arXiv subjects

R. F. Bradley

Publications and source records attributed to R. F. Bradley.

9 recordsLinked to original sources

arXiv2016

Beamforming Errors in Murchison Widefield Array Phased Array Antennas and their effects on Epoch of Reionization Science

Accurate antenna beam models are critical for radio observations aiming to isolate the redshifted 21cm spectral line emission from the Dark Ages and the Epoch of Reionization and unlock the scientific potential of 21cm cosmology. Past work has focused on characterizing mean antenna beam models using either satellite signals or astronomical sources as calibrators, but antenna-to-antenna variation due to imperfect instrumentation has remained unexplored. We characterize this variation for the Murchison Widefield Array (MWA) through laboratory measurements and simulations, finding typical deviations of order +/- 10-20% near the edges of the main lobe and in the sidelobes. We consider the ramifications of these results for image- and power spectrum-based science. In particular, we simulate visibilities measured by a 100m baseline and find that using an otherwise perfect foreground model, unmodeled beamforming errors severely limit foreground subtraction accuracy within the region of Fourier space contaminated by foreground emission (the "wedge"). This region likely contains much of the cosmological signal, and accessing it will require measurement of per-antenna beam patterns. However, unmodeled beamforming errors do not contaminate the Fourier space region expected to be free of foreground contamination (the "EOR window"), showing that foreground avoidance remains a viable strategy.

astro-ph.IM↗

arXiv2015

Measuring Phased-Array Antenna Beampatterns with High Dynamic Range for the Murchison Widefield Array using 137 MHz ORBCOMM Satellites

Detection of the fluctuations in 21 cm line emission from neutral hydrogen during the Epoch of Reionization in thousand hour integrations poses stringent requirements on calibration and image quality, both of which necessitate accurate primary beam models. The Murchison Widefield Array (MWA) uses phased array antenna elements which maximize collecting area at the cost of complexity. To quantify their performance, we have developed a novel beam measurement system using the 137 MHz ORBCOMM satellite constellation and a reference dipole antenna. Using power ratio measurements, we measure the {\it in situ} beampattern of the MWA antenna tile relative to that of the reference antenna, canceling the variation of satellite flux or polarization with time. We employ angular averaging to mitigate multipath effects (ground scattering), and assess environmental systematics with a null experiment in which the MWA tile is replaced with a second reference dipole. We achieve beam measurements over 30 dB dynamic range in beam sensitivity over a large field of view (65\% of the visible sky), far wider and deeper than drift scans through astronomical sources allow. We verify an analytic model of the MWA tile at this frequency within a few percent statistical scatter within the full width at half maximum. Towards the edges of the main lobe and in the sidelobes, we measure tens of percent systematic deviations. We compare these errors with those expected from known beamforming errors.

astro-ph.IM↗

arXiv2015

Single electron detection and spectroscopy via relativistic cyclotron radiation

It has been understood since 1897 that accelerating charges must emit electromagnetic radiation. Cyclotron radiation, the particular form of radiation emitted by an electron orbiting in a magnetic field, was first derived in 1904. Despite the simplicity of this concept, and the enormous utility of electron spectroscopy in nuclear and particle physics, single-electron cyclotron radiation has never been observed directly. Here we demonstrate single-electron detection in a novel radiofrequency spec- trometer. We observe the cyclotron radiation emitted by individual magnetically-trapped electrons that are produced with mildly-relativistic energies by a gaseous radioactive source. The relativistic shift in the cyclotron frequency permits a precise electron energy measurement. Precise beta elec- tron spectroscopy from gaseous radiation sources is a key technique in modern efforts to measure the neutrino mass via the tritium decay endpoint, and this work demonstrates a fundamentally new approach to precision beta spectroscopy for future neutrino mass experiments.

