2,642 research outputs found
Cluster Mergers, Radio Halos and Hard X-ray Tails: A Statistical Magneto-Turbulent Model
There is now firm evidence that the ICM consists of a mixture of hot plasma,
magnetic fields and relativistic particles. The most important evidences for
non-thermal phenomena in galaxy clusters comes from the diffuse Mpc-scale
synchrotron radio emission (radio halos) observed in a growing number of
massive clusters (Feretti 2003) and from hard X-ray (HXR) excess emission
(detected in a few cases) which can be explained in terms of IC scattering of
relativistic electrons off the cosmic microwave background photons
(Fusco-Femiano et al. 2003). There are now growing evidences that giant radio
halos may be naturally accounted for by synchrotron emission from relativistic
electrons reaccelerated by some kind of turbulence generated in the cluster
volume during merger events (Brunetti 2003). With the aim to investigate the
connection between thermal and non-thermal properties of the ICM, we have
developed a statistical magneto-turbulent model which describes the evolution
of the thermal and non-thermal emission from clusters. We calculate the energy
and spectrum of the magnetosonic waves generated during cluster mergers, the
acceleration and evolution of relativistic electrons and thus the resulting
synchrotron and inverse Compton spectra. Here we give a brief description of
the main results, while a more detailed discussion will be presented in a
forthcoming paper. Einstein-De Sitter cosmology, km
, , is assumed.Comment: 3 pages, 2 figures. To appear in the proceedings of IAU Colloquium
195 - "Outskirts of galaxy clusters: intense life in the suburbs", Torino,
Italy, March 12-16, 200
Statistics of Giant Radio Halos from Electron Reacceleration Models
The most important evidence of non-thermal phenomena in galaxy clusters comes
from Giant Radio Halos (GRHs), synchrotron radio sources extended over Mpc
scales, detected in a growing number of massive galaxy clusters. A promising
possibility to explain these sources is given by "in situ" stochastic
reacceleration of relativistic electrons by turbulence generated in the cluster
volume during merger events. Cassano & Brunetti (2005) have recently shown that
the expected fraction of clusters with GRHs and the increase of such a fraction
with cluster mass can be reconciled with present observations provided that a
fraction of 20-30 % of the turbulence in clusters is in the form of
compressible modes. In this work we extend these calculations by including a
scaling of the magnetic field strength with cluster mass. We show that the
observed correlations between the synchrotron radio power of a sample of 17
GRHs and the X-ray properties of the hosting clusters are consistent with, and
actually predicted by a magnetic field dependence on the virial mass of the
form B \propto M^b, with b>0.5 and typical micro Gauss strengths of the average
B intensity. The occurrence of GRHs as a function of both cluster mass and
redshift is obtained. The most relevant findings are that the predicted
luminosity functions of GRHs are peaked around a power P_{1.4 GHz} 10^{24}
W/Hz, and severely cut-off at low radio powers due to the decrease of the
electron reacceleration in smaller galaxy clusters. We expect a total number of
GRHs to be discovered at ~mJy radio fluxes of ~100 at 1.4 GHz. Finally, the
occurrence of GRHs and their number counts at 150 MHz are estimated in view of
the fortcoming operation of low frequency observatories (LOFAR, LWA) and
compared with those at higher radio frequencies.Comment: 21 pages, 17 figures, accepted for publication in MNRA
An elusive radio halo in the merging cluster Abell 781?
