arXiv subjects

J. D. Bray

Publications and source records attributed to J. D. Bray.

29 records · Page 2Linked to original sources

arXiv2014

Comparison of algorithms for determination of rotation measure and Faraday structure I. 1100 - 1400 MHz

(abridged) We run a Faraday structure determination data challenge to benchmark the currently available algorithms including Faraday synthesis (previously called RM synthesis in the literature), wavelet, compressive sampling and $QU$-fitting. The frequency set is similar to POSSUM/GALFACTS with a 300 MHz bandwidth from 1.1 to 1.4 GHz. We define three figures of merit motivated by the underlying science: a) an average RM weighted by polarized intensity, RMwtd, b) the separation $Δϕ$ of two Faraday components and c) the reduced chi-squared. Based on the current test data of signal to noise ratio of about 32, we find that: (1) When only one Faraday thin component is present, most methods perform as expected, with occasional failures where two components are incorrectly found; (2) For two Faraday thin components, QU-fitting routines perform the best, with errors close to the theoretical ones for RMwtd, but with significantly higher errors for $Δϕ$. All other methods including standard Faraday synthesis frequently identify only one component when $Δϕ$ is below or near the width of the Faraday point spread function; (3) No methods, as currently implemented, work well for Faraday thick components due to the narrow bandwidth; (4) There exist combinations of two Faraday components which produce a large range of acceptable fits and hence large uncertainties in the derived single RMs; in these cases, different RMs lead to the same Q, U behavior, so no method can recover a unique input model.

astro-ph.IM↗

arXiv2013

LUNASKA neutrino search with the Parkes and ATCA telescopes

The Moon is used as a target volume for ultra-high energy neutrino searches with terrestrial radio telescopes. The LUNASKA project has conducted observations with the Parkes and ATCA telescopes; and, most recently, with both of them in combination. We present an analysis of the data obtained from these searches, including validation and calibration results for the Parkes-ATCA experiment, as well as a summary of prospects for future observations.

astro-ph.IM↗

arXiv2013

Noise statistics in a fast digital radio receiver: the Bedlam backend for the Parkes Radio Telescope

The digital record of the voltage in a radio telescope receiver, after frequency conversion and sampling at a finite rate, is not a perfect representation of the original analog signal. To detect and characterise a transient event with a duration comparable to the inverse bandwidth it is necessary to compensate for these effects, which modifies the statistics of the signal, making it difficult to determine the significance of a potential detection. We present an analysis of these modified statistics and demonstrate them with experimental results from Bedlam, a new digital backend for the Parkes radio telescope.

astro-ph.IM↗

arXiv2012

Observations and modelling of pulsed radio emission from CU Virginis

We present 13 cm and 20 cm radio observations of the magnetic chemically peculiar star CU Virginis taken with the Australia Telescope Compact Array. We detect two circularly polarised radio pulses every rotation period which confirm previous detections. In the first pulse, the lower frequency emission arrives before the higher frequency emission and the ordering reverses in the second pulse. In order to explain the frequency dependence of the time between the two pulses, we construct a geometric model of the magnetosphere of CU Virginis, and consider various emission angles relative to the magnetic field lines. A simple electron cyclotron maser emission model, in which the emission is perpendicular to the magnetic field lines, is not consistent with our data. A model in which the emission is refracted through cold plasma in the magnetosphere is shown to have the correct pulse arrival time frequency dependence.

astro-ph.SR↗

arXiv2011

LUNASKA simultaneous neutrino searches with multiple telescopes

The most sensitive method for detecting neutrinos at the very highest energies is the lunar Cherenkov technique, which employs the Moon as a target volume, using conventional radio telescopes to monitor it for nanosecond-scale pulses of Cherenkov radiation from particle cascades in its regolith. Multiple-antenna radio telescopes are difficult to effectively combine into a single detector for this purpose, while single antennas are more susceptible to false events from radio interference, which must be reliably excluded for a credible detection to be made. We describe our progress in excluding such interference in our observations with the single-antenna Parkes radio telescope, and our most recent experiment (taking place the week before the ICRC) using it in conjunction with the Australia Telescope Compact Array, exploiting the advantages of both types of telescope.

astro-ph.IM↗

arXiv2010

Status and Strategies of Current LUNASKA Lunar Cherenkov Observations with the Parkes Radio Telescope

LUNASKA (Lunar UHE Neutrino Astrophysics with the Square Kilometre Array) is an ongoing project conducting lunar Cherenkov observations in order to develop techniques for detecting neutrinos with the next generation of radio telescopes. Our current observing campaign is with the 64-metre Parkes radio telescope, using a multibeam receiver with 300 MHz of bandwidth from 1.2-1.5 GHz. Here we provide an overview of the various factors that must be considered in the signal processing for such an experiment. We also briefly describe the flux limits which we expect to set with our current observations, including a directional limit for Centaurus A.

