Calibration artefacts in radio interferometry. I. Ghost sources in WSRT data

This work investigates a particular class of artefacts, or ghost sources, in radio interferometric images. Earlier observations with (and simulations of) the Westerbork Synthesis Radio Telescope (WSRT) suggested that these were due to calibration with incomplete sky models. A theoretical framework is derived that validates this suggestion, and provides predictions of ghost formation in a two-source scenario. The predictions are found to accurately match the result of simulations, and qualitatively reproduce the ghosts previously seen in observational data. The theory also provides explanations for many previously puzzling features of these artefacts (regular geometry, PSF-like sidelobes, seeming independence on model flux), and shows that the observed phenomenon of flux suppression affecting unmodelled sources is due to the same mechanism. We demonstrate that this ghost formation mechanism is a fundamental feature of calibration, and exhibits a particularly strong and localized signature due to array redundancy. To some extent this mechanism will affect all observations (including those with non-redundant arrays), though in most cases the ghosts remain hidden below the noise or masked by other instrumental artefacts. The implications of such errors on future deep observations are discussed.Comment: 19 pages, 15 figures, submitted to MNRA

Stream-field interactions in the magnetic accretor AO Piscium

UV spectra of the magnetic accretor AO Psc show absorption features for half the binary orbit. The absorption is unlike the wind-formed features often seen in similar stars. Instead, we attribute it to a fraction of the stream that overflows the impact with the accretion disk. Rapid velocity variations can be explained by changes in the trajectory of the stream depending on the orientation of the white-dwarf's magnetic field. Hence we are directly observing the interaction of an accretion stream with a rotating field. We compare this behavior to that seen in other intermediate polars and in SW Sex stars.Comment: Accepted for ApJ; 6 page

Late-night simulation: Opinions of fourth- and fifth-year medical students at the University of the Free State, Bloemfontein, South Africa

Background. Sleep deprivation is a problem for medical students and practitioners due to long and late working hours, which may result in a decline in their performance in practising medicine. Objectives. To investigate whether educational practices require altering with regard to the time at which simulation classes are presented, or identify any other possible suggestions for improving the preparation of students for shift work in their profession as medical doctors as a potential solution to reduce sleep-deprivation-related adverse outcomes. Methods. In this quantitative cross-sectional study, an anonymous questionnaire was distributed to 111 fourth-year and 141 fifth-year medical students at the Faculty of Health Sciences, University of the Free State (UFS), Bloemfontein, South Africa, during the second half of 2018. The researchers interpreted the responses and the Department of Biostatistics, UFS, analysed the data. Results. The majority of the fourth-year (88.6%) and fifth-year (90.4%) student groups responded that late-night simulation classes between 01h00 and 04h00 would not be beneficial to their preparation for shift work. The motivation for negative responses was that it might worsen sleep deprivation due to time constraints in an already demanding course. The fourth-year (61.4%) and fifth-year (80.5%) student groups did not regard simulation as realistic and felt that late-night simulation training sessions would not prepare them better for future shift work. However, both groups believed ‘practice makes perfect’ and, as such, their confidence with procedures would improve as they practise more during simulation. Conclusion. The majority of students were negative towards the idea of late-night simulation classes, because of the effect it would have on their already full programme. Students are familiar with the effects of sleep deprivation and felt that late-night simulation classes would add pressure to their busy lives and worsen their sleep deprivation. Further investigation and practical testing would be required to conclude the impact of late-night simulation classes in preparation for shift work of medical doctors and the resultant effect on clinical performance

Help-seeking by substance dependants presenting to healthcare professionals in the Free State Province

