9,665 research outputs found
Radio Properties of the Shapley Concentration. III. Merging Clusters in the A3558 Complex
We present the results of a 22 cm radio survey carried out with the A3558
complex, a chain formed by the merging ACO clusters A3556-A3558-A3562 and
thetwo groups SC1327-312 and SC1323-313, located in the central region of the
complex, a chain formed by the merging ACO clusters A3556-A3558-A3562 and the
two groups SC1327-312 and SC1323-313, located in the central region of the
Shapley Concentration. The purpose of our survey is to study the effects of
cluster mergers on the statistical properties of radio galaxies and to
investigate the connection between mergers and the presence of radio halos and
relic sources. We found that the radio source counts in the A3558 complex are
consistent with the background source counts. Furthermore, we found that no
correlation exists between the local density and the radio source power, and
that steep spectrum radio galaxies are not segregated in denser optical
regions. The radio luminosity function for elliptical and S0 galaxies is
significantly lower than that for cluster type galaxies and for those not
selected to be in clusters at radio powers logP(1.4) > 22.5, implying that the
probability of a galaxy becoming a radio source above this power limit is lower
in the Shapley Concentration compared with any other environment. The detection
of a head-tail source in the centre of A3562, coupled with careful inspection
of the 20 cm NRAO VLA Sky Survey (NVSS) and of 36 cm MOST observations, allowed
us to spot two extended sources in the region between A3562 and SC1329-313,
i.e. a candidate radio halo at the centre of A3562, and low brightness extended
emission around a 14.96 magnitude Shapley galaxy.Comment: 18 pages, 14 figures. Accepted for publication on 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
Two-sided radio emission in ON231 (W Comae)
Recent radio images of the BL Lac object ON231 (W Com, 1219+285) show
remarkable new features in the source structure compared to those previously
published. The images were obtained from observations made with the European
VLBI Network plus MERLIN at 1.6 GHz and 5 GHz after the exceptional optical
outburst occurred in Spring 1998. The up-to-date B band historic light curve of
ON231 is also presented together with the R band luminosity evolution in the
period 1994--1999. We identify the source core in the radio images with the
brightest component having the flattest spectrum. A consequence of this
assumption is the existence of a two--sided emission in ON231 not detected in
previous VLBI images. A further new feature is a large bend in the jet at about
10 mas from the core. The emission extends for about 20 mas after the bend,
which might be due to strong interaction with the environment surrounding the
nucleus. We suggest some possible interpretations to relate the changes in the
source structure with the optical and radio flux density variation in the frame
of the unification model.Comment: 8 pages, 8 figure
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&
Discovery of a giant radio halo in a new Planck galaxy cluster PLCKG171.9-40.7
We report the discovery of a giant radio halo in a new, hot, X-ray luminous
galaxy cluster recently found by Planck, PLCKG171.9-40.7. The radio halo was
found using Giant Metrewave Radio Telescope observations at 235 MHz and 610
MHz, and in the 1.4 GHz data from a NRAO Very Large Array Sky Survey pointing
that we have reanalyzed. The diffuse radio emission is coincident with the
cluster X-ray emission, has an extent of ~1 Mpc and a radio power of ~5x 10^24
W/Hz at 1.4 GHz. Its integrated radio spectrum has a slope of alpha~1.8 between
235 MHz and 1.4 GHz, steeper than that of a typical giant halo. The analysis of
the archival XMM-Newton X-ray data shows that the cluster is hot (~10 keV) and
disturbed, consistent with X-ray selected clusters hosting radio halos. This is
the first giant radio halo discovered in one of the new clusters found by
Planck.Comment: 9 pages, 8 figures and 4 tables. Corrected Figure 7. Matches ApJ
published versio
Black hole and the adiabatic phase
An open system consisting of a scalar field bound to a Kerr black hole whose
mass () and specific angular momentum () are slowly (adiabatically)
perturbed is considered. The adiabatically induced phase and the conditions for
the validity of the adiabatic approximation are obtained. The effect of closed
cycles in parameter space (, plane) on the energy levels of both stable
and unstable scalar field bound states, together with other quantities of
interest, is illustrated. Lastly it is noted that the black hole wavefunction
will acquire an equal and opposite phase to that of matter thus leading to a
change of its effective action (entropy).Comment: Plain TeX, 12 page
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&
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