[Show abstract][Hide abstract] ABSTRACT: We present an analysis of the diffuse emission at 5 GHz in the first quadrant of the Galactic plane using two months of preliminary
intensity data taken with the C-Band All-Sky Survey (C-BASS) northern instrument at the Owens Valley Radio Observatory, California.
Combining C-BASS maps with ancillary data to make temperature–temperature plots, we find synchrotron spectral indices of β = −2.65 ± 0.05
between 0.408 and 5 GHz and β = −2.72 ± 0.09 between 1.420 and 5 GHz for −10° < |b| < −4°, 20° < l < 40°. Through the subtraction of a radio recombination line free–free template, we determine the synchrotron spectral index
in the Galactic plane (|b| < 4°) to be β = −2.56 ± 0.07 between 0.408 and 5 GHz, with a contribution of 53 ± 8 per cent from free–free emission at
5 GHz. These results are consistent with previous low-frequency measurements in the Galactic plane. By including C-BASS data
in spectral fits, we demonstrate the presence of anomalous microwave emission (AME) associated with the H ii complexes W43, W44 and W47 near 30 GHz, at 4.4σ, 3.1σ and 2.5σ, respectively. The CORNISH (Co-Ordinated Radio ‘N’ Infrared
Survey for High mass star formation) VLA 5-GHz source catalogue rules out the possibility that the excess emission detected
around 30 GHz may be due to ultracompact H ii regions. Diffuse AME was also identified at a 4σ level within 30° < l < 40°, −2° < b < 2° between 5 and 22.8 GHz.
Full-text · Article · Jan 2015 · Monthly Notices of the Royal Astronomical Society
[Show abstract][Hide abstract] ABSTRACT: Proper modelling of astronomical receivers is vital: it describes the systematic errors in the raw data, guides the receiver
design process, and assists data calibration. In this paper, we describe a method of analytically modelling the full signal
and noise behaviour of arbitrarily complex radio receivers. We use electrical scattering matrices to describe the signal behaviour
of individual components in the receiver, and noise correlation matrices to describe their noise behaviour. These are combined
to produce the full receiver model. We apply this approach to a specified receiver architecture: a hybrid of a continuous
comparison radiometer and correlation polarimeter designed for the C-Band All-Sky Survey. We produce analytic descriptions
of the receiver Mueller matrix and noise temperature, and discuss how imperfections in crucial components affect the raw data.
Many of the conclusions drawn are generally applicable to correlation polarimeters and continuous comparison radiometers.
Preview · Article · Oct 2014 · Monthly Notices of the Royal Astronomical Society
[Show abstract][Hide abstract] ABSTRACT: Simulations of SKA1-low were performed to estimate the noise level in images
produced by the telescope over a frequency range 50-600 MHz, which extends the
50-350 MHz range of the current baseline design. The root-mean-square (RMS)
deviation between images produced by an ideal, error-free SKA1-low and those
produced by SKA1-low with varying levels of uncorrelated gain and phase errors
was simulated. The residual in-field and sidelobe noise levels were assessed.
It was found that the RMS deviations decreased as the frequency increased. The
residual sidelobe noise decreased by a factor of ~5 from 50 to 100 MHz, and
continued to decrease at higher frequencies, attributable to wider strong
sidelobes and brighter sources at lower frequencies. The thermal noise limit is
found to range between ~10 - 0.3 $\mu$Jy and is reached after ~100-100 000 hrs
integration, depending on observation frequency, with the shortest integration
time required at ~100 MHz.
[Show abstract][Hide abstract] ABSTRACT: The C-Band All-Sky Survey is a project to map the full sky in total intensity and linear polarization at 5 GHz. The northern
component of the survey uses a broad-band single-frequency analogue receiver fitted to a 6.1-m telescope at the Owens Valley
Radio Observatory in California, USA. The receiver architecture combines a continuous-comparison radiometer and a correlation
polarimeter in a single receiver for stable simultaneous measurement of both total intensity and linear polarization, using
custom-designed analogue receiver components. The continuous-comparison radiometer measures the temperature difference between
the sky and temperature-stabilized cold electrical reference loads. A cryogenic front-end is used to minimize receiver noise,
with a system temperature of ≈30 K in both linear polarization and total intensity. Custom cryogenic notch filters are used
to counteract man-made radio frequency interference. The radiometer 1/f noise is dominated by atmospheric fluctuations, while the polarimeter achieves a 1/f noise knee frequency of 10 mHz, similar to the telescope azimuthal scan frequency.
