4,006 research outputs found
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
Study of the solar signal in mean Central Europe temperature series from 1760 to 1998
We used a new series, highly reliable and representing the mean surface temperature of Central Europe for the period 1760-1998, to study Sun-Climate relationships. The results indicate that the influence of solar activity is evident only on a long time scale, in particular for the period 1860-present. On a short time scale it is not directly evident. From the spectral analysis we deduced that
the strength of solar signal in the temperature series has an intermittent behaviour. We proposed a mechanism of resonance between the two non-linear systems, the Sun and Earth climate, to explain our results
The turbulent pressure support in galaxy clusters revisited
Due to their late formation in cosmic history, clusters of galaxies are not
fully in hydrostatic equilibrium and the gravitational pull of their mass at a
given radius is expected not to be entirely balanced by the thermal gas
pressure. Turbulence may supply additional pressure, and recent (X-ray and SZ)
hydrostatic mass reconstructions claim a pressure support of of
the total pressure at . In this work we show that, after carefully
disentangling bulk from small-scale turbulent motions in high-resolution
simulations of galaxy clusters, we can constrain which fraction of the gas
kinetic energy effectively provides pressure support in the cluster's
gravitational potential. While the ubiquitous presence of radial inflows in the
cluster can lead to significant bias in the estimate of the non-thermal
pressure support, we report that only a part of this energy effectively acts as
a source of pressure, providing a support of the order of of the
total pressure at .Comment: 5 pages, 5 pages, accepted, to appear in MNRAS Letter
On The Non Thermal Emission and Acceleration of Electrons in Coma and Other Clusters of Galaxies
Some clusters of galaxies in addition to thermal bremsstrahlung (TB), emit
diffuse radiation from the intercluster medium (ICM) at radio, EUV and hard
x-ray (HXR) ranges. The radio radiation is due to synchrotron by relativistic
electrons, and the inverse Compton (IC) scattering by the cosmic microwave
background radiation of the same electrons is the most natural source for the
HXR and perhaps the EUV emissions. However, simple estimates give a weaker
magnetic field than that suggested by Faraday rotation measurements.
Consequently, non-thermal bremsstrahlung (NTB) and TB have also been suggested
as sources of these emissions. We show that NTB cannot be the source of the
HXRs and that the difficulty with the low magnetic field in the IC model is
alleviated if we take into account the effects of observational bias,
nonisotropic pitch angle distribution and spectral breaks. We derive a spectrum
for the radiating electrons and discuss acceleration scenarios. We show that
continuous and in situ acceleration in the ICM of the background thermal
electrons requires unreasonably high energy input and acceleration of injected
relativistic electrons gives rise to a much flatter spectrum than desired,
unless a large fraction of electrons escape the ICM, in which case one obtains
EUV and HXR emissions extending well beyond the boundaries of the cluster. A
continuous emission by a cooling spectrum resulting from interaction with ICM
of electrons accelerated elsewhere also suffers from similar shortcomings. The
most likely scenario appears to be an episodic injection-acceleration model,
whereby one obtains a time dependent spectrum that for certain phases of its
evolution satisfies all the requirements.Comment: 27 pages, one Table, Four Figures. Latex AAS v5.0. Accepted by Ap
Turbulent pressure support and hydrostatic mass-bias in the intracluster medium
The degree of turbulent pressure support by residual gas motions in galaxy
clusters is not well known. Mass modelling of combined X-ray and Sunyaev
Zel'dovich observations provides an estimate of turbulent pressure support in
the outer regions of several galaxy clusters. Here, we test two different
filtering techniques to disentangle bulk from turbulent motions in
non-radiative high-resolution cosmological simulations of galaxy clusters using
the cosmological hydro code ENZO. We find that the radial behavior of the ratio
of non-thermal pressure to total gas pressure as a function of cluster-centric
distance can be described by a simple polynomial function. The typical
non-thermal pressure support in the centre of clusters is 5%, increasing
to 15% in the outskirts, in line with the pressure excess found in recent
