Browsing by Author "Mcbride, Vanessa"

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    Capturing transients: an application of biostatistics to astronomy
    (2022) van Dyk, Anke; Mcbride, Vanessa; Groot, Paul
    Capture-recapture has been identified as a possible use case for estimating the underlying size of astrophysical transient populations. In this work, we present a series of exploratory analyses using capture-recapture methods from biostatistics. In the first of three separate analyses, we reproduce results of Laycock (2017). Strategically sampled X-ray lightcurves of simulated populations of high mass X-ray binaries (HMXBs) are used to probe estimator behaviour and efficiency. Overall, these statistically closed population estimators converge to the input population with increasing number of observations, yet estimator efficiency is shown to be significantly be affected by sampling strategy. I then employ nonstandard estimator models to account for variations in capture probability of individuals within the population, categorised into ‘behavioural', ‘temporal', and ‘heterogeneous' effects. In the second analysis, we present a methodology for closed population capture-recapture analysis using real data from the OGLE-IV XROM survey. The data samples consisted of observations that were grouped into epochs. The large variation in quiescent magnitude of the population creates heterogeneity in the capture probability of sources which requires non-standard modelling. Estimation of population size is therefore limited by the choice of observational magnitude threshold. Bias corrected estimation proves to be potentially useful in this context. In the third and final investigation, we present a ‘robust design' approach with a population of Dwarf Nova located towards and in the Galactic Bulge identified from the OGLE-II, -III, and -IV phases. This approach combines closed and open population practices that allows new individuals identified between the survey phases to be added to the study sample for dynamical estimation. These investigations provide a future course for population size estimation of transients and variable stellar population alongside population synthesis simulations. The generation of capture histories remain non-trivial through the choice of observation grouping, brightness scale, and imposed flux threshold. Further, there remain several unexplored avenues of inquiry and refinement for this methodology pertaining to astronomy using explanatory variables in the modelling. Recommendations are made for further exploration of the topic.
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    Characterising radio sources in the Small Magellanic Cloud using citizen science and optical spectroscopy
    (2025) De Beer, Gideon; Mcbride, Vanessa
    For several years, the Small Magellanic Cloud (SMC) has been extensively studied in multiple wavelengths, revealing rich populations of radio sources within and beyond the nearby dwarf galaxy. With radio interfer-ometers' ever-increasing sensitivity and resolution, greater source populations will be discovered. Two regions of the SMC dwarf galaxy were observed using MeerKAT, with 10873 radio sources identified across them. In this dissertation, we characterise the radio source population of these two regions. This was approached using citizen science and optical spectroscopic follow-up. From the 10873 radio sources, 1429 were selected as subjects for classification in the citizen science project. This selection was based on the radio signal-to-noise ratio. The citizen science approach used information from the radio images, combined with optical cutouts from the SuperCOSMOS Sky Survey. The results show a high level of agreement among volunteer classi-fiers on questions concerning basic morphological characteristics. Around 63 per cent of all classifications show complete agreement between at least 5 classifiers in determining whether the displayed source was a radio point or an extended source and whether there is a likely optical counterpart aligned with the radio emission for the 1429 subjects. 41 radio sources were selected from the original 10873 for spectroscopy, by cross-matching the radio source list to the GAIA optical catalogue and making a magnitude cut. Optical spectroscopic follow-up of 23 sources from these 41 was conducted in November 2023. The reduced spectra from the observations were analysed to identify the optical sources, determine whether the optical and radio emissions were coincident, and identify what mechanism might be causing the radio emission if the optical source is considered likely to be the optical counterpart to the radio source. Characterisation revealed 13 stars, 3 eclipsing binaries and 2 nebulae. Between the two analysis methods, with their own independently se-lected source sample pools, 6 sources were found to overlap. The resultant analysis between the methods gave complementary source characterisation results. Approximately 912 radio sources have been characterised in total, at least by radio morphology and the likely presence of an optical counterpart, during this project.
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    Optical and gamma-ray study of gamma-ray binaries
    (2018) Monageng, Itumeleng; Mcbride, Vanessa; Böttcher, Markus; Mohamed, Shazrene
    Gamma-ray binary stars are intriguing interacting stellar systems that exhibit emission across the whole electromagnetic spectrum. This thesis is focused on optical and gamma-ray emission of gamma-ray binary stars (GRBis). I have performed radial velocity measurements of 1FGL J1018.6-5856 to derive the orbital parameters of the system. In particular, I derive, for the first time, the orbital eccentricity which, together with other parameters, enables us to attain the orbital geometry of the system. I also use the derived parameters to put constraints on the mass of the compact object. For a large range of orbital inclination angles, a neutron star is favoured, while a black hole is allowed for very low inclination angles. I have also explored the long term variability of the Be disc in LS I +61 303. Using the measured parameters of the Hα emission line together with a semi-analytical model which describes the motions of particles in the disc, I examine long term geometric changes of the Be disc in LS I +61 303. I have explored different possible scenarios for the Be disc changes by studying the variability of the eccentricity and argument of pericenter on the superorbital timescale. One of the effects we have examined from the implication of our results is the Kozai Lidov mechanism, which has so far only been explored theoretically. I have explored the Bethe-Heitler mechanism for the production of gamma-ray emission, where energetic protons collide with stellar photons to produce energetic electron-positron pairs which upscatter photons to high energies. This mechanism has been neglected in studies of photo-hadronic processes due to the larger radiative efficiency of pion production. I demonstrate, however, that the BetheHeitler process can dominate the radiative output at lower proton energies where pairs can form but not pions. I calculate the spectrum and modulated flux in the context of the pulsar wind scenario, with photon-photon absorption and cascading effects taken into account. The effects of the ambient magnetic field around the massive star are also considered, which result in quasi-isotropic gamma-ray emission, and the orbital modulation of the flux resulting in photon-photon absorption. The work presented in this thesis encompasses different observational and theoretical aspects of gamma-ray binaries in which I use data and modeling to address some of the puzzling features of these systems. These include understanding the nature of the compact object, and thereby the physical mechanism driving the multiwavelength emission, the variability of the Be disc and its long term interaction with the compact object, and the origin of high energy emission. The results and methods presented have potential implications not only for understanding these objects, but also for other classes of interacting binaries.