6,458 research outputs found
A giant radio halo in the massive and merging cluster Abell 1351
We report on the detection of diffuse radio emission in the X-ray luminous
and massive galaxy cluster A1351 (z=0.322) using archival Very Large Array data
at 1.4 GHz. Given its central location, morphology, and Mpc-scale extent, we
classify the diffuse source as a giant radio halo. X-ray and weak lensing
studies show A1351 to be a system undergoing a major merger. The halo is
associated with the most massive substructure. The presence of this source is
explained assuming that merger-driven turbulence may re-accelerate high-energy
particles in the intracluster medium and generate diffuse radio emission on the
cluster scale. The position of A1351 in the logP - logL plane
is consistent with that of all other radio-halo clusters known to date,
supporting a causal connection between the unrelaxed dynamical state of massive
() clusters and the presence of giant radio halos.Comment: 4 pages, 3 figures, proof corrections include
The very steep spectrum radio halo in Abell 697
In this paper we present a detailed study of the giant radio halo in the
galaxy cluster Abell 697, with the aim to constrain its origin and connection
with the cluster dynamics. We performed high sensitivity GMRT observations at
325 MHz, which showed that the radio halo is much brighter and larger at this
frequency, compared to previous 610 MHz observations. In order to derive the
integrated spectrum in the frequency range 325 MHz--1.4 GHz, we re--analysed
archival VLA data at 1.4 GHz and made use of proprietary GMRT data at 610 MHz.
{Our multifrequency analysis shows that the total radio spectrum of the giant
radio halo in A\,697 is very steep, with . %\pm0.1$. Due to energy arguments, a hadronic origin of the
halo is disfavoured by such steep spectrum. Very steep spectrum halos in
merging clusters are predicted in the case that the emitting electrons are
accelerated by turbulence, observations with the upcoming low frequency arrays
will be able to test these expectations.}Comment: 10 pages, 8 figures, A&A in pres
Can giant radio halos probe the merging rate of galaxy clusters?
Radio and X-ray observations of galaxy clusters probe a direct link between
cluster mergers and giant radio halos (RH), suggesting that these sources can
be used as probes of the cluster merging rate with cosmic time. In this paper
we carry out an explorative study that combines the observed fractions of
merging clusters (fm) and RH (fRH) with the merging rate predicted by
cosmological simulations and attempt to infer constraints on merger properties
of clusters that appear disturbed in X-rays and of clusters with RH. We use
morphological parameters to identify merging systems and analyze the currently
largest sample of clusters with radio and X-ray data (M500>6d14 Msun, and
0.2<z<0.33, from the Planck SZ cluster catalogue). We found that in this sample
fm~62-67% while fRH~44-51%. The comparison of the theoretical f_m with the
observed one allows to constrain the combination (xi_m,tau_m), where xi_m and
tau_m are the minimum merger mass ratio and the timescale of merger-induced
disturbance. Assuming tau_m~ 2-3 Gyr, as constrained by simulations, we find
that the observed f_m matches the theoretical one for xi_m~0.1-0.18. This is
consistent with optical and near-IR observations of clusters in the sample
(xi_m~0.14-0.16). The fact that RH are found only in a fraction of merging
clusters may suggest that merger events generating RH are characterized by
larger mass ratio; this seems supported by optical/near-IR observations of RH
clusters in the sample (xi_min~0.2-0.25). Alternatively, RH may be generated in
all mergers but their lifetime is shorter than \tau_m (by ~ fRH/fm). This is an
explorative study, however it suggests that follow up studies using the
forthcoming radio surveys and adequate numerical simulations have the potential
to derive quantitative constraints on the link between cluster merging rate and
RH at different cosmic epochs and for different cluster masses.Comment: 10 pages, 3 figures, accepted for publication in A&
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&
An unlikely radio halo in the low X-ray luminosity galaxy cluster RXC J1514.9-1523
We report the discovery of a giant radio halo in the galaxy cluster RXC
J1514.9-1523 at z=0.22 with a relatively low X-ray luminosity, erg s. This faint, diffuse
radio source is detected with the Giant Metrewave Radio Telescope at 327 MHz.
