3,228 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
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&
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
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
GMRT Radio Halo Survey in galaxy clusters at z = 0.2 -- 0.4. II.The eBCS clusters and analysis of the complete sample
We present the results of the GMRT cluster radio halo survey. The main
purposes of our observational project are to measure which fraction of massive
galaxy clusters in the redshift range z=0.2--0.4 hosts a radio halo, and to
constrain the expectations of the particle re--acceleration model for the
origin of the non--thermal radio emission. We selected a complete sample of 50
clusters in the X-ray band from the REFLEX (27) and the eBCS (23) catalogues.
In this paper we present Giant Metrewave Radio Telescope (GMRT) observations at
610 MHz for all clusters still lacking high sensitivity radio information, i.e.
16 eBCS and 7 REFLEX clusters, thus completing the radio information for the
whole sample. The typical sensitivity in our images is in the range 1Jy b. We found a radio halo in A697, a diffuse
peripheral source of unclear nature in A781, a core--halo source in Z7160, a
candidate radio halo in A1682 and ``suspect'' central emission in Z2661.
Including the literature information, a total of 10 clusters in the sample host
a radio halo. A very important result of our work is that 25 out of the 34
clusters observed with the GMRT do not host extended central emission at the
sensitivity level of our observations, and for 20 of them firm upper limits to
the radio power of a giant radio halo were derived. The GMRT Radio Halo Survey
shows that radio halos are not common, and our findings on the fraction of
giant radio halos in massive clusters are consistent with the statistical
expectations based on the re--acceleration model. Our results favour primary to
secondary electron models.Comment: A&A in press, 17 pages, 12 figures, 4 tables Version with high
quality figures available on web at
http://www.ira.inaf.it/~tventuri/pap/Venturi_web.pd
Cluster mergers and non-thermal phenomena: expectations from a statistical magneto-turbulent model
The most important evidences for non-thermal phenomena in galaxy clusters
comes from the spectacular synchrotron radio emission diffused over Mpc scale
observed in a growing number of massive clusters. A promising possibility to
explain giant radio halos is given by the presence of relativistic electrons
reaccelerated by some kind of turbulence generated in the cluster volume during
merger events. With the aim to investigate the connection between thermal and
non-thermal properties of the ICM, in this paper we develope a statistical
magneto-turbulent model which describes in a self-consistent way the evolution
of the thermal ICM and that of the non-thermal emission from clusters. Making
use of the extended Press & Schechter formalism, we follow cluster mergers and
estimate the injection rate of the fluid turbulence generated during these
energetic events. We then calculate the evolution of the spectrum of the
relativistic electrons in the ICM during the cluster life by taking into
account both the electron-acceleration due to the merger-driven turbulence and
the relevant energy losses of the electrons. We end up with a synthetic
population of galaxy clusters for which the evolution of the ICM and of the
non-thermal spectrum emitted by the accelerated electrons is calculated. The
generation of detectable non-thermal radio and hard X-ray emission in the
simulated clusters is found to be possible during major merger events for
reliable values of the model parameters. In addition the occurrence of radio
halos as a function of the mass of the parent clusters is calculated and
compared with observations. In this case it is found that the model
expectations are in good agreement with observations.Comment: 18 pages, 8 figures. Accepted for publication in MNRA
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