Introduction. Current data regarding treatment needs in South Africa for substance dependence are based on admissions to specialised treatment centres. The data therefore do not include patients presenting to independent healthcare workers and state hospitals. Aim. The aim of this study was to investigate help-seeking behaviour for substance dependence disorders from the perspective of healthcare professionals at various levels of the referral chain in Free State Province. Design. A descriptive study was performed. Setting. Treatment environments in Free State Province accessible to substance-dependent persons. Participants. General practitioners, private psychiatrists, prescribing healthcare professionals at state hospitals and treatment centres, and non-prescribing therapists responsible for management of substance-dependent persons. Measurements. A questionnaire was used to determine the level of help-seeking experienced by the participants regarding various classes of psycho-active substances. Findings. The presentation of alcohol, cannabis, opioid, benzodiazepine, ecstasy, cocaine and inhalant dependency at general practitioners, private psychiatrists, treatment centres and non-prescribing therapists was compared. Different patterns of help-seeking for substance dependence from the various professional groups were detected. Regarding alcohol dependence, 40.3% of private general medical practitioners reported being confronted with alcohol dependence at least once per month, compared with 100% of treatment centre representatives and private psychiatrists, 70.6% of state hospitals and 53.8% of non-prescribers. State hospitals reported the highest frequency for contact with cases of cannabis dependence, compared with the other professional groups, while psychiatrists reported the highest contact with cases involving benzodiazepine, cocaine and ecstasy. Therapists reported a higher level of contact with inhalant dependence, while this group virtually never presented at general practitioners and private psychiatrists. Conclusion. Help-seeking data can be used in conjunction with other data, such as treatment demand data, to inform and adapt policies and practice. Variations in the manifestation of help-seeking behaviour at various groups may be important pointers to conditions that influence help-seeking, and therefore should be investigated further

Segmenting the brt market in unequal societies: lessons for south africa from literature

Developing countries have a history of poor public transport on which mainly lower-income travellers depend. Following the success of Bus Rapid Transit (BRT) systems implemented in several Latin American countries, this mode of transport has enjoyed increased popularity world-wide. It is regarded as an affordable public transport solution for large passenger volumes in a high-quality system. In South Africa, BRT was viewed as a solution for all market segments by providing a high-quality frequent service to attract car users to public transport, as well as to the lower income market, by providing a much-improved quality of service compared to current public transport modes. However, this idealised vision for BRT has not materialised, and its implementation and operation has been met with numerous challenges not least of which is disappointing patronage demand. A key question that arises is whether BRT can accommodate the divergent travel preferences of the different traveller market segments and whether disaggregated segmentation can assist in designing a BRT system that could cater for these divergent needs. The paper summarises the results of a literature review conducted as part of a research project to explore how BRT systems should accommodate the travel needs and preferences of different market segments in unequal societies such as in South Africa. The literature review considered the need and benefits for segmentation; previous methods of identifying market segments internationally and in the South African travel market when planning public transport. In addition, an analysis of the mode choice behaviour of various socio-economic segments of South African commuters is provided and conclusions offered for future segmentation efforts.Papers presented virtually at the 39th International Southern African Transport Conference on 05 -07 July 202

Simulations of Spinodal Nucleation in Systems with Elastic Interactions

Systems with long-range interactions quenched into a metastable state near the pseudospinodal exhibit nucleation that is qualitatively different than the classical nucleation observed near the coexistence curve. We have observed nucleation droplets in our Langevin simulations of a two-dimensional model of martensitic transformations and have determined that the structure of the nucleating droplet differs from the stable martensite structure. Our results, together with experimental measurements of the phonon dispersion curve, allow us to predict the nature of the droplet. These results have implications for nucleation in many solid-solid transitions and the structure of the final state

Analytical treatment of interacting Fermi gas in arbitrary dimensional harmonic trap

We study normal state properties of an interacting Fermi gas in an isotropic harmonic trap of arbitrary dimensions. We exactly calculate the first-order perturbation terms in the ground state energy and chemical potential, and obtain simple analytic expressions of the total energy and chemical potential. At zero temperature, we find that Thomas-Fermi approximation agrees well with exact results for any dimension even though system is dilute and small, i.e. when the Thomas-Fermi approximation is generally expected to fail. In the high temperature (classical) region, we find interaction energy decreases in proportion to T^(-d/2), where T is temperature and d is dimension of the system. Effect of interaction in the ground state in two and three-dimensional systems is also discussed.Comment: 15 pages, 4 figure

Nucleation in Systems with Elastic Forces

Systems with long-range interactions when quenced into a metastable state near the pseudo-spinodal exhibit nucleation processes that are quite different from the classical nucleation seen near the coexistence curve. In systems with long-range elastic forces the description of the nucleation process can be quite subtle due to the presence of bulk/interface elastic compatibility constraints. We analyze the nucleation process in a simple 2d model with elastic forces and show that the nucleation process generates critical droplets with a different structure than the stable phase. This has implications for nucleation in many crystal-crystal transitions and the structure of the final state