2,458 research outputs found
Planck pre-launch status: Expected LFI polarisation capability
We present a system-level description of the Low Frequency Instrument (LFI) considered as a differencing polarimeter, and evaluate its expected performance. The LFI is one of the two instruments on board the ESA Planck mission to study the cosmic microwave background. It consists of a set of 22 radiometers sensitive to linear polarisation, arranged in orthogonally-oriented pairs connected to 11 feed horns operating at 30, 44 and 70 GHz. In our analysis, the generic Jones and Mueller-matrix formulations for polarimetry are adapted to the special case of the LFI. Laboratory measurements of flight components are combined with optical simulations of the telescope to investigate the values and uncertainties in the system parameters affecting polarisation response. Methods of correcting residual systematic errors are also briefly discussed. The LFI has beam-integrated polarisation efficiency >99% for all detectors, with uncertainties below 0.1%. Indirect assessment of polarisation position angles suggests that uncertainties are generally less than 0°.5, and this will be checked in flight using observations of the Crab nebula. Leakage of total intensity into the polarisation signal is generally well below the thermal noise level except for bright Galactic emission, where the dominant effect is likely to be spectral-dependent terms due to bandpass mismatch between the two detectors behind each feed, contributing typically 1–3% leakage of foreground total intensity. Comparable leakage from compact features occurs due to beam mismatch, but this averages to < 5 × 10^(-4) for large-scale emission. An inevitable feature of the LFI design is that the two components of the linear polarisation are recovered from elliptical beams which differ substantially in orientation. This distorts the recovered polarisation and its angular power spectrum, and several methods are being developed to correct the effect, both in the power spectrum and in the sky maps. The LFI will return a high-quality measurement of the CMB polarisation, limited mainly by thermal noise. To meet our aspiration of measuring polarisation at the 1% level, further analysis of flight and ground data is required. We are still researching the most effective techniques for correcting subtle artefacts in polarisation; in particular the correction of bandpass mismatch effects is a formidable challenge, as it requires multi-band analysis to estimate the spectral indices that control the leakage
A multiband study of Hercules A. II. Multifrequency VLA imaging
We have mapped the powerful radio galaxy Hercules A at six frequencies
spanning 1295 to 8440 MHz using the VLA in all four configurations. Here we
discuss the structure revealed in total intensity, spectral index,
polarization, and projected magnetic field. Our observations clearly reveal the
relation between the bright jets, radio source. The jets and rings form a
coherent structure with a dramatically flatter spectrum than the surrounding
lobes and bridge, strongly suggesting that they represent a recently renewed
outburst from the active nucleus. The spectrum of the lobes is also steeper
than in typical radio sources, and steepens further towards the centre. The
compact core is optically thin and also has a remarkably steep spectrum (alpha
\~ -1.2). There is some evidence that the old lobe material has been swept up
and compressed ahead of the new outburst. We interpret the dramatic asymmetry
in the bright structure, and more subtle differences between diffuse lobe
structures, in terms of relativistic beaming combined with front-to-back
light-travel delays which mean that we view the two lobes at different stages
of the outburst. After correcting for Faraday rotation the projected magnetic
field closely follows the edge of the lobes, the jets, and the rings; the field
pattern in the two lobes is broadly similar. We confirm a strong asymmetry in
depolarization and Faraday rotation, with the jet side the less depolarized and
the flatter spectrum, consistent with general correlations between these
asymmetries. The spectral index asymmetry is clearly present in the `old' lobe
material and so, at least in this case, is not due to beaming; but it can be
understood in terms of the light-travel delay.Comment: 25 pages, 18 figures, accepted for publication in MNRAS. For
associated jpeg files, see http://www.jb.man.ac.uk/~jpl/hera
Global Cosmological Parameters Determined Using Classical Double Radio Galaxies
A sample of 20 powerful extended radio galaxies with redshifts between zero
and two were used to determine constraints on global cosmological parameters.
Data for six radio sources were obtained from the VLA archive, analyzed, and
combined with the sample of 14 radio galaxies used previously by Guerra & Daly
to determine cosmological parameters. The results are consistent with our
previous results, and indicate that the current value of the mean mass density
of the universe is significantly less than the critical value. A universe with
of unity is ruled out at 99.0% confidence, and the best fitting
values of in matter are and
assuming zero space curvature and zero cosmological
constant, respectively. Note that identical results obtain when the low
redshift bin, which includes Cygnus A, is excluded; these results are
independent of whether the radio source Cygnus A is included. The method does
not rely on a zero-redshift normalization.