Deep radio observations of the galaxy cluster Abell 781 have been carried out
using the Giant Metrewave Radio Telescope at 325 MHz and have been compared to
previous 610 MHz observations and to archival VLA 1.4 GHz data. The radio
emission from the cluster is dominated by a diffuse source located at the
outskirts of the X-ray emission, which we tentatively classify as a radio
relic. We detected residual diffuse emission at the cluster centre at the level
of S(325 MHz)~15-20 mJy. Our analysis disagrees with Govoni et al. (2011), and
on the basis of simple spectral considerations we do not support their claim of
a radio halo with flux density of 20-30 mJy at 1.4 GHz. Abell 781, a massive
and merging cluster, is an intriguing case. Assuming that the residual emission
is indicative of the presence of a radio halo barely detectable at our
sensitivity level, it could be a very steep spectrum source.Comment: 5 pages, 4 figures, 1 table - Accepted for publication on Monthly
Notices of the Royal Astronomical Society Letter
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
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&
On the radio -- X-ray luminosity correlation of radio halos at low radio frequency - Application of the turbulent re-acceleration model
In this paper we show expectations on the radio--X-ray luminosity correlation
of radio halos at 120 MHz. According to the "turbulent re-acceleration
scenario", low frequency observations are expected to detect a new population
of radio halos that, due to their ultra-steep spectra, are missed by present
observations at ~ GHz frequencies. These radio halos should also be less
luminous than presently observed halos hosted in clusters with the same X-ray
luminosity. Making use of Monte Carlo procedures, we show that the presence of
these ultra-steep spectrum halos at 120 MHz causes a steepening and a
broadening of the correlation between the synchrotron power and the cluster
X-ray luminosity with respect to that observed at 1.4 GHz. We investigate the
role of future low frequency radio surveys, and find that the upcoming LOFAR
surveys will be able to test these expectations.Comment: e.g.: 8 pages, 7 figures, accepted for publication in A&
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
Unveiling radio halos in galaxy clusters in the LOFAR era
Giant radio halos are mega-parsec scale synchrotron sources detected in a
fraction of massive and merging galaxy clusters. Radio halos provide one of the
most important pieces of evidence for non-thermal components in large scale
structure. Statistics of their properties can be used to discriminate among
various models for their origin. Therefore, theoretical predictions of the
occurrence of radio halos are important as several new radio telescopes are
about to begin to survey the sky at low frequencies with unprecedented
sensitivity. In this paper we carry out Monte Carlo simulations to model the
formation and evolution of radio halos in a cosmological framework. We extend
previous works on the statistical properties of radio halos in the context of
the turbulent re-acceleration model. First we compute the fraction of galaxy
clusters that show radio halos and derive the luminosity function of radio
halos. Then, we derive differential and integrated number count distributions
of radio halos at low radio frequencies with the main goal to explore the
potential of the upcoming LOFAR surveys. By restricting to the case of clusters
at redshifts <0.6, we find that the planned LOFAR all sky survey at 120 MHz is
expected to detect about 350 giant radio halos. About half of these halos have
spectral indices larger than 1.9 and substantially brighten at lower
frequencies. If detected they will allow for a confirmation that turbulence
accelerates the emitting particles. We expect that also commissioning surveys,
such as MSSS, have the potential to detect about 60 radio halos in clusters of
the ROSAT Brightest Cluster Sample and its extension (eBCS). These surveys will
allow us to constrain how the rate of formation of radio halos in these
clusters depends on cluster mass.Comment: 12 pages, 12 figures, accepted for publication in Astronomy and
Astrophysic
The Extended GMRT Radio Halo Survey I: New upper limits on radio halos and mini-halos
A fraction of galaxy clusters host diffuse radio sources called radio halos,
radio relics and mini-halos. We present the sample and first results from the
Extended GMRT Radio Halo Survey (EGRHS)- an extension of the GMRT Radio Halo
Survey (GRHS, Venturi et al. 2007, 2008). It is a systematic radio survey of
galaxy clusters selected from the REFLEX and eBCS X-ray catalogs . Analysis of
GMRT data at 610/ 235/ 325 MHz on 12 galaxy clusters are presented. We report
the detection of a newly discovered mini-halo in the cluster RXJ1532.9+3021 at
610 MHz. A small scale relic (~200 kpc) is suspected in the cluster Z348. We do
not detect cluster-scale diffuse emission in 11 clusters. Robust upper limits
on the detection of radio halo of size of 1 Mpc are determined. We also present
upper limits on the detections of mini-halos in a sub-sample of cool-core
clusters. The upper limits for radio halos and mini-halos are plotted in the
radio power- X-ray luminosity plane and the correlations are discussed. Diffuse
extended emission, not related to the target clusters, but detected as
by-products in the sensitive images of two of the cluster fields (A689 and
RXJ0439.0+0715) are reported. Based on the information about the presence of
radio halos (or upper limits), available on 48 clusters out of the total sample
of 67 clusters (EGRHS+GRHS), we find that ~23% of the clusters host radio
halos. The radio halo fraction rises to ~31%, when only the clusters with X-ray
luminosities >8x10^44 erg/s are considered. Mini-halos are found in ~50 % of
cool-core clusters. A qualitative examination of the X-ray images of the
clusters with no diffuse radio emission indicates that a majority of these
clusters do not show extreme dynamical disturbances and supports the idea that
mergers play an important role in the generation of radio halos/relics.Comment: 21 pages, 18 figures, 3 tables, accepted for publication in A&
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