arXiv subjects

S. Webb

Publications and source records attributed to S. Webb.

4 recordsLinked to original sources

arXiv2022

Radio Transients and Variables in the Tenth Deeper, Wider, Faster Observing Run

The Deeper, Wider, Faster (DWF) program coordinates observations with telescopes across the electromagnetic spectrum, searching for transients on timescales of milliseconds to days. The tenth DWF observing run was carried out in near real-time during September 2021 and consisted of six consecutive days of observations of the NGC 6744 galaxy group and a field containing the repeating fast radio burst FRB190711 with the Australian Square Kilometre Array Pathfinder, the Dark Energy Camera, the Hard X-ray Modulation Telescope and the Parkes 64m "Murriyang" radio telescope. In this work we present the results of an image-domain search for transient, variable and circularly polarised sources carried out with ASKAP using data from the observing run, along with test observations prior to the run and follow-up observations carried out during and after the run. We identified eight variable radio sources, consisting of one pulsar, six stellar systems (five of which exhibit circularly polarised emission) and one previously uncatalogued source. Of particular interest is the detection of pulses from the ultra-cool dwarf SCR J1845-6357 with a period of $14.2\pm 0.3$ h, in good agreement with the known optical rotation period, making this the slowest rotating radio-loud ultra-cool dwarf discovered.

astro-ph.HE↗

arXiv2022

Mathematical modelling of adjuvant-enhanced active ingredient leaf uptake of pesticides

The global importance of effective and affordable pesticides to optimise crop yield and to support health of our growing population cannot be understated. But to develop new products or refine existing ones in response to climate and environmental changes is both time-intensive and expensive which is why the agrochemical industry is increasingly interested in using mechanistic models as part of their formulation development toolbox. In this work, we develop such a model to describe uptake of pesticide spray droplets across the leaf surface. We simplify the leaf structure by identifying the outer cuticle as the main barrier to uptake; the result is a novel, hybrid model in which two well-mixed compartments are separated by a membrane in which we describe the spatio-temporal distribution of the pesticide. This leads to a boundary value partial differential equation problem coupled to a pair of ordinary differential equation systems which we solve numerically. We also simplify the pesticide formulation into two key components: the Active Ingredient which produces the desired effect of the pesticide and an Adjuvant which is present in the formulation to facilitate effective absorption of the Active Ingredient into the leaf. This approach gives rise to concentration-dependent diffusion. We take an intuitive approach to parameter estimation using a small experimental data set and subsequently demonstrate the importance of the concentration-dependent diffusion in replicating the data. Finally, we demonstrate the need for further work to identify how the physicochemical properties of pesticides affect flow into and across the leaf surface.

math.DS↗

arXiv2021

Tapered helical undulator system for high efficiency energy extraction from a high brightness electron beam

In this paper we discuss the design choices and construction strategy of the tapered undulator system designed for a high energy extraction efficiency experiment in the ultraviolet region of the electromagnetic spectrum planned for installation at the Argonne National Laboratory Linac Extension Area (LEA) beamline. The undulator is comprised of 4 sections pure permanent magnet Halbach array separated by short break sections, each one of them housing a focusing quadrupole doublet and a phase shifter. The quadrupoles use a novel hybrid design which allows one to vary the gradient and match the beam transversely. The undulator tapering profile is optimized to maximize the energy conversion efficiency from a 343 MeV 1 kA beam into coherent 257.5 nm radiation taking into account the longitudinal current profile generated by the linac.

physics.acc-ph↗

arXiv2020

OzDES multi-object fibre spectroscopy for the Dark Energy Survey: Results and second data release

We present a description of the Australian Dark Energy Survey (OzDES) and summarise the results from its six years of operations. Using the 2dF fibre positioner and AAOmega spectrograph on the 3.9-metre Anglo-Australian Telescope, OzDES has monitored 771 AGN, classified hundreds of supernovae, and obtained redshifts for thousands of galaxies that hosted a transient within the 10 deep fields of the Dark Energy Survey. We also present the second OzDES data release, containing the redshifts of almost 30,000 sources, some as faint as $r_{\mathrm AB}=24$ mag, and 375,000 individual spectra. These data, in combination with the time-series photometry from the Dark Energy Survey, will be used to measure the expansion history of the Universe out to $z\sim1.2$ and the masses of hundreds of black holes out to $z\sim4$. OzDES is a template for future surveys that combine simultaneous monitoring of targets with wide-field imaging cameras and wide-field multi-object spectrographs.

astro-ph.CO↗