434 research outputs found
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&
Shock acceleration as origin of the radio relic in A521?
We present new high sensitivity observations of the radio relic in A521
carried out with the Giant Metrewave Radio Telescope at 327 MHz and with the
Very Large Array at 4.9 and 8.5 GHz. We imaged the relic at these frequencies
and carried out a detailed spectral analysis, based on the integrated radio
spectrum between 235 MHz and 4.9 GHz, and on the spectral index image in the
frequency range 327-610 MHz. To this aim we used the new GMRT observations and
other proprietary as well as archival data. We also searched for a possible
shock front co-located with the relic on a short archival Chandra X-ray
observation of the cluster. The integrated spectrum of the relic is consistent
with a single power law; the spectral index image shows a clear trend of
steepening going from the outer portion of the relic toward the cluster centre.
We discuss the origin of the source in the light of the theoretical models for
the formation of cluster radio relics. Our results on the spectral properties
of the relic are consistent with acceleration of relativistic electrons by a
shock in the intracluster medium. This scenario is further supported by our
finding of an X-ray surface brightness edge coincident with the outer border of
the radio relic. This edge is likely a shock front.Comment: 13 pages, 12 figures, accepted for publication in A&
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&
Localization Transition of Biased Random Walks on Random Networks
We study random walks on large random graphs that are biased towards a
randomly chosen but fixed target node. We show that a critical bias strength
b_c exists such that most walks find the target within a finite time when
b>b_c. For b<b_c, a finite fraction of walks drifts off to infinity before
hitting the target. The phase transition at b=b_c is second order, but finite
size behavior is complex and does not obey the usual finite size scaling
ansatz. By extending rigorous results for biased walks on Galton-Watson trees,
we give the exact analytical value for b_c and verify it by large scale
simulations.Comment: 4 pages, includes 4 figure
AGN Feedback in groups and clusters of galaxies
The lack of very cool gas at the cores of groups and clusters of galaxies,
even where the cooling time is significantly shorter than the Hubble time, has
been interpreted as evidence of sources that re-heat the intergalactic medium.
Most studies of rich clusters adopt AGN feedback to be this source of heating.
From ongoing GMRT projects involving clusters and groups, we demonstrate how
low-frequency GMRT radio observations, together with Chandra/XMM-Newton X-ray
data, present a unique insight into the nature of feedback, and of the energy
transfer between the AGN and the IGM.Comment: 5 pages, 3 figures, To appear in ASP Conference Series, Vol. 407, The
Low-Frequency Radio Universe, Eds: D. J. Saikia, D. A. Green, Y. Gupta and T.
Venturi (Invited talk, conference held at NCRA-TIFR, Pune, INDIA, 8-12
December, 2008
Weighted distances in scale-free preferential attachment models
We study three preferential attachment models where the parameters are such
that the asymptotic degree distribution has infinite variance. Every edge is
equipped with a non-negative i.i.d. weight. We study the weighted distance
between two vertices chosen uniformly at random, the typical weighted distance,
and the number of edges on this path, the typical hopcount. We prove that there
are precisely two universality classes of weight distributions, called the
explosive and conservative class. In the explosive class, we show that the
typical weighted distance converges in distribution to the sum of two i.i.d.
finite random variables. In the conservative class, we prove that the typical
weighted distance tends to infinity, and we give an explicit expression for the
main growth term, as well as for the hopcount. Under a mild assumption on the
weight distribution the fluctuations around the main term are tight.Comment: Revised version, results are unchanged. 30 pages, 1 figure. To appear
in Random Structures and Algorithm
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