Browsing by Author "Woudt, Patrick"
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- ItemOpen AccessAccretion processes in cataclysmic variable stars: insights from optical transient surveys(2024) Motsoaledi, Mokhine; Woudt, Patrick; Buckley, David; Warner, BrianCataclysmic variable (CV) stars are binary stars mostly characterised by accretion from a main sequence donor star to a white dwarf star. Multiple CV subclasses exist with varia tions in the nature of the CV, many of which have accretion discs surrounding the accretor. This study focuses on two sub-classes of CVs, namely AM Canum Venaticorum (AM CVn) stars with helium accretion from a (semi-)degenerate helium star or white dwarf donor star, and magnetic CVs, specifically polars, which have the strongest magnetic fields of the accreting white dwarfs in CVs and lack accretion discs. I make use of optical transient surveys to explore individual objects, as well as the global population of polars, with a focus on the Catalina Real-time Transient Survey (CRTS). Follow-up observations were carried out with the 1-m and 1.9-m telescopes and the 10-m Southern African Large Telescope (SALT) at the South African Astronomical Observatory (SAAO). Observations of 7 outbursting AM CVn stars with orbital periods ranging from
- ItemOpen AccessAST3002F: Reduction Guide (2018)(2018) Titus, Naomi; Woudt, PatrickThe AST3002F spectroscopy practical involves the reduction and analysis of SALT spectroscopy of candidate white dwarfs in the Kepler field. The data are associated with a SALT Science project (2016-1- SCI-038). The reduction guide describes, step-by-step, the reduction process for the spectroscopic practical which includes a number of steps, namely: - a visual inspection of the spectra - removal of cosmic rays - wavelength calibration - sky subtraction - extraction of the spectrum of the primary target - extraction of the spectrum of other stars near the target
- ItemOpen AccessDetecting optical transients and variables with MeerLICHT(2018) Paterson, Kerry; Woudt, Patrick; Warner, Brian; Groot, P. J.For a long time, the discovery of a transient in a one wavelength triggered observations in other wavelengths. Due to various constraints such as telescope access, scheduling and availability, and the need for human intervention, follow up observations of the transient event could take place long after the initial discovery. Due to the time-critical nature of some transients, with prompt emission that can fade rapidly, this delay in follow up observations can result in important information about the event being missed. Thus, the concept of MeerLICHT was born. MeerLICHT is an fully robotic, optical telescope whose main goal is the detection of transients in real-time. In collaboration with two legacy-style large survey projects (ThunderKAT and MeerTRAP) on MeerKAT (South Africa’s precursor to the Square Kilometre Array), MeerLICHT is the first fully dedicated telescope to follow another telescope in a different wavelength. Through linked-pointing, MeerLICHT and MeerKAT will provide simultaneous optical and radio data of the transient sky. This will provide invaluable information on emission at both wavelengths, including: the relation and evolution of emission from both wavelength windows, the discovery of optical counterparts of radio transients, the characterization of radio transients, and an early warning of radio transients using optical transients. This thesis contains the work on the development and implementation of the data processing pipeline for MeerLICHT. This data processing pipeline was developed for the automatic processing of data from MeerLICHT/BlackGEM for transient detection in real time.
- ItemOpen AccessMeerKAT observations of novalike cataclysmic variables(2020) Hewitt, Danté Michael; Pretorius, M L; Woudt, PatrickI have conducted a systematic survey of eleven nearby novalike cataclysmic variables in the radio band, using the MeerKAT radio interferometer. Radio emission is detected from four out of the eleven systems: IM Eri, RW Sex, V3885 Sgr and V603 Aql. While RW Sex, V3885 Sgr and V603 Aql had been previously detected, this is the first reported radio detection of IM Eri. These observations have doubled the sample of non-magnetic cataclysmic varaibles with sensitive radio data. I observe that at these radio detection limits, a specific optical luminosity & 2.2 × 1018 erg s−1 Hz−1 (corresponding to MV . 6.0) is required to produce a radio detection. I also find that the X-ray and radio luminosities of the detected novalikes are on an extension of the LX ∝ L ∼0.7 R power law originally proposed for non-pulsating neutron star low-mass X-ray binaries. No other correlations are found between the radio emission and emission in other wavebands or any other system parameters for the existing sample of radio-detected novalikes. In-band (0.9–1.7 GHz) radio spectral indices are measured, and are found to be consistent with reports from earlier work. I construct broad spectral energy distributions for this sample from published multi-wavelength data, and use them to place constraints on the mass transfer rates of these eleven systems. Finally, I also present the results of time-resolved optical spectroscopy of two little-studied systems in the sample: V5662 Sgr and LSIV -08 3. I obtain orbital periods for these two systems that are consistent with previous measurements in the literature.