X-ray observations. While the complex dynamics of the ICM makes it impossible
to reconstruct a simple correlation between turbulent motions and hydrostatic
bias, we find that a relation between them can be established using the median
properties of a sample of objects. Moreover, we estimate the contribution of
radial accelerations to the non-thermal pressure support and conclude that it
decreases moving outwards from 40% (in the core) to 15% (in the cluster's
outskirts). Adding this contribution to one provided by turbulence, we show
that it might account for the entire observed hydrostatic bias in the innermost
regions of the clusters, and for less than 80% of it at .Comment: 20 pages; 21 figures; Substantial Revision; MNRAS in pres
Extreme summer temperatures in Western Europe
Abstract. We discuss the evolution of summer temperature extremes over Western Europe during 1961–2004 in the context of current climate warming. Using a parametric approach, we investigate the role of properties and changes in probability density functions of daily temperatures in modifying the frequency of severe, isolated events. In this perspective, the recent intensification of extremely warm events over Europe turns out to be well consistent with a pure, nonuniform shift of mean values, with no room for conjectures about increasing temperature variability
A KAT-7 view of a low-mass sample of galaxy clusters
Radio observations over the last two decades have provided evidence that
diffuse synchrotron emission in the form of megaparsec-scale radio halos in
galaxy clusters is likely tracing regions of the intracluster medium where
relativistic particles are accelerated during cluster mergers. In this paper we
present results of a survey of 14 galaxy clusters carried out with the
7-element Karoo Array Telescope at 1.86 GHz, aimed to extend the current
studies of radio halo occurrence to systems with lower masses (M M). We found upper limits at the Watt Hz level for of the sample, confirming that
bright radio halos in less massive galaxy clusters are statistically rare.Comment: 7 pages, 4 figures. Conference proceeding of "The many facets of
extragalactic radio surveys: towards new scientific challenges", 20-23
October 2105, Bologna, Ital
Supersymmetric Field-Theoretic Models on a Supermanifold
We propose the extension of some structural aspects that have successfully
been applied in the development of the theory of quantum fields propagating on
a general spacetime manifold so as to include superfield models on a
supermanifold. We only deal with the limited class of supermanifolds which
admit the existence of a smooth body manifold structure. Our considerations are
based on the Catenacci-Reina-Teofillatto-Bryant approach to supermanifolds. In
particular, we show that the class of supermanifolds constructed by
Bonora-Pasti-Tonin satisfies the criterions which guarantee that a
supermanifold admits a Hausdorff body manifold. This construction is the
closest to the physicist's intuitive view of superspace as a manifold with some
anticommuting coordinates, where the odd sector is topologically trivial. The
paper also contains a new construction of superdistributions and useful results
on the wavefront set of such objects. Moreover, a generalization of the
spectral condition is formulated using the notion of the wavefront set of
superdistributions, which is equivalent to the requirement that all of the
component fields satisfy, on the body manifold, a microlocal spectral condition
proposed by Brunetti-Fredenhagen-K\"ohler.Comment: Final version to appear in J.Math.Phy
The Italian air force sea level pressure data set (1951-2000)
A set of 39 homogenised sea level pressure records, extracted from the Italian Air Force dataset (1951-2000), is introduced and analysed for trends. The data consist of 3-hourly observations. Daily mean pressures are obtained using a method that allows biases to be avoided due to the presence of a high fraction of days that do not have all 8 observations. Trend analysis is performed on seasonal and yearly basis and concerns both the individual station records and the series of their averages. The results show a highly significant positive trend in winter and yearly air pressure all over Italy. It is mainly due to a
change-point around 1980. The Italian air pressure records are also compared with the NCAR/NCEP and UKMO gridded data sets. The results give evidence that gridded data capture
most of the trend and variability of air pressure over Italy, even if NCAR/NCEP data display some significant inhomogeneities with respect to the station records
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