astro-ph.HE↗

arXiv2010

Observations of radio pulses from CU Virginis

The magnetic chemically peculiar star CU Virginis is a unique astrophysical laboratory for stellar magnetospheres and coherent emission processes. It is the only known main sequence star to emit a radio pulse every rotation period. Here we report on new observations of the CU Virginis pulse profile in the 13 and 20\,cm radio bands. The profile is known to be characterised by two peaks of 100$\%$ circularly polarised emission that are thought to arise in an electron-cyclotron maser mechanism. We find that the trailing peak is stable at both 13 and 20\,cm, whereas the leading peak is intermittent at 13\,cm. Our measured pulse arrival times confirm the discrepancy previously reported between the putative stellar rotation rates measured with optical data and with radio observations. We suggest that this period discrepancy might be caused by an unknown companion or by instabilities in the emission region. Regular long-term pulse timing and simultaneous multi-wavelength observations are essential to clarify the behaviour of this emerging class of transient radio source.

astro-ph.SR↗

arXiv2010

LUNASKA Experiment Observational Limits on UHE Neutrinos from Centaurus A and the Galactic Center

We present the first observational limits to the ultra-high energy (UHE) neutrino flux from the Galactic Center, and from Centaurus A which is the nearest active galactic nucleus (AGN). These results are based on our "Lunar UHE Neutrino Astrophysics using the Square Kilometer Array" (LUNASKA) project experiments at the Australia Telescope Compact Array (ATCA). We also derive limits for the previous experiments and compare these limits with expectations for acceleration and super-heavy dark matter models of the origin of UHE cosmic rays.

astro-ph.HE↗

arXiv2010

LUNASKA experiments using the Australia Telescope Compact Array to search for ultra-high energy neutrinos and develop technology for the lunar Cherenkov technique

We describe the design, performance, sensitivity and results of our recent experiments using the Australia Telescope Compact Array (ATCA) for lunar Cherenkov observations with a very wide (600 MHz) bandwidth and nanosecond timing, including a limit on an isotropic neutrino flux. We also make a first estimate of the effects of small-scale surface roughness on the effective experimental aperture, finding that contrary to expectations, such roughness will act to increase the detectability of near-surface events over the neutrino energy-range at which our experiment is most sensitive (though distortions to the time-domain pulse profile may make identification more difficult). The aim of our "Lunar UHE Neutrino Astrophysics using the Square Kilometer Array" (LUNASKA) project is to develop the lunar Cherenkov technique of using terrestrial radio telescope arrays for ultra-high energy (UHE) cosmic ray (CR) and neutrino detection, and in particular to prepare for using the Square Kilometer Array (SKA) and its path-finders such as the Australian SKA Pathfinder (ASKAP) and the Low Frequency Array (LOFAR) for lunar Cherenkov experiments.

astro-ph.HE↗

arXiv2009

Experimental set-up of the LUNASKA lunar Cherenkov observations at the ATCA

This contribution describes the experimental set-up implemented by the LUNASKA project at the Australia Telescope Compact Array (ATCA) to enable the radio-telescope to be used to search for pulses of coherent Cherenkov radiation from UHE particle interactions in the Moon with an unprecedented bandwidth, and hence sensitivity. Our specialised hardware included analogue de-dispersion filters to coherently correct for the dispersion expected of a ~nanosecond pulse in the Earth's ionosphere over our wide (600 MHz) bandwidth, and FPGA-based digitising boards running at 2.048 GHz for pulse detection. The trigger algorithm is described, as are the methods used discriminate between terrestrial RFI and true lunar pulses. We also outline the next stage of hardware development expected to be used in our 2010 observations.

astro-ph.HE↗

arXiv2009

Results of LUNASKA lunar Cherenkov observations at the ATCA

The lunar Cherenkov technique is a method to use radio-telescopes to detect ultra-high energy cosmic rays (CR) and neutrinos ($ν$). By observing the short-duration ($\sim$few nanosecond) pulses of coherent Cherenkov radiation emitted from particle cascades via the Askaryan Effect in the Moon's outer layers (nominally the regolith), the primary particles initiating the cascades may be identified. Our collaboration (LUNASKA) aims to develop the technique to be used with the next generation of giant radio-arrays. Here, we present the results of our two preliminary UHE particle searches using this technique with three antennas at the Australia Telescope Compact Array (ATCA) during February and May 2008.

astro-ph.HE↗