725 research outputs found
Effect of gain and phase errors on SKA1-low imaging quality from 50-600 MHz
Simulations of SKA1-low were performed to estimate the noise level in images
produced by the telescope over a frequency range 50-600 MHz, which extends the
50-350 MHz range of the current baseline design. The root-mean-square (RMS)
deviation between images produced by an ideal, error-free SKA1-low and those
produced by SKA1-low with varying levels of uncorrelated gain and phase errors
was simulated. The residual in-field and sidelobe noise levels were assessed.
It was found that the RMS deviations decreased as the frequency increased. The
residual sidelobe noise decreased by a factor of ~5 from 50 to 100 MHz, and
continued to decrease at higher frequencies, attributable to wider strong
sidelobes and brighter sources at lower frequencies. The thermal noise limit is
found to range between ~10 - 0.3 Jy and is reached after ~100-100 000 hrs
integration, depending on observation frequency, with the shortest integration
time required at ~100 MHz.Comment: 23 pages, 11 figures Typo correcte
Bayesian evidence-driven likelihood selection for sky-averaged 21-cm signal extraction
We demonstrate that the Bayesian evidence can be used to find a good
approximation of the true likelihood function of a dataset, a goal of the
likelihood-free inference (LFI) paradigm. As a concrete example, we use forward
modelled sky-averaged 21-cm signal antenna temperature datasets where we
artificially inject noise structures of various physically motivated forms. We
find that the Gaussian likelihood performs poorly when the noise distribution
deviates from the Gaussian case e.g. heteroscedastic radiometric or
heavy-tailed noise. For these non-Gaussian noise structures, we show that the
generalised normal likelihood is on a similar Bayesian evidence scale with
comparable sky-averaged 21-cm signal recovery as the true likelihood function
of our injected noise. We therefore propose the generalised normal likelihood
function as a good approximation of the true likelihood function if the noise
structure is a priori unknown
Bayesian evidence-driven diagnosis of instrumental systematics for sky-averaged 21-cm cosmology experiments
We demonstrate the effectiveness of a Bayesian evidence-based analysis for
diagnosing and disentangling the sky-averaged 21-cm signal from instrumental
systematic effects. As a case study, we consider a simulated REACH pipeline
with an injected systematic. We demonstrate that very poor performance or
erroneous signal recovery is achieved if the systematic remains unmodelled.
These effects include sky-averaged 21-cm posterior estimates resembling a very
deep or wide signal. However, when including parameterised models of the
systematic, the signal recovery is dramatically improved in performance. Most
importantly, a Bayesian evidence-based model comparison is capable of
determining whether or not such a systematic model is needed as the true
underlying generative model of an experimental dataset is in principle unknown.
We, therefore, advocate a pipeline capable of testing a variety of potential
systematic errors with the Bayesian evidence acting as the mechanism for
detecting their presence
Drought affects the performance of native oak seedlings more strongly than competition with invasive crested wattle seedlings
Two of the most important processes threatening vulnerable plant species are competitive displacement by invasive alien species and water stress due to global warming. Quercus lusitanica, an oak shrub species with remarkable conservation interest, could be threatened by the expansion of the invasive alien tree Paraserianthes lophantha. However, it is unclear how competition would interact with predicted reductions in water availability due to global climate change. We set up a full factorial experiment to examine the direct interspecific competition between P. lophantha and Q. lusitanica seedlings under control and water-limited conditions. • We measured seed biomass, germination, seedling emergence, leaf relative growth rate, biomass, root/shoot ratio, predawn shoot water potential and mortality to assess the individual and combined effects of water stress and interspecific competition on both species. • Our results indicate that, at seedling stage, both species experience competitive effects and responses. However, water stress exhibited a stronger overall effect than competition. Although both species responded strongly to water stress, the invasive P. lophantha exhibited significantly less drought stress than the native Q. lusitanica based on predawn shoot water potential measurements. • The findings of this study suggest that the competition with invasive P. lophantha in the short term must not be dismissed, but that the long-term conservation of the native shrub Q. lusitanica could be compromised by increased drought as a result of global change. Our work sheds light on the combined effects of biological invasions and climate change that can negatively affect vulnerable plant species