physics.ins-det↗

arXiv2009

Large Instrument Development for Radio Astronomy

This white paper offers cautionary observations about the planning and development of new, large radio astronomy instruments. Complexity is a strong cost driver so every effort should be made to assign differing science requirements to different instruments and probably different sites. The appeal of shared resources is generally not realized in practice and can often be counterproductive. Instrument optimization is much more difficult with longer lists of requirements, and the development process is longer and less efficient. More complex instruments are necessarily further behind the technology state of the art because of longer development times. Including technology R&D in the construction phase of projects is a growing trend that leads to higher risks, cost overruns, schedule delays, and project de-scoping. There are no technology breakthroughs just over the horizon that will suddenly bring down the cost of collecting area. Advances come largely through careful attention to detail in the adoption of new technology provided by industry and the commercial market. Radio astronomy instrumentation has a very bright future, but a vigorous long-term R&D program not tied directly to specific projects needs to be restored, fostered, and preserved.

astro-ph.IM↗

arXiv2009

Observing Neutral Hydrogen Above Redshift 6: The "Global" Perspective

Above redshift 6, the dominant source of neutral hydrogen in the Universe shifts from localized clumps in and around galaxies and filaments to a pervasive, diffuse component of the intergalactic medium (IGM). This transition tracks the global neutral fraction of hydrogen in the IGM and can be studied, in principle, through the redshifted 21 cm hyperfine transition line. During the last half of the reionization epoch, the mean (global) brightness temperature of the redshifted 21 cm emission is proportional to the neutral fraction, but at earlier times (10 < z < 25), the mean brightness temperature should probe the spin temperature of neutral hydrogen in the IGM. Measuring the (of order 10 mK) mean brightness temperature of the redshifted 21 cm line as a function of frequency (and hence redshift) would chart the early evolution of galaxies through the heating and ionizing of the IGM by their stellar populations. Experiments are already underway to accomplish this task or, at least, provide basic constraints on the evolution of the mean brightness temperature. We provide a brief overview of one of these projects, the Experiment to the Detect the Global EOR Signature (EDGES), and discuss prospects for future results.

astro-ph.GA↗

arXiv2006

A High Resolution Search for Dark-Matter Axions

We have performed a high resolution search for galactic halo axions in cold flows using a microwave cavity detector. The analysis procedure and other details of this search are described. No axion signal was found in the mass range 1.98-2.17 micro-eV. We place upper limits on the density of axions in local discrete flows based on this result.

astro-ph↗

arXiv2005

Results of a Search for Cold Flows of Dark Matter Axions

Theoretical arguments predict that the distribution of cold dark matter in spiral galaxies has peaks in velocity space associated with non-thermalized flows of dark matter particles. We searched for the corresponding peaks in the spectrum of microwave photons from axion to photon conversion in the cavity detector of dark matter axions. We found none and place limits on the density of any local flow of axions as a function of the flow velocity dispersion over the axion mass range 1.98 to 2.17 $μ$eV.

astro-ph↗

arXiv2005

Mitigation of Pulsed Interference to Redshifted HI and OH Observations between 960 and 1215 MHz

The neutral hydrogen 21-cm spectral line (1420.4 MHz) and the four 18-cm lines of the hydroxyl molecule (1612-1720 MHz) are observable at redshifts which put their measured line frequencies well below their protected frequency bands. Part of the redshift ranges (z = 0.171-0.477 for HI and z = 0.37-0.73 for OH) fall in the 960 to 1215 MHz band that is allocated to aircraft navigation. Most of the signals in this band are pulsed emissions of low duty cycle so much of the time between pulses is interference free. This paper outlines the structure and measured properties of signals in this band and demonstrates a signal processing strategy that is effective at removing the pulsed signals from spectra at sensitivities produced by several hours of integration.

astro-ph↗

arXiv2003

An Improved RF Cavity Search for Halo Axions

The axion is a hypothetical elementary particle and cold dark matter candidate. In this RF cavity experiment, halo axions entering a resonant cavity immersed in a static magnetic field convert into microwave photons, with the resulting photons detected by a low-noise receiver. The ADMX Collaboration presents new limits on the axion-to-photon coupling and local axion dark matter halo mass density from a RF cavity axion search in the axion mass range 1.9-2.3 microeV, broadening the search range to 1.9-3.3 microeV. In addition, we report first results from an improved analysis technique.

astro-ph↗