- ItemOpen AccessRadio observations as a tool to study shock interactions and mass ejections in novae(2021) Nyamai, Mumbua; Woudt, Patrick; Ribeiro, V A R M; Chomiuk, LThis thesis presents radio frequency studies of three novae, namely V445 Puppis, V3890 Sagitarii and V339 Delphini. The new data, in this thesis, represent some of the most detailed and comprehensive radio light curves to date. In these systems a thermonuclear eruption occurs on the surface a white dwarf following extensive periods of accretion of material from a companion star. The result of the thermonuclear eruption is the explosive ejection of the outer layers of the accumulated material. Since the ejected material emits at radio wavelengths at some point in its evolution, radio data obtained for the three objects are utilised to test the hypothesis of radio emission models. First, radio data of V445 Pup are presented. V445 Pup is the only helium nova observed to date; its eruption in late 2000 showed high velocities up to 8500 km s°1, and a remarkable bipolar morphology cinched by an equatorial dust disc. Here we present multi-frequency radio observations of V445 Pup obtained with the Very Large Array (VLA) spanning 1.5°43.3 GHz, to 2001 January and 2008 March (ª 89 ° 2700 days after eruption). The radio light curve is dominated by synchrotron emission over these seven years, and shows four distinct radio flares. Resolved radio images obtained in the VLA's A configuration show that the synchrotron emission hugs the equatorial disc, and comparisons to near-IR images of the nova clearly demonstrate that it is the densest ejecta—not the fastest ejecta—that are the sites of the synchrotron emission in V445 Pup. The data are consistent with a model where the synchrotron emission is produced by a wind from the white dwarf impacting the dense equatorial disc, resulting in shocks and particle acceleration. The individual synchrotron flares may be associated with density enhancements in the equatorial disc and/or velocity variations in the wind from the white dwarf. This overall scenario is similar to a common picture of shock production in hydrogen-rich classical novae, but V445 Pup is remarkable in that these shocks persist for almost a decade, much longer than the weeks or months for which shocks are typically observed in classical novae. Second, the radio observations following the 2019 August eruption of V3890 Sgr obtained with the MeerKAT radio telescope at 1.28 GHz are presented. The radio light curve spans from day 1 to 300 after discovery and is dominated by synchrotron emission produced as the expanding ejected nova envelope interacts with the dense wind from an evolved companion in the binary system. The radio emission is detected early on day 6 and increases rapidly with the radio flux peaking after day 15. The radio luminosity increases due to a decrease in the opacity of the circumstellar material in front of the shocked material and fades as the density of the surrounding medium decreases and the velocity of the shock losses energy and decelerates. Modelling of the light curve provides an estimated mass-loss rate of M ˙ wind º 10°8 MØ yr°1 for the red giant wind. V3890 Sgr hosts a massive white dwarf, shows presence of structured circumbinary material consistent with the surrounding environment present in some supernovae type Ia (SNe Ia) and therefore it is possible to be a SNe Ia progenitor. Finally, radio observations of V339 Del obtained using the Karl G. Jansky Very Large Array and the Arcminute Microkelvin Imager Large Array (AMI) following the 2013 eruption are presented. The AMI data were obtained every two days resulting in the most detailed radio light curve of a classical nova known to date. The radio data are used to test the hypothesis that the observations were due to a bipolar shaped ejecta as suggested from emission line modelling at optical wavelength. Here, the morphology is utilised in predicting the ejected mass of V339 Del during the eruption. The radio light curve is modelled in the morpho-kinematical program SHAPE using a linearly expanding ionized ejecta, assuming that the nova emission is dominated by thermal free-free emission. Considering a bipolar geometry for the nova ejecta, the radio light curve fluxes can be replicated using an ejected mass of Mej º 0.1°8£10°5 MØ. The mass estimate of the ejecta compares well with estimates of emission line modelling in optical spectroscopy and also ∞-ray emission modelling. High resolution radio images of V339 Del obtained with the VLA's A configuration initially show a spherical structure when the ejecta is optically thick and later a ring structure when the ejecta is optically thin. The change in optical depth and apparent morphology is a further indication of a non-spherical geometry. V339 Del shows evidence of shock powered emission in the first 100 days after the nova eruption. This thesis emphasizes the role of radio data towards a better understanding of the effects of thermonuclear eruptions in novae. This work highlights the importance of obtaining well sampled radio light curves of novae which give more insight into the physical processes that follow nova explosions, which may have been overlooked before due to sparsely sampled radio light curves. Resolved radio images of novae are also crucial in determining the morphology of the ejecta hence provide clues on the approach to modelling radio emission from novae. This is particularly relevant given the new generation of sensitive radio telescope arrays such as MeerKAT, the next generation Very Large Array, and ultimately the Square Kilometre Array.