The radio properties of each source are also used to determine the density of
the gas in the vicinity of the source, and the beam power of the source. The
six new radio sources have physical characteristics similar to those found for
the original 14 sources. The density of the gas around these radio sources is
typical of gas in present day clusters of galaxies. The beam powers are
typically about .Comment: 39 pages includes 21 figures, accepted to Ap
Transfer learning for radio galaxy classification
In the context of radio galaxy classification, most state-of-the-art neural
network algorithms have been focused on single survey data. The question of
whether these trained algorithms have cross-survey identification ability or
can be adapted to develop classification networks for future surveys is still
unclear. One possible solution to address this issue is transfer learning,
which re-uses elements of existing machine learning models for different
applications. Here we present radio galaxy classification based on a 13-layer
Deep Convolutional Neural Network (DCNN) using transfer learning methods
between different radio surveys. We find that our machine learning models
trained from a random initialization achieve accuracies comparable to those
found elsewhere in the literature. When using transfer learning methods, we
find that inheriting model weights pre-trained on FIRST images can boost model
performance when re-training on lower resolution NVSS data, but that inheriting
pre-trained model weights from NVSS and re-training on FIRST data impairs the
performance of the classifier. We consider the implication of these results in
the context of future radio surveys planned for next-generation radio
telescopes such as ASKAP, MeerKAT, and SKA1-MID
Spinning compact binary inspiral II: Conservative angular dynamics
We establish the evolution equations of the set of independent variables
characterizing the 2PN rigorous conservative dynamics of a spinning compact
binary, with the inclusion of the leading order spin-orbit, spin-spin and mass
quadrupole - mass monopole effects, for generic (noncircular, nonspherical)
orbits. More specifically, we give a closed system of first order ordinary
differential equations for the orbital elements of the osculating ellipse and
for the angles characterizing the spin orientations with respect to the
osculating orbit. We also prove that (i) the relative angle of the spins stays
constant for equal mass black holes, irrespective of their orientation, and
(ii) the special configuration of equal mass black holes with equal, but
antialigned spins, both laying in the plane of motion (leading to the largest
recoil found in numerical simulations) is preserved at 2PN level of accuracy,
with leading order spin-orbit, spin-spin and mass quadrupolar contributions
included.Comment: v2: 19 pages, extended, improved, published versio
Racial Residential Segregation in Ohio's Eigh Largest Cities: 1950-1980
Author Institution: Department of Urban Studies, The University of AkronRacial residential segregation patterns in the eight largest Ohio cities are examined from 1950-1980 to determine if certain gains which blacks recently have made in other areas of life have been translated into improved residential integration. Data were collected from the 1950, I960, 1970, and 1980 United States Censuses of Population and Housing at the census tract level. The index of dissimilarity is used to measure residential segregation at both the central city and outside central city levels of the Standard Metropolitan Statistical area. Results indicate improvement in residential segregation during the 1970s for both the central cities and the suburbs, although the levels of segregation remain high overall. Cleveland, home to nearly one-fourth the black population of the state, remains highly segregated, both in the central city and in its suburbs
Synchrotron Polarization at High Galactic Latitude
We present preliminary results from mapping the high-latitude Galactic
polarization with the Effelsberg Telescope at 21 cm. Structures on the
resulting maps are mostly on the scale of several degrees. The results show
detection of polarization over most of the field, at the level of tens of
percent of the synchrotron emission. The evidence of more structure in Stokes Q
and U rather than in suggests the existence of Faraday
rotation.Comment: To be published in the proceedings of "The Cosmic Microwave
Background and its Polarization", New Astronomy Reviews, (eds. S. Hanany and
K.A. Olive
Computational Fluid Dynamics Modelling of Natural Convection in Copper Electrorefining
A computational fluid dynamics (CFD) model of copper electrorefining is discussed. Copper electrorefining takes place in a rectangular geometry, with two electrodes opposing each other, and a source and sink of copper ions at the respective electrodes. The resultant gradients in the copper concentration lead to buoyancy forces, and natural convection develops. The transport of copper ions is coupled to the Navier-Stokes equations in a CFD software package ANSYS CFX (version 11). Validation of the CFD model is provided for several cases varying in size, from a small laboratory scale to large industrial scale, including one that has not been compared with a CFD model previously. The larger scale systems are analysed in terms of the Rayleigh number, and we clarify that the important length scale for turbulence onset is the width of the cell, in addition to the cell height. Clarification of the appropriate turbulence model is given
Discovery of X-ray pulsations from IGR J16320-4751 = AX J1631.9-4752
We report a discovery of strong modulations of the X-ray flux detected from
IGR J16320-4751 = AX J1631.9-4752 with a period of P~1300 sec. We reanalyzed
the data of an XMM-Newton ToO performed soon after the discovery of the source
by INTEGRAL and found the modulation at a period of P=1309+/-40 sec with a high
significance. Modulations of the source flux with two possible periods of ~1300
and ~1500 sec were identified in the ASCA archival data. It is very likely that
the modulation can be interpreted as X-ray pulsations, favouring a pulsar as
the compact object in IGR/AX J16320-4752. Thus for the moment this source
became the fourth source from a new class of highly absorbed binary systems for
which the pulsations are observed.Comment: accepted for the publication in A&A Letters, 4 pages, 4 figure
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