1,512 research outputs found
A VSA search for the extended Sunyaev-Zel'dovich Effect in the Corona Borealis Supercluster
We present interferometric imaging at 33 GHz of the Corona Borealis
supercluster, using the extended configuration of the Very Small Array. A total
area of 24 deg^2 has been imaged, with an angular resolution of 11 arcmin and a
sensitivity of 12 mJy/beam. The aim of these observations is to search for
Sunyaev-Zel'dovich (SZ) detections from known clusters of galaxies in this
supercluster and for a possible extended SZ decrement due to diffuse warm/hot
gas in the intercluster medium. We measure negative flux values in the
positions of the ten richest clusters in the region. Collectively, this implies
a 3.0-sigma detection of the SZ effect. In the clusters A2061 and A2065 we find
decrements of approximately 2-sigma. Our main result is the detection of two
strong and resolved negative features at -70+-12 mJy/beam (-157+-27 microK) and
-103+-10 mJy/beam (-230+-23 microK), respectively, located in a region with no
known clusters, near the centre of the supercluster. We discuss their possible
origins in terms of primordial CMB anisotropies and/or SZ signals related to
either unknown clusters or to a diffuse extended warm/hot gas distribution. Our
analyses have revealed that a primordial CMB fluctuation is a plausible
explanation for the weaker feature (probability of 37.82%). For the stronger
one, neither primordial CMB (probability of 0.33%) nor SZ can account alone for
its size and total intensity. The most reasonable explanation, then, is a
combination of both primordial CMB and SZ signal. Finally, we explore what
characteristics would be required for a filamentary structure consisting of
warm/hot diffuse gas in order to produce a significant contribution to such a
spot taking into account the constraints set by X-ray data.Comment: 16 pages, 10 figures. Accepted in MNRA
First results from the Very Small Array -- I. Observational methods
The Very Small Array (VSA) is a synthesis telescope designed to image faint
structures in the cosmic microwave background on degree and sub-degree angular
scales. The VSA has key differences from other CMB interferometers with the
result that different systematic errors are expected. We have tested the
operation of the VSA with a variety of blank-field and calibrator observations
and cross-checked its calibration scale against independent measurements. We
find that systematic effects can be suppressed below the thermal noise level in
long observations; the overall calibration accuracy of the flux density scale
is 3.5 percent and is limited by the external absolute calibration scale.Comment: 9 pages, 10 figures, MNRAS in press (Minor revisions
Maximum-likelihood astrometric geometry calibration of interferometric telescopes: application to the Very Small Array
Interferometers require accurate determination of the array configuration in
order to produce reliable observations. A method is presented for finding the
maximum-likelihood estimate of the telescope geometry, and of other
instrumental parameters, astrometrically from the visibility timelines obtained
from observations of celestial calibrator sources. The method copes
systematically with complicated and unconventional antenna and array
geometries, with electronic bandpasses that are different for each antenna
radiometer, and with low signal-to-noise ratios for the calibrators. The
technique automatically focusses on the geometry errors that are most
significant for astronomical observation. We apply this method to observations
made with the Very Small Array and constrain some 450 telescope parameters,
such as the antenna positions, effective observing frequencies and correlator
amplitudes and phase shifts; this requires only ~ 1 h of CPU time on a typical
workstation.Comment: 9 pages, 8 figures, submitted to MNRA
The highest frequency detection of a radio relic : 16 GHz AMI observations of the 'Sausage' cluster
This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society: Letters. © 2014 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.We observed the cluster CIZA J2242.8+5301 with the Arcminute Microkelvin Imager at 16 GHz and present the first high radio-frequency detection of diffuse, non-thermal cluster emission. This cluster hosts a variety of bright, extended, steep-spectrum synchrotron-emitting radio sources, associated with the intracluster medium, called radio relics. Most notably, the northern, Mpc-wide, narrow relic provides strong evidence for diffusive shock acceleration in clusters. We detect a puzzling, flat-spectrum, diffuse extension of the southern relic, which is not visible in the lower radio-frequency maps. The northern radio relic is unequivocally detected and measures an integrated flux of 1.2 ± 0.3 mJy. While the low-frequency (<2 GHz) spectrum of the northern relic is well represented by a power law, it clearly steepens towards 16 GHz. This result is inconsistent with diffusive shock acceleration predictions of ageing plasma behind a uniform shock front. The steepening could be caused by an inhomogeneous medium with temperature/density gradients or by lower acceleration efficiencies of high energy electrons. Further modelling is necessary to explain the observed spectrum.Peer reviewe
Searching for non-Gaussianity in the VSA data
We have tested Very Small Array (VSA) observations of three regions of sky
for the presence of non-Gaussianity, using high-order cumulants, Minkowski
functionals, a wavelet-based test and a Bayesian joint power
spectrum/non-Gaussianity analysis. We find the data from two regions to be
consistent with Gaussianity. In the third region, we obtain a 96.7% detection
of non-Gaussianity using the wavelet test. We perform simulations to
characterise the tests, and conclude that this is consistent with expected
residual point source contamination. There is therefore no evidence that this
detection is of cosmological origin. Our simulations show that the tests would
be sensitive to any residual point sources above the data's source subtraction
level of 20 mJy. The tests are also sensitive to cosmic string networks at an
rms fluctuation level of (i.e. equivalent to the best-fit observed
value). They are not sensitive to string-induced fluctuations if an equal rms
of Gaussian CDM fluctuations is added, thereby reducing the fluctuations due to
the strings network to rms . We especially highlight the usefulness
of non-Gaussianity testing in eliminating systematic effects from our data.Comment: Minor corrections; accepted for publication to MNRA
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