The source is barely detected at 1.4 GHz in a NVSS pointing that we have
reanalyzed. The integrated radio spectrum of the halo is quite steep, with a
slope \alpha = 1.6 between 327 MHz and 1.4 GHz. While giant radio halos are
common in more X-ray luminous cluster mergers, there is a less than 10%
probability to detect a halo in systems with L_X \ltsim 8 \times 10^{44} erg
s. The detection of a new giant halo in this borderline luminosity
regime can be particularly useful for discriminating between the competing
theories for the origin of ultrarelativistic electrons in clusters.
Furthermore, if our steep radio spectral index is confirmed by future deeper
radio observations, this cluster would provide another example of the recently
discovered population of ultra-steep spectrum radio halos, predicted by the
model in which the cluster cosmic ray electrons are produced by turbulent
reacceleration.Comment: 4 pages, 2 figures - Accepted for publication on A&A Research Note
Dynamical locality of the nonminimally coupled scalar field and enlarged algebra of Wick polynomials
We discuss dynamical locality in two locally covariant quantum field
theories, the nonminimally coupled scalar field and the enlarged algebra of
Wick polynomials. We calculate the relative Cauchy evolution of the enlarged
algebra, before demonstrating that dynamical locality holds in the nonminimally
coupled scalar field theory. We also establish dynamical locality in the
enlarged algebra for the minimally coupled massive case and the conformally
coupled massive case.Comment: 39p
X-ray Emission Processes in Extragalactic Jets, Lobes and Hot Spots
This paper is a brief review of the processes responsible for X-ray emission
from radio jets, lobes and hot spots. Possible photons in inverse Compton
scattering models include the radio synchrotron radiation itself (i.e.
synchrotron self-Compton [SSC] emission), the cosmic microwave background
(CMB), the galaxy starlight and radiation from the active nucleus. SSC emission
has been detected from a number of hot spots. Scattering of the CMB is expected
to dominate for jets (and possibly hot spots) undergoing bulk relativistic
motion close to the direction towards the observer. Scattering of infrared
radiation from the AGN should be observable from radio lobes, especially if
they are close to the active nucleus. Synchrotron radiation is detected in some
sources, most notably the jet of M87. I briefly discuss why different hot spots
emit X-rays by different emission mechanisms and the nature of the synchrotron
spectra.Comment: To be published in the proceedings of the Bologna conference ``The
Physics of Relativistic Jets in the Chandra and XMM Era'', New Astronomy
Revie
The cluster relic source in A521
We present high sensitivity radio observations of the merging cluster A521,
at a mean redsfhit z=0.247. The observations were carried out with the GMRT at
610 MHz and cover a region of 1 square degree, with a sensitivity limit
of = 35 Jy b. The most relevant result of these
observations is the presence of a radio relic at the cluster periphery, at the
edge of a region where group infalling into the main cluster is taking place.
Thanks to the wealth of information available in the literature in the optical
and X-ray bands, a multi--band study of the relic and its surroundings was
performed. Our analysis is suggestive of a connection between this source and
the complex ongoing merger in the A521 region. The relic might be ``revived'
fossil radio plasma through adiabatic compression of the magnetic field or
shock re--acceleration due to the merger events. We also briefly discussed the
possibility that this source is the result of induced ram pressure stripping of
radio lobes associated with the nearby cluster radio galaxy J0454--1016a.
Allowing for the large uncertainties due to the small statistics, the number of
radio emitting early--type galaxies found in A521 is consistent with the
expectations from the standard radio luminosity function for local (z0.09)
cluster ellipticals.Comment: 30 pages 8 figures, 5 tables, accepted by New Astronom
In-Situ Particle Acceleration in Extragalactic Radio Hot Spots: Observations Meet Expectations
We discuss, in terms of particle acceleration, the results from optical VLT
observations of hot spots associated with radio galaxies. On the basis of
observational and theoretical grounds, it is shown that:
1. relatively low radio-radio power hot spots are the optimum candidates for
being detected at optical waves. This is supported by an unprecedented optical
detection rate of 70% out of a sample of low radio power hot spots.
2. the shape of the synchrotron spectrum of hot spots is mainly determined by
the strength of the magnetic field in the region. In particular, the break
frequency, related to the age of the oldest electrons in the hot spots, is
found to increase with decreasing synchrotron power and magnetic field
strength.
Both observational results are in agreement with an in-situ particle
acceleration scenario.Comment: 5 pages, TeX (or Latex, etc), 4 figures, to appear in MNRAS Letter,
Updated reference
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
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