Full-text · Article · Dec 2013 · Monthly Notices of the Royal Astronomical Society
[Show abstract][Hide abstract] ABSTRACT: We describe the development of two circularly symmetric antennas with high polarization purity and low spill-over. Both were designed to be used in an all-sky polarization and intensity survey at 5 GHz (the C-Band All-Sky Survey, C-BASS). The survey requirements call for very low cross-polar signal levels and far-out sidelobes. Two different existing antennas, with 6.1-m and 7.6-m diameter primaries, were adapted by replacing the feed and secondary optics, resulting in identical beam performances of 0.73° FWHM, cross-polarization better than - 50 dB, and far-out sidelobes below -70 dB. The polarization purity was realized by using a symmetric low-loss dielectric foam support structure for the secondary mirror, avoiding the need for secondary support struts. Ground spill-over was largely reduced by using absorbing baffles around the primary and secondary mirrors, and by the use of a low-sidelobe profiled corrugated feedhorn. The 6.1-m antenna and receiver have been completed and tested. Results show that the co-polar beam matches the design simulations very closely in the main beam and down to levels of - 80 dB in the backlobes. With the absorbing baffles in place the far-out ( >; 100°) sidelobe response is reduced below -90 dB. Cross-polar response could only be measured down to a noise floor of - 20 dB but is also consistent with the design simulations. Temperature loading and groundspill due to the secondary support were measured at less than 1 K.
Full-text · Article · Jan 2013 · IEEE Transactions on Antennas and Propagation
[Show abstract][Hide abstract] ABSTRACT: We present the design and testing of a 2–20-GHz continuum-band analog lag correlator with 16 frequency channels for astronomical interferometry. The correlator has been designed for future use with a prototype single-baseline interferometer operating at 185–275 GHz. The design uses a broad-band Wilkinson divider tree with integral thin-film resistors implemented on an alumina substrate, and custom-made broad-band InGaP/GaAs Gilbert Cell multipliers. The prototype correlator has been fully bench-tested, together with the necessary readout electronics for acquisition of the output signals. The results of these measurements show that the response of the correlator is well behaved over the band. An investigation of the noise behavior also shows that the signal-to-noise ratio of the system is not limited by the correlator performance.
Full-text · Article · Aug 2012 · IEEE Transactions on Instrumentation and Measurement
[Show abstract][Hide abstract] ABSTRACT: We describe an upgrade to the Cosmic Background Imager instrument to increase its surface brightness sensitivity at small
angular scales. The upgrade consisted of replacing the 13 0.9-m antennas with 1.4-m antennas incorporating a novel combination
of design features, which provided excellent sidelobe and spillover performance for low manufacturing cost. Off-the-shelf
spun primaries were used, and the secondary mirrors were oversized and shaped relative to a standard Cassegrain in order to
provide an optimum compromise between aperture efficiency and low spillover lobes. Low-order distortions in the primary mirrors
were compensated for by custom machining of the secondary mirrors. The secondaries were supported on a transparent dielectric
foam cone to minimize scattering. The antennas were tested in the complete instrument, and the beam shape and spillover noise
contributions were as expected. We demonstrate the performance of the telescope and the intercalibration with the previous
system using observations of the Sunyaev–Zel’dovich effect in the cluster Abell 1689. The enhanced instrument has been used
to study the cosmic microwave background, the Sunyaev–Zel’dovich effect and diffuse Galactic emission.
Full-text · Article · Aug 2011 · Monthly Notices of the Royal Astronomical Society
[Show abstract][Hide abstract] ABSTRACT: We present a parametrized model of the intracluster medium that is suitable for jointly analysing pointed observations of
the Sunyaev–Zel'dovich (SZ) effect and X-ray emission in galaxy clusters. The model is based on assumptions of hydrostatic
equilibrium, the Navarro, Frenk and White model for the dark matter and a softened power-law profile for the gas entropy.
We test this entropy-based model against high and low signal-to-noise ratio ratio mock observations of a relaxed and recently
merged cluster from N-body/hydrodynamic simulations, using Bayesian hyperparameters to optimize the relative statistical weighting of the mock
SZ and X-ray data. We find that it accurately reproduces both the global values of the cluster temperature, total mass and
gas mass fraction (fgas), and the radial dependencies of these quantities outside the core (r > 100 kpc). For reference, we also provide a comparison with results from the single isothermal β model. We confirm previous
results that the single isothermal β model can result in significant biases in derived cluster properties.
