981 research outputs found
Diffuse radio emission in MACS J0025.41222: the effect of a major merger on bulk separation of ICM components
Mergers of galaxy clusters are among the most energetic events in the
Universe. These events have significant impact on the intra-cluster medium,
depositing vast amounts of energy - often in the form of shocks - as well as
heavily influencing the properties of the constituent galaxy population. Many
clusters have been shown to host large-scale diffuse radio emission, known
variously as radio haloes and relics. These sources arise as a result of
electron (re-)acceleration in cluster-scale magnetic fields, although the
processes by which this occurs are still poorly understood. We present new,
deep radio observations of the high-redshift galaxy cluster MACS
J0025.41222, taken with the GMRT at 325 MHz, as well as new analysis of all
archival X-ray observations. We aim to investigate the potential of
diffuse radio emission and categorise the radio population of this cluster,
which has only been covered previously by shallow radio surveys. We produce
low-resolution maps of MACS J0025.41222 through a combination of uv-tapering
and subtracting the compact source population. Radial surface brightness and
mass profiles are derived from the data. We also derive a 2D map of
the ICM temperature. For the first time, two sources of diffuse radio emission
are detected in MACS J0025.41222, on linear scales of several hundred kpc.
Given the redshift of the cluster and the assumed cosmology, these sources
appear to be consistent with established trends in power scaling relations for
radio relics. The X-ray temperature map presents evidence of an asymmetric
temperature profile and tentative identification of a temperature jump
associated with one relic. We classify the pair of diffuse radio sources in
this cluster as a pair of radio relics, given their consistency with scaling
relations, location toward the cluster outskirts, and the available X-ray data.Comment: 20 pages, 15 figures, accepted for publication in A&
Optical nonlinearity enhancement of graded metallic films
The effective linear and third-order nonlinear susceptibility of graded
metallic films with weak nonlinearity have been investigated. Due to the simple
geometry, we were able to derive exactly the local field inside the graded
structures having a Drude dielectric gradation profile. We calculated the
effective linear dielectric constant and third-order nonlinear susceptibility.
We investigated the surface plasmon resonant effect on the optical absorption,
optical nonlinearity enhancement, and figure of merit of graded metallic films.
It is found that the presence of gradation in metallic films yields a broad
resonant plasmon band in the optical region, resulting in a large enhancement
of the optical nonlinearity and hence a large figure of merit. We suggest
experiments be done to check our theoretical predictions, because graded
metallic films can be fabricated more easily than graded particles.Comment: 11 pages, 2 eps figures, submitted to Applied Physics Letter
Early Science with the Karoo Array Telescope: a Mini-Halo Candidate in Galaxy Cluster Abell 3667
Abell 3667 is among the most well-studied galaxy clusters in the Southern
Hemisphere. It is known to host two giant radio relics and a head-tail radio
galaxy as the brightest cluster galaxy. Recent work has suggested the
additional presence of a bridge of diffuse synchrotron emission connecting the
North-Western radio relic with the cluster centre. In this work, we present
full-polarization observations of Abell 3667 conducted with the Karoo Array
Telescope at 1.33 and 1.82 GHz. Our results show both radio relics as well as
the brightest cluster galaxy. We use ancillary higher-resolution data to
subtract the emission from this galaxy, revealing a localised excess, which we
tentatively identify as a radio mini-halo. This mini-halo candidate has an
integrated flux density of mJy beam at 1.37 GHz,
corresponding to a radio power of P
W Hz, consistent with established trends in mini-halo power scaling.Comment: 17 pages, 10 figures, accepted MNRA
Investigating the source of Planck-detected AME: high resolution observations at 15 GHz
The Planck 28.5 GHz maps were searched for potential Anomalous Microwave
Emission (AME) regions on the scale of or smaller, and several
new regions of interest were selected. Ancillary data at both lower and higher
frequencies were used to construct spectral energy distributions (SEDs), which
seem to confirm an excess consistent with spinning dust models. Here we present
higher resolution observations of two of these new regions with the Arcminute
Microkelvin Imager Small Array (AMI SA) between 14 and 18 GHz to test for the
presence of a compact (10 arcmin or smaller) component. For
AME-G107.1+5.2, dominated by the {\sc Hii} region S140, we find evidence for
the characteristic rising spectrum associated with the either the spinning dust
mechanism for AME or an ultra/hyper-compact \textsc{Hii} region across the AMI
frequency band, however for AME-G173.6+2.8 we find no evidence for AME on
scales of arcmin.Comment: 13 pages, 8 figures, 4 tables. Submitted to Advances in Astronomy AME
Special Issu
Terahertz response of dipolar impurities in polar liquids: On anomalous dielectric absorption of protein solutions
A theory of radiation absorption by dielectric mixtures is presented. The
coarse-grained formulation is based on the wavevector-dependent correlation
functions of molecular dipoles of the host polar liquid and a density-density
structure factor of the positions of the solutes. A nonlinear dependence of the
absorption coefficient on the solute concentration is predicted and originates
from the mutual polarization of the liquid surrounding the solutes by the
collective field of the solute dipoles aligned along the radiation field. The
theory is applied to terahertz absorption of hydrated saccharides and proteins.
While the theory gives an excellent account of the observations for saccharides
without additional assumptions and fitting parameters, experimental absorption
coefficient of protein solutions significantly exceeds theoretical calculations
within standard dielectric models and shows a peak against the protein
concentration. A substantial polarization of protein's hydration shell is
required to explain the differences between standard theories and experiment.
