2,235 research outputs found
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
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
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&
On the occurrence of Radio Halos in galaxy clusters - Insight from a mass-selected sample
Giant radio halos (RH) are diffuse Mpc-scale synchrotron sources detected in
a fraction of massive and merging galaxy clusters. An unbiased study of the
statistical properties of RHs is crucial to constrain their origin and
evolution. We aim at investigating the occurrence of RHs and its dependence on
the cluster mass in a SZ-selected sample of galaxy clusters, which is as close
as possible to be a mass-selected sample. Moreover, we analyse the connection
between RHs and merging clusters. We select from the Planck SZ catalogue
(Planck Collaboration XXIX 2014) clusters with
at z=0.08-0.33 and we search for the presence of RHs using the NVSS for z<0.2
and the GMRT RH survey (GRHS, Venturi et al. 2007, 2008) and its extension
(EGRHS, Kale et al. 2013, 2015) for 0.2<z<0.33. We use archival Chandra X-ray
data to derive information on the clusters dynamical status. We confirm that RH
clusters are merging systems while the majority of clusters without RH are
relaxed, thus supporting the idea that mergers play a fundamental role in the
generation of RHs. We find evidence for an increase of the fraction of clusters
with RHs with the cluster mass and this is in line with expectations derived on
the basis of the turbulence re-acceleration scenario. Finally, we discuss the
effect of the incompleteness of our sample on this result.Comment: 11 pages, 7 figures, accepted for publication in A&
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
New giant radio sources and underluminous radio halos in two galaxy clusters
The aim of this work is to analyse the radio properties of the massive and
dynamical disturbed clusters Abell 1451 and Zwcl 0634.1+4750, especially
focusing on the possible presence of diffuse emission. We present new GMRT 320
MHz and JVLA 1.5 GHz observations of these two clusters. We found that both
Abell 1451 and Zwcl 0634.1+4750 host a radio halo with a typical spectrum
(). Similarly to a few other cases reported in the recent
literature, these radio halos are significantly fainter in radio luminosity
with respect to the current radio power-mass correlations and they are smaller
than classical giant radio halos. These underluminous sources might contribute
to shed light on the complex mechanisms of formation and evolution of radio
halos. Furthermore, we detected a candidate radio relic at large distance from
the cluster center in Abell 1451 and a peculiar head tail radio galaxy in Zwcl
0634.1+4750, which might be interacting with a shock front.Comment: 15 pages, 13 figures, accepted for publication in A&
Is the Sunyaev-Zeldovich effect responsible for the observed steepening in the spectrum of the Coma radio halo ?
The spectrum of the radio halo in the Coma cluster is measured over almost
two decades in frequency. The current radio data show a steepening of the
spectrum at higher frequencies, which has implications for models of the radio
halo origin. There is an on-going debate on the possibility that the observed
steepening is not intrinsic to the emitted radiation, but is instead caused by
the SZ effect. Recently, the Planck satellite measured the SZ signal and its
spatial distribution in the Coma cluster allowing to test this hypothesis.
Using the Planck results, we calculated the modification of the radio halo
spectrum by the SZ effect in three different ways. With the first two methods
we measured the SZ-decrement within the aperture radii used for flux
measurements of the halo at the different frequencies. First we adopted the
global compilation of data from Thierbach et al. and a reference aperture
radius consistent with those used by the various authors. Second we used the
available brightness profiles of the halo at different frequencies to derive
the spectrum within two fixed apertures, and derived the SZ-decrement using
these apertures. As a third method we used the quasi-linear correlation between
the y and the radio-halo brightness at 330 MHz discovered by Planck to derive
the modification of the radio spectrum by the SZ-decrement in a way that is
almost independent of the adopted aperture radius. We found that the spectral
modification induced by the SZ-decrement is 4-5 times smaller than that
necessary to explain the observed steepening. Consequently a break or cut-off
in the spectrum of the emitting electrons is necessary to explain current data.
We also show that, if a steepening is absent from the emitted spectrum, future
deep observations at 5 GHz with single dishes are expected to measure a halo
flux in a 40 arcmin radius that would be 7-8 times higher than currently seen.Comment: 8 pages, 6 figures, accepted in Astronomy and Astrophysics (date of
acceptance 19/08/2013
The Extended GMRT Radio Halo Survey II: Further results and analysis of the full sample
The intra-cluster medium contains cosmic rays and magnetic fields that are
manifested through the large scale synchrotron sources, termed as radio halos,
relics and mini-halos. The Extended Giant Metrewave Radio Telescope (GMRT)
Radio Halo Survey (EGRHS) is an extension of the GMRT Radio Halo Survey (GRHS)
designed to search for radio halos using GMRT 610/235 MHz observations. The
GRHS+EGRHS consists of 64 clusters in the redshift range 0.2 -- 0.4 that have
an X-ray luminosity larger than 5x10^44 erg/s in the 0.1 -- 2.4 keV band and
with declinations > -31 deg in the REFLEX and eBCS X-ray cluster catalogues. In
this second paper in the series, GMRT 610/235 MHz data on the last batch of 11
galaxy clusters and the statistical analysis of the full sample are presented.
A new mini-halo in RXJ2129.6+0005 and candidate diffuse sources in Z5247, A2552
and Z1953 are discovered. A unique feature of this survey are the upper limits
on the detections of 1 Mpc sized radio halos; 4 new are presented here making a
total of 31 in the survey. Of the sample, 58 clusters that have adequately
sensitive radio information were used to obtain the most accurate occurrence
fractions so far. The occurrence of radio halos in our X-ray selected sample is
~22%, that of mini-halos is 13% and that of relics is ~5%. The radio power -
X-ray luminosity diagrams for the radio halos and mini-halos with the
detections and upper limits are presented. The morphological estimators namely,
centroid shift (w), concentration parameter (c) and power ratios (P_3/P_0)
derived from the Chandra X-ray images are used as proxies for the dynamical
states of the GRHS+EGRHS clusters. The clusters with radio halos and mini-halos
occupy distinct quadrants in the c-w, c-P_3/P_0 and w - P_3/P_0 planes,
corresponding to the more and less morphological disturbance, respectively. The
non-detections span both the quadrants.Comment: 24 pages, 5 tables, 25 figures, accepted for publication in A&
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