Full-text · Article · Dec 2010 · Monthly Notices of the Royal Astronomical Society
[Show abstract][Hide abstract] ABSTRACT: The C-Band All-Sky Survey (C-BASS) aims to produce sensitive, all-sky maps of diffuse Galactic emission at 5 GHz in total intensity and linear polarization. These maps will be used (with other surveys) to separate the several astrophysical components contributing to microwave emission, and in particular will allow an accurate map of synchrotron emission to be produced for the subtraction of foregrounds from measurements of the polarized Cosmic Microwave Background. We describe the design of the analog instrument, the optics of our 6.1 m dish at the Owens Valley Radio Observatory, the status of observations, and first-look data. Comment: 10 pages, 11 figures, published in Proceedings of SPIE MIllimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy V (2010), Vol. 7741, 77411I-1 - 77411I-10
Full-text · Article · Aug 2010 · Proceedings of SPIE - The International Society for Optical Engineering
[Show abstract][Hide abstract] ABSTRACT: The Very Small Array (VSA) has been used to survey the ℓ∼ 27° to region of the Galactic plane at a resolution of 13 arcmin. This ℓ-range covers a section through the Local, Sagittarius and the Cetus spiral arms. The survey consists of 44 pointings of the VSA, each with an rms sensitivity of ∼90 mJy beam−1. These data are combined in a mosaic to produce a map of the area. The majority of the sources within the map are H ii regions.The main aim of the programme was to investigate the anomalous radio emission from the warm dust in individual H ii regions of the survey. This programme required making a spectrum extending from GHz frequencies to the far-infrared (FIR) IRAS frequencies for each of nine strong sources selected to lie in unconfused areas. It was necessary to process each of the frequency maps with the same u, v coverage as was used for the VSA 33 GHz observations. The additional radio data were at 1.4, 2.7, 4.85, 8.35, 10.55, 14.35 and 94 GHz in addition to the 100, 60, 25 and 12 μm IRAS bands. From each spectrum the free–free, thermal dust and anomalous dust emission were determined for each H ii region. The mean ratio of 33 GHz anomalous flux density to FIR 100 μm flux density for the nine selected H ii regions was ΔS(33 GHz)/S(100 μm) = 1.10 ± 0.21 × 10−4. When combined with six H ii regions previously observed with the VSA and the Cosmic Background Imager, the anomalous emission from warm dust in H ii regions is detected with a 33 GHz emissivity of 4.65 ± 0.40 μK (MJy sr−1)−1 (11.5σ). This level of anomalous emission is 0.3 to 0.5 of that detected in cool dust clouds.A radio spectrum of the H ii region anomalous emission covering GHz frequencies is constructed. It has the shape expected for spinning dust composed of very small grains. The anomalous radio emission in H ii regions is on average 41 ± 10 per cent of the radio continuum at 33 GHz. Another result is that the excess (i.e. non-free–free) emission from H ii regions at 94 GHz correlates strongly with the 100 μm emission; it is also inversely correlated with the dust temperature. Both these latter results are as expected for very large grain dust emission. The anomalous emission on the other hand is expected to originate in very small spinning grains and correlates more closely with the 25 μm emission.
No preview · Article · Jun 2010 · Monthly Notices of the Royal Astronomical Society
[Show abstract][Hide abstract] ABSTRACT: We present observations between 14.2 and 17.9GHz of 12 reported supernova remnants (SNRs) made with the Arcminute Microkelvin Imager Small Array (AMI SA). In conjunction with data from the literature at lower radio frequencies, we determine spectra of these objects. For well-studied SNRs (Cas A, Tycho's SNR, 3C58 and the Crab Nebula), the results are in good agreement with spectra based on previous results. For the less well-studied remnants, the AMI SA observations provide higher-frequency radio observations than previously available, and better constrain their radio spectra. The AMI SA results confirm a spectral turnover at ~=11GHz for the filled-centre remnant G74.9+1.2. We also see a possible steepening of the spectrum of the filled-centre remnant G54.1+0.3 within the AMI SA frequency band compared with lower frequencies. We confirm that G84.9+0.5, which had previously been identified as a SNR, is rather an HII region and has a flat radio spectrum. We kindly request that any reference to this paper cites `AMI Consortium: Hurley-Walker et al. 2009'. Issuing author: E-mail: firstname.lastname@example.org
Full-text · Article · Jun 2009 · Monthly Notices of the Royal Astronomical Society
[Show abstract][Hide abstract] ABSTRACT: Accurate calibration of data is essential for the current generation of cosmic microwave background (CMB) experiments. Using data from the Very Small Array (VSA), we describe procedures which will lead to an accuracy of 1 per cent or better for experiments such as the VSA and CBI. Particular attention is paid to the stability of the receiver systems, the quality of the site and frequent observations of reference sources. At 30 GHz the careful correction for atmospheric emission and absorption is shown to be essential for achieving 1 per cent precision.
