Browsing by Author "Groot, Paul Joseph"

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    Gamma-ray bursts with MeerLICHT
    (2024) De Wet, Simon; Groot, Paul Joseph
    The subject of this thesis is the multi-wavelength observational study of gamma-ray burst (GRB) afterglows. The driving force behind this work is the fully robotic MeerLICHT optical telescope which is able to rapidly slew to the position of a GRB in the sky and obtain multi-lter follow-up observations of the early phases of the afterglow, when non-standard behaviour may occur. Following the introduction, the thesis consists of four main chapters: a chapter outlining the more than two year GRB follow-up programme undertaken with MeerLICHT in which we followed-up 29 bursts, and three chapters presenting detailed studies on individual GRBs. The rst of these studies focuses on GRB 210731A. Starting 286 seconds post-trigger, MeerLICHT obtained a highly unusual light curve consisting of three peaks, which we interpreted as being due to energy injection. Through multi-wavelength theoretical modeling we found that a forward shock model within a stellar wind medium could explain all of our X-ray, optical and radio data, but not our 1.4 GHz upper limits. We suggested that a possible thermal electron population might explain the additional opacity at lower radio frequencies. The subject of the second detailed study is GRB 220627A, a rare burst consisting of two gamma-ray emission episodes separated by almost 1000 s. The discovery of the optical afterglow by MeerLICHT led to spectroscopic observations which secured the burst redshift to z=3.08, making this the most distant ultra-long GRB to date. Our modelling and afterglow analysis showed that GRB 220627A does not appear to have a dierent progenitor compared to the wider long GRB propulation. The third detailed study encompasses GRB 210702A. This burst was unique for being the rst GRB with a clear rebrightening in its millimeter light curve which we attempted to explain via energy injection or a reverse shock from a late-time shell collision. Prior to the millimeter rebrightening, we found that the X-ray, optical and millimeter data could be reconciled within a standard forward shock model in a stellar wind medium, however, similar to other bursts with extensive radio data sets, no standard model could explain all of our radio data.
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    Log mining to develop a diagnostic and prognostic framework for the MeerLICHT telescope
    (2022) Roelf, Timothy Brian; Groot, Paul Joseph; Rakotonirainy, Rosephine Georgina
    In this work we present the approach taken to address the problems anomalous fault detection and system delays experienced by the MeerLICHT telescope. We make use of the abundantly available console logs, that record all aspects of the telescope's function, to obtain information. The MeerLICHT operational team must devote time to manually inspecting the logs during system downtime to discover faults. This task is laborious, time inefficient given the large size of the logs, and does not suit the time-sensitive nature of many of the surveys the telescope partakes in. We used the novel approach of the Hidden Markov model, to address the problems of fault detection and system delays experienced by the MeerLICHT. We were able to train the model in three separate ways, showing some success at fault detection and none at the addressing the system delays.
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    Searching for Ultra-compact binary stars with the MeerLICHT Southern All-Sky Survey
    (2022) Lees, Robert; Groot, Paul Joseph; Woudt, Patrick
    An 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.
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    Spectroscopic and photometric observations and analysis of compact binaries
    (2023) Brink, Jaco; Groot, Paul Joseph
    I present the results of time resolved optical spectroscopic and photometric observations of three recently discovered transient compact binary systems which comprise all three types of accretors, namely a white dwarf, neutron star and black hole. One system is a Cataclysmic Variable (CV), with an accreting white dwarf, namely 2SXPS J062339.9−265751, while two are Low-Mass X-Ray Binaries (LMXBs). One of these, Swift J1357.2−0933 is thought to contain an accreting black hole, while the other, CXOU J1109−6502, contains a neutron star and is also a Transitional Millisecond Pulsar (tMSP) candidate. All of these sources were observed as part of the Southern African Large Telescope (SALT) transient follow-up program. The CV, 2SXPS J062339.9−265751, was recently found in the eROSITA all-sky survey and was shown to be a Novalike CV system, although its true nature is not well established and photometry I present here supports a possible Intermediate Polar classification. Swift J1357.2−0933 was observed by SALT since it was discovered to be in outburst in 2019 and CXOU J1109−6502 was observed as part of a follow-up campaign, also utilizing SALT, on tMSP candidates. One major aim of this thesis is to study the time varying nature of the optical spectra of the three objects and particularly to measure the emission lines parameters and to determine the radial velocities of the systems. For 2SXPS J062339.9− 265751, spectroscopy was undertaken using the SAAO 1.9 metre telescope. In addition, time series high speed photometry, over many epochs, was also obtained and analyzed for 2SXPS J062339.9−265751, using the two SAAO 1 metre telescopes of the South African Astronomical Observatory (SAAO). The aim of the spectroscopic and photometric observations of 2SXPS J062339.9−265751 was to help understand the physical nature of the system, and determine whether it is a magnetic CV (i.e. an intermediate polar) or an over-luminous non-magnetic novalike CV. The derived Lomb- Scargle radial velocity periodograms of 2SXPS J062339.9−265751 were used to determine the orbital period of the system, which is 3.164 ± 0.036 h. This is significantly different to the photometric period determined by TESS of 3.941 ± 0.010 h which may indicate the presence of a superhump period. 2SXPS J062339.9−265751 displays some similarities to other novalike systems, such as V341 Ara and ASAS J071404+7004.3, some of which also show similar shallow broad absorption lines with strong central emission peaks. From my analysis of multiple epochs (from 1 November 2020 to 2 March 2021) of high speed photometry of 2SXPS J062339.9−265751, comprising 56.5 hours of data, I show evidence for periodicty at P = 24.905 ± 0.003 min, which I interpret as evidence that the system is a member of the intermediate polar class of magnetic CVs. For the LMXB black hole system, Swift J1357.2−0933, the observed transient blue-shifted absorption lines were analyzed to determine the radial velocity changes of the out-flowing material, previously attributed to a hot wind outflow. Persistent weak double-peaked emission lines, centred near the rest velocity, were also detected at times, most likely being produced by the accretion disk. These emission lines do not show any modulation throughout an observation, although the separation between the peaks do vary slightly from epoch to epoch. The Hα emission line is also seen to change from a broad line to showing a P Cygni profile at certain epochs. Time resolved spectra of the tMSP candidate, CXOU J1109−6502, were analysed and radial velocity measurements were determined for the Hα emission line. This emission line displayed a multi-component nature and showed dramatic changes from epoch to epoch, and even throughout a single night, from broad emission to narrow emission lines at the rest wavelength.