9,385 research outputs found
Two-sided radio emission in ON231 (W Comae)
Recent radio images of the BL Lac object ON231 (W Com, 1219+285) show
remarkable new features in the source structure compared to those previously
published. The images were obtained from observations made with the European
VLBI Network plus MERLIN at 1.6 GHz and 5 GHz after the exceptional optical
outburst occurred in Spring 1998. The up-to-date B band historic light curve of
ON231 is also presented together with the R band luminosity evolution in the
period 1994--1999. We identify the source core in the radio images with the
brightest component having the flattest spectrum. A consequence of this
assumption is the existence of a two--sided emission in ON231 not detected in
previous VLBI images. A further new feature is a large bend in the jet at about
10 mas from the core. The emission extends for about 20 mas after the bend,
which might be due to strong interaction with the environment surrounding the
nucleus. We suggest some possible interpretations to relate the changes in the
source structure with the optical and radio flux density variation in the frame
of the unification model.Comment: 8 pages, 8 figure
Occurrence of radio minihalos in a mass-limited sample of galaxy clusters
We investigate the occurrence of radio minihalos --- diffuse radio sources of
unknown origin observed in the cores of some galaxy clusters --- in a
statistical sample of 58 clusters drawn from the Planck Sunyaev-Zel'dovich
cluster catalog using a mass cut (). We
supplement our statistical sample with a similarly-sized non-statistical sample
mostly consisting of clusters in the ACCEPT X-ray catalog with suitable X-ray
and radio data, which includes lower-mass clusters. Where necessary (for 9
clusters), we reanalyzed the Very Large Array archival radio data to determine
if a mihinalo is present. Our total sample includes all 28 currently known and
recently discovered radio minihalos, including 6 candidates. We classify
clusters as cool-core or non-cool core according to the value of the specific
entropy floor in the cluster center, rederived or newly derived from the
Chandra X-ray density and temperature profiles where necessary (for 27
clusters). Contrary to the common wisdom that minihalos are rare, we find that
almost all cool cores - at least 12 out of 15 (80%) - in our complete sample of
massive clusters exhibit minihalos. The supplementary sample shows that the
occurrence of minihalos may be lower in lower-mass cool-core clusters. No
minihalos are found in non-cool-cores or "warm cores". These findings will help
test theories of the origin of minihalos and provide information on the
physical processes and energetics of the cluster cores.Comment: 34 pages, accepted for publication in ApJ. Added a section
"Definition of a minihalo" and an appendix "Radio size and average surface
brigthtness of minihalos and halos
Squeezing arguments and the plurality of informal notions
In this paper we argue that squeezing arguments à la Kreisel fail to univocally capture an informal or intuitive notion of validity. This suggests a form of logical pluralism, at a conceptual level, not only among but also within logical systems
Interdependence between integrable cosmological models with minimal and non-minimal coupling
We consider the relation between exact solutions of cosmological models
having minimally and non-minimally coupled scalar fields. This is done for a
particular class of solvable models which, in the Einstein frame, have
potentials depending on hyperbolic functions and in the Jordan frame, where the
non-minimal coupling is conformal, possess a relatively simple dynamics. We
show that a particular model in this class can be generalized to the cases of
closed and open Friedmann universes and still exhibits a simple dynamics.
Further we illustrate the conditions for the existences of bounces in some
sub-classes of the set of integrable models we have considered.Comment: 15 pages, v2: figures and references added, accepted for publication
in CQ
Integrable cosmological models with non-minimally coupled scalar fields
We obtain general solutions for some flat Friedmann universes filled with a
scalar field in induced gravity models and models including the
Hilbert-Einstein curvature term plus a scalar field conformally coupled to
gravity. As is well known, these models are connected to minimally coupled
models through the combination of a conformal transformation and a
transformation of the scalar field. The explicit forms of the self-interaction
potentials for six exactly solvable models are presented here. We obtain the
general solution for one of the integrable models, namely, the induced gravity
model with a power-law potential for the self-interaction of the scalar field.
We argue that although being mathematically in a one-to-one correspondence with
the solutions in the minimally coupled models, the solutions in the
corresponding non-minimally coupled models are physically different. This is
because the cosmological evolutions seen by an internal observer connected with
the cosmic time can be quite different. The study of a few induced gravity
models with particular potentials gives us an explicit example of such a
difference.Comment: 20 pages, v3: references added, accepted for publication in CQ
Radio halos in merging clusters of galaxies
We present the preliminary results of 235 MHz, 327 MHz and 610 MHz
observations of the galaxy cluster A3562 in the core of the Shapley
Concentration. The purpose of these observations, carried out with the Giant
Metrewave Radio Telescope (GMRT, Pune, India) was to study the radio halo
located at the centre of A3562 and determine the shape of its radio spectrum at
low frequencies, in order to understand the origin of this source. In the
framework of the re--acceleration model, the preliminary analysis of the halo
spectrum suggests that we are observing a young source (few yrs) at the
beginning of the re--acceleration phase.Comment: 3 pages, 2 figures. Proceedings of IAU Colloquium 195 - Outskirts of
Galaxy Clusters: intense life in the suburb
Testing the radio halo-cluster merger scenario. The case of RXCJ2003.5-2323
We present a combined radio, X-ray and optical study of the galaxy cluster
RXCJ2003.5-2323. The cluster hosts one of the largest, most powerful and
distant giant radio halos known to date, suggesting that it may be undergoing a
strong merger process. The aim of our multiwavelength study is to investigate
the radio-halo cluster merger scenario. We studied the radio properties of the
giant radio halo in RXCJ2003.5-2323 by means of new radio data obtained at 1.4
GHz with the Very Large Array, and at 240 MHz with the Giant Metrewave Radio
Telescope, in combination with previously published GMRT data at 610 MHz. The
dynamical state of the cluster was investigated by means of X-ray Chandra
observations and optical ESO--NTT observations. Our study confirms that
RXCJ2003.5-2323 is an unrelaxed cluster. The unusual filamentary and clumpy
morphology of the radio halo could be due to a combination of the filamentary
structure of the magnetic field and turbulence in the inital stage of a cluster
merger.Comment: 10 page, 10 figures, accepted for publication on A&
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