The sources for which a 1 per cent relative flux density calibration was achieved included Cas A, Cyg A, Tau A and NGC 7027 and the planets Venus, Jupiter and Saturn. A flux density, or brightness temperature in the case of the planets, was derived at 33 GHz relative to Jupiter which was adopted as the fundamental calibrator. A spectral index at ∼30 GHz is given for each.
Cas A, Tau A, NGC 7027 and Venus were examined for variability. Cas A was found to be decreasing at 0.394 ± 0.019 per cent yr−1 over the period 2001 March to 2004 August. In the same period Tau A was decreasing at 0.22 ± 0.07 per cent yr−1. A survey of the published data showed that the planetary nebula NGC 7027 decreased at 0.16 ± 0.04 per cent yr−1 over the period 1967–2003. Venus showed an insignificant (1.5 ± 1.3 per cent) variation with Venusian illumination. The integrated polarization of Tau A at 33 GHz was found to be 7.8 ± 0.6 per cent at position angle =148°± 3°.
Preview · Article · Jul 2008 · Monthly Notices of the Royal Astronomical Society
[Show abstract][Hide abstract] ABSTRACT: Accurate calibration of data is essential for the current generation of CMB experiments. Using data from the Very Small Array (VSA), we describe procedures which will lead to an accuracy of 1 percent or better for experiments such as the VSA and CBI. Particular attention is paid to the stability of the receiver systems, the quality of the site and frequent observations of reference sources. At 30 GHz the careful correction for atmospheric emission and absorption is shown to be essential for achieving 1 percent precision. The sources for which a 1 percent relative flux density calibration was achieved included Cas A, Cyg A, Tau A and NGC7027 and the planets Venus, Jupiter and Saturn. A flux density, or brightness temperature in the case of the planets, was derived at 33 GHz relative to Jupiter which was adopted as the fundamental calibrator. A spectral index at ~30 GHz is given for each. Cas A,Tau A, NGC7027 and Venus were examined for variability. Cas A was found to be decreasing at $0.394 \pm 0.019$ percent per year over the period March 2001 to August 2004. In the same period Tau A was decreasing at $0.22\pm 0.07$ percent per year. A survey of the published data showed that the planetary nebula NGC7027 decreased at $0.16\pm 0.04$ percent per year over the period 1967 to 2003. Venus showed an insignificant ($1.5 \pm 1.3$ percent) variation with Venusian illumination. The integrated polarization of Tau A at 33 GHz was found to be $7.8\pm 0.6$ percent at pa $ = 148^\circ \pm 3^\circ$.}
[Show abstract][Hide abstract] ABSTRACT: We describe the source subtraction strategy and observations for the extended Very Small Array (VSA), a cosmic microwave background interferometer operating at 33 GHz. A total of 453 sources were monitored at 33 GHz using a dedicated source subtraction baseline. 131 sources brighter than 20 mJy were directly subtracted from the VSA visibility data. Some characteristics of the subtracted sources, such as spectra and variability, are discussed. The 33-GHz source counts are estimated from a sample selected at 15 GHz. The selection of VSA fields in order to avoid bright sources introduces a bias into the observed counts. This bias is corrected and the resulting source count is estimated to be complete in the flux-density range 20-114 mJy. The 33-GHz source counts are used to calculate a correction to the VSA power spectrum for sources below the subtraction limit.
Full-text · Article · Apr 2008 · Monthly Notices of the Royal Astronomical Society
[Show abstract][Hide abstract] ABSTRACT: — We describe a novel heterodyne interferometer currently under construction at Oxford. The instrument employs new techniques in heterodyne interferometry, with the aim of achieving very high brightness sensitivity in the millimetre band. It is a single-baseline tracking interferometer for operation in the frequency range 185-275 GHz with two 0.4m offset parabolic antennas separated by a 0.5 m baseline. Each antenna feeds an SIS mixer with a 2-20 GHz IF band, driven by a phase-switched LO source. The IF signals from the mixers are processed by a 2-20 GHz analogue complex correlator. The primary science goal of this instrument is to measure the spectrum of the Sunyaev-Zel'dovich effect in galaxy clusters. In particular we intend to measure the frequency of the S-Z null near 217 GHz, which allows the gas temperature of the cluster to be determined. Measuring the spectrum of the S-Z effect requires very high brightness sensitivity with moderate spatial and spectral resolution.