- ItemOpen AccessSearching for Ultra-compact binary stars with the MeerLICHT Southern All-Sky Survey(2022) Lees, Robert; Groot, Paul Joseph; Woudt, PatrickAn effort was made to expand the known population of AM Canum Venaticorum (AM CVn) stars, a rare type of ultra-compact binary star. This was done by searching through the MeerLICHT Southern All-Sky Survey (ML SASS) data with a candidate selection process which made use of a selection technique based on the colours of stars. The candidates found were then observed with an extensive follow-up observation campaign which included both high-speed photometric observations and longslit spectroscopy. Over 20 weeks of observing were conducted, I conducted 9 of them myself and the rest were handled by other members of the MeerLICHT working group. These observations were conducted with multiple telescopes at the South African Astronomical Observatory (SAAO) in Sutherland. Over 100 targets were observed photometrically and over 150 spectra were obtained, out of a possible 251 primary candidates. The photometric lightcurves were then analysed by producing Lomb-Scargle periodograms for each of the observed target lightcurves searching for variability on the order of 5 to 65 minutes. All of the observed spectra were processed and searched for certain hydrogen, helium and metal absorption and emission lines. Unfortunately, no new AM CVns were found, although a new potential magnetic cataclysmic variable (CV) was discovered and one unknown but highly interesting spectrum was discovered. The lack of AM CVns was not unexpected as these binary stars are exceptionally rare with a little over 70 currently known AM CVns in the entire sky, and our statistical analysis predicted that we would have only detected approximately one AM CVn this far into the follow-up campaign. Potential further work and methods to improve both the candidate selection process and the follow-up campaign are outlined.
- ItemOpen AccessThe broad spectral energy distributions of accreting white dwarfs(2023) Lancaster, Tamara; Woudt, PatrickCataclysmic Variable stars (CVs) are excellent examples of multi-wavelength objects. They have been detected at all frequencies, from radio to γ-ray. However, while large sets of multi-wavelength data exist, few studies have been performed on the broad spectral energy distributions (SEDs) of these objects. I perform such a study here, and present the largest collection of broad CV SEDs yet constructed. I define a sample of well-studied, nearby cataclysmic variables of several different CV classes, and gather existing multi-wavelength data over a broad frequency domain. I identify the contributions to the spectrum and compare the spectra between classes. I then use the SEDs to constrain the mass accretion rates by modelling the accretion disc as a series of concentric annuli, each of which radiates as a black body with effective temperature appropriate to the radius of that annulus. The resulting bestfit accretion rates are compared to published values of mass accretion rates for the systems. My method of modelling the emission from a CV accretion disc as a sum of blackbodies produces accretion rate estimates that are reliable to within a factor of 5 for those systems that have steady state discs. However, the data are significantly underfitted by these simple models in all cases. While this method is a useful tool for modelling spectra and finding accretion rates, it cannot be applied to a small amount of data quickly and simply. It must be performed on as large a dataset as possible, and other contributions to the spectrum must be accounted for. As a result, an attempt to obtain accretion rate estimates for a larger, volume-limited sample of non-magnetic CVs was unsuccessful. I also investigate the ratios of flux densities in different wavebands. My data reveal two previously reported correlations, namely that shorter period CVs have higher X-ray to optical flux ratios and that magnetic CVs are X-ray bright compared to non-magnetic CVs. In addition, I find that polars are radio bright compared to IPs and non-magnetic CVs, and illustrate this, for the first time.
