. In particular,
different values of the index of the power spectrum produce very different
total intensity and polarization brightness distributions.Comment: 19 pages, 11 figures. Accepted for publication in A&
Modelling and verifying contract-oriented systems in Maude
We address the problem of modelling and verifying contractoriented systems, wherein distributed agents may advertise and stipulate contracts, but — differently from most other approaches to distributed agents — are not assumed to always behave “honestly”. We describe an executable specification in Maude of the semantics of CO2, a calculus for contract-oriented systems [6]. The honesty property [5] characterises those agents which always respect their contracts, in all possible execution contexts. Since there is an infinite number of such contexts, honesty cannot be directly verified by model-checking the state space of an agent (indeed, honesty is an undecidable property in general [5]). The main contribution of this paper is a sound verification technique for honesty. To do that, we safely over-approximate the honesty property by abstracting from the actual contexts a process may be engaged with. Then, we develop a model-checking technique for this abstraction, we describe an implementation in Maude, and we discuss some experiments with it
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