3,913 research outputs found
Application of a Bayesian Method to Absorption Spectral-Line Finding in Simulated ASKAP Data
The large spectral bandwidth and wide field of view of the Australian SKA
Pathfinder radio telescope will open up a completely new parameter space for
large extragalactic HI surveys. Here we focus on identifying and parametrising
HI absorption lines which occur in the line of sight towards strong radio
continuum sources. We have developed a method for simultaneously finding and
fitting HI absorption lines in radio data by using multi-nested sampling, a
Bayesian Monte Carlo algorithm. The method is tested on a simulated ASKAP data
cube, and is shown to be reliable at detecting absorption lines in low
signal-to-noise data without the need to smooth or alter the data. Estimation
of the local Bayesian evidence statistic provides a quantitative criterion for
assigning significance to a detection and selecting between competing
analytical line-profile models.Comment: 8 pages, 5 figures and 1 table; accepted for publication in PAS
Further Observational Evidence for a Critical Ionising Luminosity in Active Galaxies
We report the results of a survey for HI 21-cm absorption at redshifts of z >
2.6 in a new sample of radio sources with the Green Bank and Giant Metrewave
Radio Telescopes. From a total of 25 targets, we report zero detections in the
16 for which optical depth limits could be obtained. Based upon the detection
rate for z > 0.1 associated absorption, we would expect approximately four
detections. Of the 11 which have previously not been searched, there is
sufficient source-frame optical/ultra-violet photometry to determine the
ionising photon rate for four. Adding these to the literature, the hypothesis
that there is a critical rate of logQ = 56 ionising photons per second is now
significant at ~7 sigma. This reaffirms our assertion that searching z > 3
active galaxies for which optical redshifts are available selects sources in
which the ultra-violet luminosity is sufficient to ionise all of the neutral
gas in the host galaxy.Comment: Accepted by MNRA
A search for 21 cm HI absorption in AT20G compact radio galaxies
We present results from a search for 21 cm associated HI absorption in a
sample of 29 radio sources selected from the Australia Telescope 20 GHz survey.
Observations were conducted using the Australia Telescope Compact Array
Broadband Backend, with which we can simultaneously look for 21 cm absorption
in a redshift range of 0.04 < z < 0.08, with a velocity resolution of 7 km/s .
In preparation for future large-scale H I absorption surveys we test a
spectral-line finding method based on Bayesian inference. We use this to assign
significance to our detections and to determine the best-fitting number of
spectral-line components. We find that the automated spectral-line search is
limited by residuals in the continuum, both from the band-pass calibration and
spectral-ripple subtraction, at spectral-line widths of \Deltav_FWHM > 103 km/s
. Using this technique we detect two new absorbers and a third, previously
known, yielding a 10 per cent detection rate. Of the detections, the
spectral-line profiles are consistent with the theory that we are seeing
different orientations of the absorbing gas, in both the host galaxy and
circumnuclear disc, with respect to our line-of-sight to the source. In order
to spatially resolve the spectral-line components in the two new detections,
and so verify this conclusion, we require further high-resolution 21 cm
observations (~0.01 arcsec) using very long baseline interferometry.Comment: 16 pages, 8 figures and 5 tables; accepted for publication in MNRAS
(version 2 based on proof corrections
Greenhouse gas balance over thaw-freeze cycles in discontinuous zone permafrost
Peat in the discontinuous permafrost zone contains a globally significant reservoir of carbon that has undergone multiple permafrost-thaw cycles since the end of the mid-Holocene (~3700 years before present). Periods of thaw increase C decomposition rates which leads to the release of CO2 and CH4 to the atmosphere creating potential climate feedback. To determine the magnitude and direction of such feedback, we measured CO2 and CH4 emissions and modeled C accumulation rates and radiative fluxes from measurements of two radioactive tracers with differing lifetimes to describe the C balance of the peatland over multiple permafrost-thaw cycles since the initiation of permafrost at the site. At thaw features, the balance between increased primary production and higher CH4 emission stimulated by warmer temperatures and wetter conditions favors C sequestration and enhanced peat accumulation. Flux measurements suggest that frozen plateaus may intermittently (order of years to decades) act as CO2 sources depending on temperature and net ecosystem respiration rates, but modeling results suggest that—despite brief periods of net C loss to the atmosphere at the initiation of thaw—integrated over millennia, these sites have acted as net C sinks via peat accumulation. In greenhouse gas terms, the transition from frozen permafrost to thawed wetland is accompanied by increasing CO2 uptake that is partially offset by increasing CH4 emissions. In the short-term (decadal time scale) the net effect of this transition is likely enhanced warming via increased radiative C emissions, while in the long-term (centuries) net C deposition provides a negative feedback to climate warming
Design study of Software-Implemented Fault-Tolerance (SIFT) computer
Software-implemented fault tolerant (SIFT) computer design for commercial aviation is reported. A SIFT design concept is addressed. Alternate strategies for physical implementation are considered. Hardware and software design correctness is addressed. System modeling and effectiveness evaluation are considered from a fault-tolerant point of view
Incidence of keratitis of varying severity among contact lens wearers
AIM: To determine the incidence of non-severe keratitis (NSK) and severe keratitis (SK) among wearers of current generation contact lenses. METHODS: A 12 month, prospective, hospital based epidemiological study was conducted by examining all contact lens wearers presenting with a corneal infiltrate/ulcer to a hospital centre in Manchester. A clinical severity matrix was used to differentiate between NSK and SK, based on the severity of signs and symptoms. The size of the hospital catchment population and the wearing modalities (daily wear (DW) or extended wear (EW)) and lens types being used were estimated from relevant demographic and market data. RESULTS: During the survey period, 80 and 38 patients presented with NSK and SK, respectively. The annual incidences (cases per 10,000 wearers) for each wearing modality and lens type were: DW rigid--NSK 5.7, SK 2.9; DW hydrogel daily disposable--NSK 9.1, SK 4.9; DW hydrogel (excluding daily disposable)--NSK 14.1, SK 6.4; DW silicone hydrogel--NSK 55.9, SK 0.0; EW rigid--NSK 0.0, SK 0.0; EW hydrogel--NSK 48.2, SK 96.4; EW silicone hydrogel--NSK 98.8, SK 19.8. The difference in SK between EW hydrogel and EW silicone hydrogel was significant (p = 0.04). CONCLUSIONS: A clinical severity matrix has considerable utility in assessing contact lens related keratitis. There is a significantly higher incidence of SK in wearers who sleep in contact lenses compared with those who only use lenses during the waking hours. Those who choose to sleep in lenses should be advised to wear silicone hydrogel lenses, which carry a five times decreased risk of SK for extended wear compared with hydrogel lenses
Deflections in Magnet Fringe Fields
A transverse multipole expansion is derived, including the longitudinal
components necessarily present in regions of varying magnetic field profile. It
can be used for exact numerical orbit following through the fringe field
regions of magnets whose end designs introduce no extraneous components, {\it
i.e.} fields not required to be present by Maxwell's equations. Analytic
evaluations of the deflections are obtained in various approximations. Mainly
emphasized is a ``straight-line approximation'', in which particle orbits are
treated as straight lines through the fringe field regions. This approximation
leads to a readily-evaluated figure of merit, the ratio of r.m.s. end
deflection to nominal body deflection, that can be used to determine whether or
not a fringe field can be neglected. Deflections in ``critical'' cases (e.g.
near intersection regions) are analysed in the same approximation.Comment: To be published in Physical Review
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