- ItemOpen AccessThe near infrared luminosity function of the Norma Cluster(2007) Skelton, R.E; Kraan-Korteweg, Renee; Woudt, PatrickIn this thesis, a near infrared KB-band photometric catalogue of galaxies in the Nonna cluster is presented. The survey of the cluster covers an area of '" 0.8h702 Mpc2, and is estimated to be complete to a magnitude of MK. ⋨-19 mag; this extends into the dwarf regime, 6 magnitudes below M'k •. The catalogue contains 390 objects, 235 of which are classified as likely or definite galaxies and 155 as candidate galaxies. The K.,-band luminosity function (LF) is constructed from the photometric sample, using a smaller spectroscopic survey to estimate the number of member galaxies at each magnitude. A Schechter function provides a reasonable fit to the data, with a characteristic magnitude of M'k. = -25.36 ± 0.74 mag and faint-end slope of a = -1.25 ± 0.10. There is a distinct dip in the LF at MK. '" -22 mag. The shape of the LF is similar to those found in previous determinations of the cluster LF, in both optical and near infrared. The Schechter parameters obtained by flagging the dip point agree well with those of recent field LFs. This suggests that both the shape of the bright end and the faint end slope are relatively insensitive to environment.
- ItemOpen AccessThe very high energy gamma ray burst GRB190114C as observed by MeerKAT(2024) Diretse, Reikantseone; Woudt, Patrick; van der Horst, AlexanderWe present the MeerKAT study of the highly energetic gamma ray burst (GRB), GRB190114C, which was the first GRB to have been recorded at teraelectronvolt (TeV) energies by the MAGIC telescope. We have observed this GRB with MeerKAT from as early as within the first day of the burst event, up to nearly a year at over 350 days. Our 13 epoch light curve is the most sampled radio light curve of this GRB at low frequencies (1.28 GHz) in comparison to published results. We detail the evolution of GRB190114C as a forward shock in a homogeneous medium for about 100 days post-burst, followed by a jet-break likely occurring simultaneously with the passage of the characteristic peak frequency across our observing band. We constrain the host galaxy component, and compare our modelled host value to fluxes extrapolated from pre-burst images of the field. As it is often for GRB observations at low radio frequencies, the evolution of GRB190114C is highly affected by strong interstellar scintillation, which we quantitatively constrain as long lasting refractive scattering. We then use the calculated scintillation parameters to put an upper limit on the radial size of the fireball, and compare this to other GRBs. Beyond the GRB, we take a commensal approach to study its intriguing and expansive background. We survey the field for potential radio transients or variables. We apply the LOFAR Transients Pipeline (TraP) in this pursuit. The pipeline has been adapted for MeerKAT data. We discover 11 potential transient or variable candidates. We detail their light curves, images, multi-wavelength counterparts and classifications to reveal their nature. We find that most of the candidates we find are likely active galactic nuclei (AGNs), and their variability in this context is reasonably due to interstellar scintillation as derived for the field
- ItemOpen AccessXMM-Newton Survey of the Magellanic Bridge(2020) Le Roux, Ryan; Townsend, Lee; McBride, Vanessa; Woudt, PatrickWe aim to characterise the X-ray binary population as a function of the local stellar population (in terms of age, metallicity, and stellar density) in the Magellanic Bridge, the interconnecting region between the Small and Large Magellanic Cloud. Gardiner and Noguchi (1996) suggest that closest approach between Small and Large Magellanic Cloud, as evidenced by dramatic phase shift in star formation, occurred approximately 200 Myr ago. During the approach, gas had been tidally stripped (most likely from the Small Magellanic Cloud) into the interconnecting Bridge. According to models of star formation history (Harris, 2007), alongside optical surveys of the Bridge (Skowron et al., 2014), there is strong evidence to suggest that the young, low metallicity stellar population formed in situ, rather than being tidally stripped from either Magellanic Cloud. Three fields located near the Western Bridge observed by Harris (2007) were also observed with the XMMNewton. Cross-matching between optical and X-ray sources was performed, and any interesting matches were followed up with spectroscopic analysis, using the 1.9 m telescope located in Sutherland. A Be/X-ray Binary (BeXRB) candidate is discussed, and if confirmed, will be the furthest known BeXRB from the SMC.