32 research outputs found
A new approach to multi-frequency synthesis in radio interferometry
We present a new approach to multi-frequency synthesis in radio astronomy.
Using Bayesian inference techniques, the new technique estimates the sky
brightness and the spectral index simultaneously. In principle, the bandwidth
of a wide-band observation can be fully exploited for sensitivity and
resolution, currently only limited by higher order effects like spectral
curvature. Employing this new approach, we further present a multi-frequency
extension to the imaging algorithm RESOLVE. In simulations, this new algorithm
outperforms current multi-frequency imaging techniques like MS-MF-CLEAN.Comment: 13 pages, 5 fugures, submitted to Astronomy and Astrophysic
Imprints of magnetic power and helicity spectra on radio polarimetry statistics
Statistical properties of turbulent magnetic fields in radio-synchrotron
sources should imprint on the statistics of polarimetric observables. In search
of these imprints, we calculate correlation and cross-correlation functions
from a set of observables containing the total intensity I, the polarized
intensity P and the Faraday depth phi. The correlation functions are evaluated
for all combinations of observables up to fourth order in the magnetic field B.
We derive these as far as possible analytically and from first principles only
using some basic assumptions such as Gaussian statistics of the underlying
magnetic field in the observed region and statistical homogeneity. We further
assume some simplifications to reduce the complexity of the calculations, as
for a start we were interested in a proof of concept. Using this statistical
approach, we show that it is in principle possible to gain information about
the helical part of the magnetic power spectrum, namely via the correlation
functions and . Using this insight, we
construct an easy-to-use test for helicity, called LITMUS (Local Inference Test
for Magnetic fields which Uncovers heliceS). For now, all calculations are
given in a Faraday-free case, but set up in a way so that Faraday rotational
effects could be included later on.Comment: 24 pages, 4 figures; typos corrected; additional explanations in
section 1 and 2; revised and extended derivation in section 5, results
unchange
The mixed problem for the Lam\'e system in two dimensions
We consider the mixed problem for the Lam\'e system of elasticity in a
bounded Lipschitz domain . We suppose that the
boundary is written as the union of two disjoint sets, . We take traction data from the space and Dirichlet data from a
Sobolev space and look for a solution of with the
given boundary conditions. We give a scale invariant condition on and find
an exponent so that for , we have a unique solution of this
boundary value problem with the non-tangential maximal function of the gradient
of the solution in . We also establish the existence of a
unique solution when the data is taken from Hardy spaces and Hardy-Sobolev
spaces with in for some
The nature of the low-frequency emission of M51: First observations of a nearby galaxy with LOFAR
The grand-design spiral galaxy M51 was observed with the LOFAR High Frequency
Antennas (HBA) and imaged in total intensity and polarisation. This observation
covered the frequencies between 115 MHz and 175 MHz. We produced an image of
total emission of M51 at the mean frequency of 151 MHz with 20 arcsec
resolution and 0.3 mJy rms noise, which is the most sensitive image of a galaxy
at frequencies below 300 MHz so far. The integrated spectrum of total radio
emission is described well by a power law, while flat spectral indices in the
central region indicate thermal absorption. We observe that the disk extends
out to 16 kpc and see a break in the radial profile near the optical radius of
the disk. Our main results, the scale lengths of the inner and outer disks at
151 MHz and 1.4 GHz, arm--interarm contrast, and the break scales of the
radio--far-infrared correlations, can be explained consistently by CRE
diffusion, leading to a longer propagation length of CRE of lower energy. The
distribution of CRE sources drops sharply at about 10 kpc radius, where the
star formation rate also decreases sharply. We find evidence that thermal
absorption is primarily caused by HII regions. The non-detection of
polarisation from M51 at 151 MHz is consistent with the estimates of Faraday
depolarisation. Future searches for polarised emission in this frequency range
should concentrate on regions with low star formation rates.Comment: 20 pages, 18 figures, accepted for publication in A&
Evidence for particle re-acceleration in the radio relic in the galaxy cluster PLCKG287.0+32.9
Radio relics are diffuse radio sources observed in galaxy clusters, probably produced by shock acceleration during cluster-cluster mergers. Their large size, of the order of 1 Mpc, indicates that the emitting electrons need to be (re)accelerated locally. The usually invoked diffusive shock acceleration models have been challenged by recent observations and theory. We report the discovery of complex radio emission in the Galaxy cluster PLCKG287.0+32.9, which hosts two relics, a radio halo, and several radio filamentary emission. Optical observations suggest that the cluster is elongated, likely along an intergalactic filament, and displays a significant amount of substructure. The peculiar features of this radio relic are that (1) it appears to be connected to the lobes of a radio galaxy and (2) the radio spectrum steepens on either side of the radio relic. We discuss the origins of these features in the context of particle re-acceleration. © 2014. The American Astronomical Society. All rights reserved.
