Browsing by Author "Koen, C"
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Item Fitting extreme value distributions to the Zambezi river flood water levels recorded at Katima Mulilo in Namibia(University of the Western Cape, 2005) Kamwi, Innocent Silibelo; Koen, C; Blignaut, Renette; Dept. of Statistics; Faculty of ScienceThe aim of this research project was to estimate parameters for the distribution of annual maximum flood levels for the Zambezi River at Katima Mulilo. The estimation of parameters was done by using the maximum likelihood method. The study aimed to explore data of the Zambezi's annual maximum flood heights at Katima Mulilo by means of fitting the Gumbel, Weibull and the generalized extreme value distributions and evaluated their goodness of fit.Item Multifilter time-series observations of three short period ATLAS variable stars(Oxford University Press, 2019-09) Koen, CThe ‘Asteroid Terrestrial-impact Last Alert System’ discovered hundreds of thousands of new candidate variable stars. Follow-up observations of three of these are reported in this paper. The targets were selected on the basis of having high probability of being periodic (false alarm probability for period detection smaller than 10−5), short periods (P < 0.2 d), and being relatively bright (g < 17). The targets were also chosen to be either very blue (g − i < −0.4, r − z < −0.4) or very red (g − i > 2.2, r − z > 1.5) as periodic variables with these colours are relatively rare. Two of the stars are hot subdwarfs, both of which are likely reflection effect binaries. In both cases simple models suggest that the companions may have masses very close to or below 0.1 M . The third star is also a binary, which appears to consist of two M dwarfs in a near contact configuration. At 0.12 d its period is one of the shortest known for M-type binaries.Item On multiple classes of gamma-ray bursts, as deduced from autocorrelation functions or bivariate duration/hardness ratio distributions(Oxford University Press, 2012) Koen, C; Bere, AAutocorrelation functions (ACFs) of 119 gamma-ray bursts (GRBs) monitored by the Burst Alert Telescope (BAT) on Swift are calculated. Contrasting with previous results for smaller numbers of bursts from other missions, the widths of the ACFs are not bimodally distributed. Although the distribution appears slightly asymmetrical, underlying mixtures of distributions can also probably be ruled out. Factors contributing to differences between the results presented here, and those in the literature, may include the differences in energy passbands used, and the superior sensitivity of the BAT instrument (which affects e.g. the redshift distribution of the detected GRB). The second part of the paper is concerned with the fitting of mixtures of bivariate Gaussians to the joint duration/hardness ratio data of 325 GRBs. A careful analysis confirms that a three-component mixture is the statistically most acceptable, but it is shown that the implied marginal distribution of the hardness ratios does not fit the data very well. It is also stressed that mixture components cannot automatically be assumed to represent different classes of GRBs. The point is illustrated by showing two substantially different, but statistically almost equivalent, mixture models for the distribution of 571 BAT-determined GRB durations.Item Starspot modelling of the TESS light curve of CVSO 30(EDP Sciences, 2021) Koen, CAims. I aim to investigate whether the photometric variability in the candidate host star CVSO 30 can be explained by starspots. Methods. The Transiting Exoplanet Survey Satellite (TESS) light curve of CVSO 30 is separated into two independent non-sinusoidal periodic components. A starspot modelling technique is applied to each of these components. Results. Combined, the two model light curves reproduce the TESS observations to a high accuracy, obviating the need to invoke planetary transits to describe part of the variability.