66,629 research outputs found
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&
Radio mini-halos and AGN heating in cool core clusters of galaxies
The brightest cluster galaxy (BCG) in the majority of relaxed, cool core
galaxy clusters is radio loud, showing non-thermal radio jets and lobes ejected
by the central active galactic nucleus (AGN). Such relativistic plasma has been
unambiguously shown to interact with the surrounding thermal intra-cluster
medium (ICM) thanks to spectacular images where the lobe radio emission is
observed to fill the cavities in the X-ray-emitting gas. This `radio-mode AGN
feedback' phenomenon, which is thought to quench cooling flows, is widespread
and is critical to understand the physics of the inner regions of galaxy
clusters and the properties of the central BCG. At the same time,
mechanically-powerful AGN are likely to drive turbulence in the central ICM
which may contribute to gas heating and also play a role for the origin of
non-thermal emission on cluster-scales. Diffuse non-thermal emission has been
observed in a number of cool core clusters in the form of a radio mini-halo
surrounding the radio-loud BCG on scales comparable to that of the cooling
region. This contribution outlines the main points covered by the talk on these
topics. In particular, after summarizing the cooling flow regulation by AGN
heating and the non-thermal emission from cool core clusters, we present a
recent study of the largest collection of known mini-halo clusters (~ 20
objects) which investigated the scenario of a common origin of radio mini-halos
and gas heating. We further discuss the prospects offered by future radio
surveys with the Square Kilometre Array (SKA) for building large (>> 100
objects), unbiased mini-halo samples while probing at the same time the
presence of radio-AGN feedback in the host clusters.Comment: 8 pages, 3 figures. Conference proceeding of "The many facets of
extragalactic radio surveys: towards new scientific challenges", 20-23
October 2015, Bologna, Ital
Solar Physics with the Square Kilometre Array
The Square Kilometre Array (SKA) will be the largest radio telescope ever
built, aiming to provide collecting area larger than 1 km. The SKA will
have two independent instruments, SKA-LOW comprising of dipoles organized as
aperture arrays in Australia and SKA-MID comprising of dishes in South Africa.
Currently the phase-1 of SKA, referred to as SKA1, is in its late design stage
and construction is expected to start in 2020. Both SKA1-LOW (frequency range
of 50-350 MHz) and SKA1-MID Bands 1, 2, and 5 (frequency ranges of 350-1050,
950-1760, and 4600-15300 MHz, respectively) are important for solar
observations. In this paper we present SKA's unique capabilities in terms of
spatial, spectral, and temporal resolution, as well as sensitivity and show
that they have the potential to provide major new insights in solar physics
topics of capital importance including (i) the structure and evolution of the
solar corona, (ii) coronal heating, (iii) solar flare dynamics including
particle acceleration and transport, (iv) the dynamics and structure of coronal
mass ejections, and (v) the solar aspects of space weather. Observations of the
Sun jointly with the new generation of ground-based and space-borne instruments
promise unprecedented discoveries.Comment: Accepted for publication in Advances in Space Researc
The Astrophysics of Ultrahigh Energy Cosmic Rays
The origin of the highest energy cosmic rays is still unknown. The discovery
of their sources will reveal the workings of the most energetic astrophysical
accelerators in the universe. Current observations show a spectrum consistent
with an origin in extragalactic astrophysical sources. Candidate sources range
from the birth of compact objects to explosions related to gamma-ray bursts or
to events in active galaxies. We discuss the main effects of propagation from
cosmologically distant sources including interactions with cosmic background
radiation and magnetic fields. We examine possible acceleration mechanisms
leading to a survey of candidate sources and their signatures. New questions
arise from an observed hint of sky anisotropies and an unexpected evolution of
composition indicators. Future observations may reach the necessary sensitivity
to achieve charged particle astronomy and to observe ultrahigh energy photons
and neutrinos, which will further illuminate the workings of the universe at
these extreme energies. In addition to fostering a new understanding of
high-energy astrophysical phenomena, the study of ultrahigh energy cosmic rays
can constrain the structure of the Galactic and extragalactic magnetic fields
as well as probe particle interactions at energies orders of magnitude higher
than achieved in terrestrial accelerators.Comment: Draft of solicited review article; 44 pages and 12 figures; Final
version to appear in Annual Review of Astronomy and Astrophysics vol. 49
(2011
Testing the radio halo-cluster merger scenario. The case of RXCJ2003.5-2323
We present a combined radio, X-ray and optical study of the galaxy cluster
RXCJ2003.5-2323. The cluster hosts one of the largest, most powerful and
distant giant radio halos known to date, suggesting that it may be undergoing a
strong merger process. The aim of our multiwavelength study is to investigate
the radio-halo cluster merger scenario. We studied the radio properties of the
giant radio halo in RXCJ2003.5-2323 by means of new radio data obtained at 1.4
GHz with the Very Large Array, and at 240 MHz with the Giant Metrewave Radio
Telescope, in combination with previously published GMRT data at 610 MHz. The
dynamical state of the cluster was investigated by means of X-ray Chandra
observations and optical ESO--NTT observations. Our study confirms that
RXCJ2003.5-2323 is an unrelaxed cluster. The unusual filamentary and clumpy
morphology of the radio halo could be due to a combination of the filamentary
structure of the magnetic field and turbulence in the inital stage of a cluster
merger.Comment: 10 page, 10 figures, accepted for publication on A&
A sparse octree gravitational N-body code that runs entirely on the GPU processor
We present parallel algorithms for constructing and traversing sparse octrees
on graphics processing units (GPUs). The algorithms are based on parallel-scan
and sort methods. To test the performance and feasibility, we implemented them
in CUDA in the form of a gravitational tree-code which completely runs on the
GPU.(The code is publicly available at:
http://castle.strw.leidenuniv.nl/software.html) The tree construction and
traverse algorithms are portable to many-core devices which have support for
CUDA or OpenCL programming languages. The gravitational tree-code outperforms
tuned CPU code during the tree-construction and shows a performance improvement
of more than a factor 20 overall, resulting in a processing rate of more than
2.8 million particles per second.Comment: Accepted version. Published in Journal of Computational Physics. 35
pages, 12 figures, single colum
Planning the Future of U.S. Particle Physics (Snowmass 2013): Chapter 4: Cosmic Frontier
These reports present the results of the 2013 Community Summer Study of the
APS Division of Particles and Fields ("Snowmass 2013") on the future program of
particle physics in the U.S. Chapter 4, on the Cosmic Frontier, discusses the
program of research relevant to cosmology and the early universe. This area
includes the study of dark matter and the search for its particle nature, the
study of dark energy and inflation, and cosmic probes of fundamental
symmetries.Comment: 61 page
- …