Software correlators as testbeds for RFI algorithms

In-correlator techniques offer the possibility of identifying and/or excising radio frequency interference (RFI) from interferometric observations at much higher time and/or frequency resolution than is generally possible with the final visibility dataset. Due to the considerable computational requirements of the correlation procedure, cross-correlators have most commonly been implemented using high-speed digital signal processing boards, which typically require long development times and are difficult to alter once complete. "Software" correlators, on the other hand, make use of commodity server machines and a correlation algorithm coded in a high-level language. They are inherently much more flexible and can be developed - and modified - much more rapidly than purpose-built "hardware" correlators. Software correlators are thus a natural choice for testing new RFI detection and mitigation techniques for interferometers. The ease with which software correlators can be adapted to test RFI detection algorithms is demonstrated by the addition of kurtosis detection and plotting to the widely used DiFX software correlator, which highlights previously unknown short -duration RFI at the Hancock VLBA station.Comment: 6 pages, 1 figure, accepted for publication in Proceedings of Science [PoS(RFI2010)035]. Presented at RFI2010, the Third Workshop on RFI Mitigation in Radio Astronomy, 29-31 March 2010, Groningen, The Netherland

Understanding Community Impacts: A Tool for Evaluating Externalities from Local Bio-Fuels Production

The popularity of public investment in local bio-fuels production as a rural development initiative is growing. An important consideration in determining the level of public support for a plant's development, however, is accurately measuring public externalities resulting from plant activity. The purpose of this research was to first develop a set of community multipliers associated with various bio-fuel plant configurations, and then develop an easy to use tool that allows local communities to measure potential benefits based on varying levels of plant activity.

Precision VLBI astrometry: Instrumentation, algorithms and pulsar parallax determination

(Abridged) This thesis describes the development of DiFX, the first general-purpose software correlator for radio interferometry, and its use with the Australian Long Baseline Array (LBA) to complete the largest VLBI pulsar astrometry program undertaken to date in the Southern Hemisphere. This two year astrometry program has resulted in the measurement of seven new pulsar parallaxes, more than trebling the number of measured VLBI pulsar parallaxes in the Southern Hemisphere. The measurements included a determination of the distance and transverse velocity of PSR J0437-4715 with better than 1% accuracy, enabling improved tests of General Relativity, and the first significant measurement of parallax for the famous double pulsar system PSR J0737-3039A/B, which will allow tests of General Relativity in this system to proceed to the 0.01% level. The DiFX software correlator developed to enable this science has been extensively tested and is now an integral part of the upgraded LBA Major National Research Facility; furthermore, it has been selected to facilitate a substantial sensitivity upgrade for the US Very Long Baseline Array.Comment: PhD Thesis, Swinburne University, accepted January 2009. 202 pages, 51 figures. For a version with high resolution images, see http://www.aoc.nrao.edu/~adeller/AdamDellerPhDThesis.pd

Understanding Community Impacts: A Tool for Evaluating Economic Impacts from Local Bio-Fuels Production

The popularity of public investment in local bio-fuel production as a rural development initiative is growing. An important consideration in determining the level of public support for a plant\u27s development, however, is accurately measuring public benefits resulting from plant activity. The purpose of the research reported here was to first develop a set of community multipliers associated with various bio-fuel plant configurations and then to develop an easy-to-use tool that allows local communities to measure potential benefits based on varying levels of plant activity

A wider audience: Turning VLBI into a survey instrument

Radio observations using the Very Long Baseline Interferometry (VLBI) technique typically have fields of view of only a few arcseconds, due to the computational problems inherent in imaging larger fields. Furthermore, sensitivity limitations restrict observations to very compact and bright objects, which are few and far between on the sky. Thus, while most branches of observational astronomy can carry out sensitive, wide-field surveys, VLBI observations are limited to targeted observations of carefully selected objects. However, recent advances in technology have made it possible to carry out the computations required to target hundreds of sources simultaneously. Furthermore, sensitivity upgrades have dramatically increased the number of objects accessible to VLBI observations. The combination of these two developments have enhanced the survey capabilities of VLBI observations such that it is now possible to observe (almost) any point in the sky with milli-arcsecond resolution. In this talk I review the development of wide-field VLBI, which has made significant progress over the last three years.Comment: Invited review at the General Assembly of the Astronomische Gesellschaf

Very long baseline astrometry of PSR J1012+5307 and its implications on alternative theories of gravity