Occurrence of Cryptosporidium and Giardia in raw and finished drinking water in north-eastern Spain
This paper collects the first large-sample-size study on the presence of Cryptosporidium oocysts and Giardia cysts in drinking water plants at the 20 most populated towns in Aragón (north-eastern Spain). Samples of influent raw water and effluent finished water were collected from each plant during different seasons and processed according to USEPA Method 1623. Cryptosporidium oocysts and Giardia cysts were detected in samples collected from 55% and 70% plants, respectively, with nine plants being positive for both protozoa and only four plants being negative over the study period. Both parasites were identified in the raw water throughout the year, with a lower frequency in autumn and a peak in winter, at a mean concentration of 67 ± 38 oocysts per 100 l and 125 ± 241 cysts per 100 l. The turbidity of raw water was not related to the presence or concentration of (oo)cysts, and the (oo)cyst removal efficiency was not related to the type of water treatment. One or both pathogens were identified in the finished water in 7 out of 11 plants with a conventional treatment process (coagulation, flocculation, sedimentation, filtration, and disinfection processes) compared to 4 out of 9 plants that did not apply one of the pre-chlorination treatment steps. Protozoa were detected in the finished water of positive plants at a mean concentration of 88 ± 55 oocysts per 100 l and 37 ± 41 cysts per 100 l, and most of them excluded propidium iodide so were considered potentially viable. The ubiquity of these parasites in the drinking water sources and the inefficiency of conventional water treatment in reducing/inactivating them may present a serious public health issue in this geographical area
Occurrence and genetic diversity of Cryptosporidium and Giardia in urban wastewater treatment plants in north-eastern Spain
This study was designed to investigate the presence and removal efficiency of Cryptosporidium and Giardia in wastewater treatment plants at the 20 most populated towns in Aragón (north-eastern Spain). Samples of influent and effluent wastewater and dewatered sewage sludge were collected seasonally from 23 plants and processed according to USEPA Method 1623. All samples from raw and treated wastewater tested positive for Giardia, at an average concentration of 3247 ± 2039 cysts/l and 50 ± 28 cysts/l, respectively. Cryptosporidium was identified in most samples from both raw (85/92) and treated (78/92) wastewaters in a concentration significantly lower than Giardia, at both influent (96 ± 105 oocysts/l) and effluent samples (31 ± 70 oocysts/l) (P < 0.001). The (oo)cyst counts peaked in summer in most plants. The removal efficiency was higher for Giardia (1.06-log to 2.34-log) than Cryptosporidium (0.35-log to 1.8-log). Overall, high removal efficiency values were found for Giardia after secondary treatment based on activated sludge, while tertiary treatment (microfiltration, chlorination and/or ultraviolet irradiation) was needed to achieve the greatest removal or inactivation of Cryptosporidium. Most samples of treated sludge were positive for Giardia (92/92) and Cryptosporidium (45/92), at an average concentration of 20–593 cysts/g and 2–44 oocyst/g, respectively. The molecular characterization of Cryptosporidium oocysts and Giardia cysts were attempted at the SSU rRNA/GP60 and bg/tpi loci, respectively. G. duodenalis sub-assemblage AII was identified in all plants, with a large proportion of samples (15/47) harboring mixed assemblages (AII + B). Nine Cryptosporidium species and six subtypes were identified, with C. parvum IIaA15G2R1 being the most prevalent. The presence of significant numbers of (oo)cysts in samples of final effluents and treated sludge reveals the limited efficacy of conventional treatments in removing (oo)cysts and highlights the potential environmental impact and public health risks associated with disposal and reclamation of wastewater
A Bayesian approach to RFI mitigation
Interfering signals such as Radio Frequency Interference from ubiquitous
satellite constellations are becoming an endemic problem in fields involving
physical observations of the electromagnetic spectrum. To address this we
propose a novel data cleaning methodology. Contamination is simultaneously
flagged and managed at the likelihood level. It is modeled in a Bayesian
fashion through a piecewise likelihood that is constrained by a Bernoulli prior
distribution. The techniques described in this paper can be implemented with
just a few lines of code.Comment: 6 pages, 4 figures, accepted by Physical Review D (APS
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