A radio ridge connecting two galaxy clusters in a filament of the cosmic web
Galaxy clusters are the most massive gravitationally bound structures in the Universe. They grow by accreting smaller structures in a merging process that produces shocks and turbulence in the intracluster gas. We observed a ridge of radio emission connecting the merging galaxy clusters Abell 0399 and Abell 0401 with the Low-Frequency Array (LOFAR) telescope network at 140 megahertz. This emission requires a population of relativistic electrons and a magnetic field located in a filament between the two galaxy clusters. We performed simulations to show that a volume-filling distribution of weak shocks may reaccelerate a preexisting population of relativistic particles, producing emission at radio wavelengths that illuminates the magnetic ridge
An improved map of the Galactic Faraday sky
We aim to summarize the current state of knowledge regarding Galactic Faraday
rotation in an all-sky map of the Galactic Faraday depth. For this we have
assembled the most extensive catalog of Faraday rotation data of compact
extragalactic polarized radio sources to date. In the map making procedure we
use a recently developed algorithm that reconstructs the map and the power
spectrum of a statistically isotropic and homogeneous field while taking into
account uncertainties in the noise statistics. This procedure is able to
identify some rotation angles that are offset by an integer multiple of pi. The
resulting map can be seen as an improved version of earlier such maps and is
made publicly available, along with a map of its uncertainty. For the angular
power spectrum we find a power law behavior with a power law index of -2.14 for
a Faraday sky where an overall variance profile as a function of Galactic
latitude has been removed, in agreement with earlier work. We show that this is
in accordance with a 3D Fourier power spectrum P(k) proportional to k^-2.14 of
the underlying field n_e times B_r under simplifying geometrical and
statistical assumptions.Comment: 16 pages, 11 figures. Update in one data catalog. All results are
available at http://www.mpa-garching.mpg.de/ift/faraday
Observations of a nearby filament of galaxy clusters with the Sardinia Radio Telescope
We report the detection of diffuse radio emission which might be connected to
a large-scale filament of the cosmic web covering a 8deg x 8deg area in the
sky, likely associated with a z~0.1 over-density traced by nine massive galaxy
clusters. In this work, we present radio observations of this region taken with
the Sardinia Radio Telescope. Two of the clusters in the field host a powerful
radio halo sustained by violent ongoing mergers and provide direct proof of
intra-cluster magnetic fields. In order to investigate the presence of
large-scale diffuse radio synchrotron emission in and beyond the galaxy
clusters in this complex system, we combined the data taken at 1.4 GHz obtained
with the Sardinia Radio Telescope with higher resolution data taken with the
NRAO VLA Sky Survey. We found 28 candidate new sources with a size larger and
X-ray emission fainter than known diffuse large-scale synchrotron cluster
sources for a given radio power. This new population is potentially the tip of
the iceberg of a class of diffuse large-scale synchrotron sources associated
with the filaments of the cosmic web. In addition, we found in the field a
candidate new giant radio galaxy.Comment: 35 pages, 30 figures, MNRAS Accepted, A high-resolution version of
the paper can be found at the link
http://erg.oa-cagliari.inaf.it/preprints/paper_filament.pd