PSR J1012+5307, a millisecond pulsar in orbit with a helium white dwarf (WD), has been timed with high precision for about 25 years. One of the main objectives of this long-term timing is to use the large asymmetry in gravitational binding energy between the neutron star and the WD to test gravitational theories. Such tests, however, will be eventually limited by the accuracy of the distance to the pulsar. Here, we present VLBI (very long baseline interferometry) astrometry results spanning approximately 2.5 years for PSR J1012+5307, obtained with the Very Long Baseline Array as part of the MSPSRPI project. These provide the first proper motion and absolute position for PSR J1012+5307 measured in a quasi-inertial reference frame. From the VLBI results, we measure a distance of $0.83^{+0.06}_{-0.02}$kpc (all the estimates presented in the abstract are at 68% confidence) for PSR J1012+5307, which is the most precise obtained to date. Using the new distance, we improve the uncertainty of measurements of the unmodeled contributions to orbital period decay, which, combined with three other pulsars, places new constraints on the coupling constant for dipole gravitational radiation $\kappa_D=(-1.7\pm1.7)\times 10^{-4}$ and the fractional time derivative of Newton's gravitational constant $\dot{G}/G = -1.8^{\,+5.6}_{\,-4.7}\times 10^{-13}\,{\rm yr^{-1}}$ in the local universe. As the uncertainties of the observed decays of orbital period for the four leading pulsar-WD systems become negligible in $\approx10$ years, the uncertainties for $\dot{G}/G$ and $\kappa_D$ will be improved to $\leq1.5\times10^{-13}\,{\rm yr^{-1}}$ and $\leq1.0\times10^{-4}$, respectively, predominantly limited by the distance uncertainties.Comment: published in ApJ (2020ApJ...896...85D

Perceptually Motivated Wavelet Packet Transform for Bioacoustic Signal Enhancement

A significant and often unavoidable problem in bioacoustic signal processing is the presence of background noise due to an adverse recording environment. This paper proposes a new bioacoustic signal enhancement technique which can be used on a wide range of species. The technique is based on a perceptually scaled wavelet packet decomposition using a species-specific Greenwood scale function. Spectral estimation techniques, similar to those used for human speech enhancement, are used for estimation of clean signal wavelet coefficients under an additive noise model. The new approach is compared to several other techniques, including basic bandpass filtering as well as classical speech enhancement methods such as spectral subtraction, Wiener filtering, and Ephraim–Malah filtering. Vocalizations recorded from several species are used for evaluation, including the ortolan bunting (Emberiza hortulana), rhesus monkey (Macaca mulatta), and humpback whale (Megaptera novaeanglia), with both additive white Gaussian noise and environment recording noise added across a range of signal-to-noise ratios (SNRs). Results, measured by both SNR and segmental SNR of the enhanced wave forms, indicate that the proposed method outperforms other approaches for a wide range of noise conditions

Reconciling optical and radio observations of the binary millisecond pulsar PSR J1640+2224

Previous optical and radio observations of the binary millisecond pulsar PSR J1640+2224 have come to inconsistent conclusions about the identity of its companion, with some observations suggesting the companion is a low-mass helium-core (He-core) white dwarf (WD), while others indicate it is most likely a high-mass carbon-oxygen (CO) WD. Binary evolution models predict PSR J1640+2224 most likely formed in a low-mass X-ray binary (LMXB) based on the pulsar's short spin period and long-period, low-eccentricity orbit, in which case its companion should be a He-core WD with mass about $0.35 - 0.39 \, M_\odot$, depending on metallicity. If it is instead a CO WD, that would suggest the system has an unusual formation history. In this paper we present the first astrometric parallax measurement for this system from observations made with the Very Long Baseline Array (VLBA), from which we determine the distance to be $1520^{+170}_{-150}\,\mathrm{pc}$. We use this distance and a reanalysis of archival optical observations originally taken in 1995 with the Wide Field Planetary Camera 2 (WFPC2) on the Hubble Space Telescope (HST) in order to measure the WD's mass. We also incorporate improvements in calibration, extinction model, and WD cooling models. We find that the existing observations are not sufficient to tightly constrain the companion mass, but we conclude the WD mass is $>0.4\,M_\odot$ with $>90\%$ confidence. The limiting factor in our analysis is the low signal-to-noise ratio of the original HST observations.Comment: 6 pages, 5 figure

A trap-based pulsed positron beam optimised for positronium laser spectroscopy

We describe a pulsed positron beam that is optimised for positronium (Ps) laser-spectroscopy experiments. The system is based on a two-stage Surko-type buffer gas trap that produces 4 ns wide pulses containing up to 5 × 105 positrons at a rate of 0.5-10 Hz. By implanting positrons from the trap into a suitable target material, a dilute positronium gas with an initial density of the order of 107 cm−3 is created in vacuum. This is then probed with pulsed (ns) laser systems, where various Ps-laser interactions have been observed via changes in Ps annihilation rates using a fast gamma ray detector. We demonstrate the capabilities of the apparatus and detection methodology via the observation of Rydberg positronium atoms with principal quantum numbers ranging from 11 to 22 and the Stark broadening of the n = 2 → 11 transition in electric fields