[Show abstract][Hide abstract] ABSTRACT: Aims: We describe a 6-12 GHz analogue correlator that has been developed for use in radio interferometers. Methods: We use a lag-correlator technique to synthesis eight complex spectral channels. Two schemes were considered for sampling the cross-correlation function, using either real or complex correlations, and we developed prototypes for both of them. We opted for the ``add and square'' detection scheme using Schottky diodes over the more commonly used active multipliers because the stability of the device is less critical. Results: We encountered an unexpected problem, in that there were errors in the lag spacings of up to ten percent of the unit spacing. To overcome this, we developed a calibration method using astronomical sources which corrects the effects of the non-uniform sampling as well as gain error and dispersion in the correlator.
Full-text · Article · Mar 2007 · Astronomy and Astrophysics
[Show abstract][Hide abstract] ABSTRACT: Several technologies are now being considered for modulating the polarization in various B-mode instruments, including rotating quasioptical half-wave plates in front of the focal plane array, rotating waveguide half-wave plates and Faraday rotators. It is not at all clear that any of these techniques is feasible without heavy penalty in cost or performance. A potentially much more efficient method is to use a pseudo-correlation polarimeter in conjunction with a planar circuit phase switch. We investigate three different devices for use as mm-wave switches, SIS tunnel junctions, capacitively coupled superconducting nanostrips and RF MEMS. The SIS tunnel junction switches operate by switching between two different bias voltages, while the nanostrip switch operates by changing the impedance of a resonant circuit by driving the nanostrip from the superconducting to normal state. In each case the RF signal sees two substantially different complex impedance states, hence could be switched from one transmission line branch to another. In MEMS this is achieved by mechanical movement of one plate of a parallel plate capacitor system. Although RF MEMS have been reported at high microwave and low mm-wave frequencies, in this work we have investigated cryogenic MEMS for operation at high mm-wave frequencies (225 GHz) using superconducting transmission lines. We present and compare designs and simulations of the performance of phase switches based on all three switching technologies, as well as preliminary experimental results for each of the switches. Finally we also present designs of phase shift circuits that translates the on/off switching into phase modulation.
No preview · Article · Jun 2006 · Proceedings of SPIE - The International Society for Optical Engineering
[Show abstract][Hide abstract] ABSTRACT: We report the first detection of a Sunyaev–Zel'dovich (S–Z) decrement using the Arcminute Microkelvin Imager (AMI). We have
made commissioning observations towards the cluster of galaxies A1914 and detected the S–Z effect with a significance of 17σ
in a uv-tapered map at a mean frequency of 15.75 GHz. The integrated S–Z flux density of −8.6 ± 0.5 mJy is consistent with a simple
X-ray derived model for the intracluster gas. We find that the spectrum of the decrement, measured in the six channels between
13.5–18 GHz, fits well to that expected for the S–Z effect. The sensitivity of the telescope is consistent with the figures
used in our simulations of cluster surveys with AMI.
Full-text · Article · Jun 2006 · Monthly Notices of the Royal Astronomical Society Letters
[Show abstract][Hide abstract] ABSTRACT: (Abridged) We have used the Rayner & Best (1989) smooth tests of goodness-of-fit to study the Gaussianity of the Very Small Array (VSA) data. Out of the 41 published VSA individual pointings dedicated to cosmological observations, 37 are found to be consistent with Gaussianity, whereas four pointings show deviations from Gaussianity. In two of them, these deviations can be explained as residual systematic effects of a few visibility points which, when corrected, have a negligible impact on the angular power spectrum. The non-Gaussianity found in the other two (adjacent) pointings seems to be associated to a local deviation of the power spectrum of these fields with respect to the common power spectrum of the complete data set, at angular scales of the third acoustic peak (l = 700-900). No evidence of residual systematics is found in this case, and unsubstracted point sources are not a plausible explanation either. If those visibilities are removed, a cosmological analysis based on this new VSA power spectrum alone shows no differences in the parameter constraints with respect to our published results, except for the physical baryon density, which decreases by 10 percent. Finally, the method has been also used to analyse the VSA observations in the Corona Borealis supercluster region (Genova-Santos et al. 2005), which show a strong decrement which cannot be explained as primordial CMB. Our method finds a clear deviation (99.82%) with respect to Gaussianity in the second-order moment of the distribution, and which can not be explained as systematic effects. A detailed study shows that the non-Gaussianity is produced in scales of l~500, and that this deviation is intrinsic to the data (in the sense that can not be explained in terms of a Gaussian field with a different power spectrum).