When the correlation function of the total dipole moment of the protein with
its hydration shell from numerical simulations is used in the present
analytical model an absorption peak similar to that seen is experiment is
obtained. The result is sensitive to the specifics of protein-protein
interactions in solution. Numerical testing of the theory requires the
combination of terahertz dielectric and small-angle scattering measurements.Comment: 11 p
Electric field inside a "Rossky cavity" in uniformly polarized water
Electric field produced inside a solute by a uniformly polarized liquid is
strongly affected by dipolar polarization of the liquid at the interface. We
show, by numerical simulations, that the electric "cavity" field inside a
hydrated non-polar solute does not follow the predictions of standard Maxwell's
electrostatics of dielectrics. Instead, the field inside the solute tends, with
increasing solute size, to the limit predicted by the Lorentz virtual cavity.
The standard paradigm fails because of its reliance on the surface charge
density at the dielectric interface determined by the boundary conditions of
the Maxwell dielectric. The interface of a polar liquid instead carries a
preferential in-plane orientation of the surface dipoles thus producing
virtually no surface charge. The resulting boundary conditions for
electrostatic problems differ from the traditional recipes, affecting the
microscopic and macroscopic fields based on them. We show that relatively small
differences in cavity fields propagate into significant differences in the
dielectric constant of an ideal mixture. The slope of the dielectric increment
of the mixture versus the solute concentration depends strongly on which
polarization scenario at the interface is realized. A much steeper slope found
in the case of Lorentz polarization also implies a higher free energy penalty
for polarizing such mixtures.Comment: 9 pages, 8 figure
Deep observations of the Super-CLASS super-cluster at 325 MHz with the GMRT: the low-frequency source catalogue
We present the results of 325 MHz GMRT observations of a super-cluster field,
known to contain five Abell clusters at redshift . We achieve a
nominal sensitivity of Jy beam toward the phase centre. We
compile a catalogue of 3257 sources with flux densities in the range
within the entire square degree
field of view. Subsequently, we use available survey data at other frequencies
to derive the spectral index distribution for a sub-sample of these sources,
recovering two distinct populations -- a dominant population which exhibit
spectral index trends typical of steep-spectrum synchrotron emission, and a
smaller population of sources with typically flat or rising spectra. We
identify a number of sources with ultra-steep spectra or rising spectra for
further analysis, finding two candidate high-redshift radio galaxies and three
gigahertz-peaked-spectrum radio sources. Finally, we derive the
Euclidean-normalised differential source counts using the catalogue compiled in
this work, for sources with flux densities in excess of Jy. Our
differential source counts are consistent with both previous observations at
this frequency and models of the low-frequency source population. These
represent the deepest source counts yet derived at 325 MHz. Our source counts
exhibit the well-known flattening at mJy flux densities, consistent with an
emerging population of star-forming galaxies; we also find marginal evidence of
a downturn at flux densities below Jy, a feature so far only seen
at 1.4 GHz.Comment: 25 pages, 18 figures, 10 tables. Accepted for publication in MNRA
Optical nonlinearity enhancement of graded metal-dielectric composite films
We have derived the local electric field inside graded metal-dielectric
composite films with weak nonlinearity analytically, which further yields the
effective linear dielectric constant and third-order nonlinear susceptibility
of the graded structures. As a result, the composition-dependent gradation can
produce a broad resonant plasmon band in the optical region, resulting in a
large enhancement of the optical nonlinearity and hence a large figure of
merit.Comment: 11 pages, 2 figures. To be published in Europhysics Letter
Investigating Differences between Graphical and Textual Declarative Process Models
Declarative approaches to business process modeling are regarded as well
suited for highly volatile environments, as they enable a high degree of
flexibility. However, problems in understanding declarative process models
often impede their adoption. Particularly, a study revealed that aspects that
are present in both imperative and declarative process modeling languages at a
graphical level-while having different semantics-cause considerable troubles.
In this work we investigate whether a notation that does not contain graphical
lookalikes, i.e., a textual notation, can help to avoid this problem. Even
though a textual representation does not suffer from lookalikes, in our
empirical study it performed worse in terms of error rate, duration and mental
effort, as the textual representation forces the reader to mentally merge the
textual information. Likewise, subjects themselves expressed that the graphical
representation is easier to understand
AMI-LA Observations of the SuperCLASS Super-cluster
We present a deep survey of the SuperCLASS super-cluster - a region of sky
known to contain five Abell clusters at redshift - performed using
the Arcminute Microkelvin Imager (AMI) Large Array (LA) at 15.5GHz. Our
survey covers an area of approximately 0.9 square degrees. We achieve a nominal
sensitivity of Jy beam toward the field centre, finding 80
sources above a threshold. We derive the radio colour-colour
distribution for sources common to three surveys that cover the field and
identify three sources with strongly curved spectra - a high-frequency-peaked
source and two GHz-peaked-spectrum sources. The differential source count (i)
agrees well with previous deep radio source count, (ii) exhibits no evidence of
an emerging population of star-forming galaxies, down to a limit of 0.24mJy,
and (iii) disagrees with some models of the 15GHz source population.
However, our source count is in agreement with recent work that provides an
analytical correction to the source count from the SKADS Simulated Sky,
supporting the suggestion that this discrepancy is caused by an abundance of
flat-spectrum galaxy cores as-yet not included in source population models.Comment: 17 pages, 14 figures, 3 tables. Accepted for publication in MNRA
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