Browsing by Author "Mohamed, Shazrene"
Now showing 1 - 6 of 6
Results Per Page
Sort Options
- ItemOpen AccessDispersion measure variations in pulsar observations with LOFAR(2019) Ibrahim, Abubakr; Serylak, Maciej; Mohamed, ShazreneI present an analysis of the dispersion measure (DM) variations for 68 pulsars. The observations were taken using six International LOFAR Stations in Europe over the period of 3.5 years (between June 2014 and November 2017) at the centre frequency of 150 MHz with 80 MHz of bandwidth. During this time each pulsar was observed on weekly basis resulting in an average of 160 observations per source. I show that, the variations of the DM measurements show various trends along the span of the observation: increasing or decreasing, and in some cases more changes from one trend to another. I perform the structure function analysis for each of observed pulsar included in the study, in order to check if the DM variations follow the Kolmogorov power spectrum which describes the turbulence structure of the interstellar medium (ISM). I find that for a number of pulsars results show consistency with the Kolmogorov distribution (e.g. PSRs J1913−0440 and J2157+4017) while other sources show significant difference (e.g. PSRs J0108+6608 and J0614+2229). I also obtain the DM derivatives (i.e. dDM/dt) for each pulsar, in order to examine the correlation between the DM and its derivative. The result of this correlation shows a best-fit with a square-root dependence of 0.6±0.2, which is comparable with the result that was previously obtained by Hobbs et al. (2004), who shows a dependence of square-root between the DM and its derivative; with a gradient of 0.57±0.09. Also, one of the major results of this study that, thanks to the timing analysis, allowed me to produce a new timing solution for three pulsars: PSRs J0613+3731, J0815+4611 and J1740+27. This study concludes in that: i) the DM variations can be used to understand the general properties of the ISM ii) the low-frequency observations can enable us to study the dispersion effect on pulsar signals, which can be very useful for the effort of the pulsar timing array (PTA) project iii) IISM studies using pulsar timing is a powerful technique requiring careful approach to data reduction and analysis due to characteristic of the pulsars.
- ItemOpen AccessMeshless hydrodynamic simulations of young supernova remnants(2020) Mogawana, Orapeleng; Mohamed, Shazrene; van der Heyden, KThe majority of massive stars end their lives by ejecting their outer envelopes in a corecollapse supernova explosion. The collision of their ejecta with the surrounding circumstellar medium results in the formation of supernova remnants that have been detected at all wavelengths, from radio to gamma-rays. For several dozen supernova remnants, very-long-baseline radio interferometers have spatially resolved the interaction region and directly measured the expansion rates of the shocked gas; many show evidence of the interaction of supernova ejecta with the dense slow winds characteristic of the red supergiant progenitors. Understanding the dynamics and morphology of the interaction region, particularly in young supernova remnants leads to estimates of the total mass of the circumstellar medium, as well as its density distribution around the star given the value of the wind velocity. Here we studied the interaction of the supernova ejecta with different circumstellar environments to investigate the hydrodynamic evolution of young supernova remnants in the SedovTaylor phase. We used the massively parallel, multi-physics magneto-hydrodynamics (MHD) and gravity code, GIZMO, for our simulations. We chose GIZMO for its flexibility in allowing the user to choose different methods to solve the fluid equations, i.e., new Lagrangian Godunovtype schemes, e.g., Meshless Finite Volume (MFV) and Meshless Finite Mass (MFM), as well as various flavors of smoothed particle hydrodynamics (SPH), or Eulerian fixed-grid schemes. Since the majority of previous studies used the latter, we focused on an extensive comparison of all the meshless methods in solving the Sedov-Taylor blastwave test, a problem for which there is an exact solution. For our given compute resources, we found the parameters (e.g., smoothing length, number of neighbours, artificial viscosity, and particle resolution) for each meshless method that gave the best agreement with the exact solution. We then carried out 2D and 3D simulations of the hydrodynamic interaction of the supernova ejecta with varying density profiles assumed for the circumstellar medium, namely: a 1/r 2 density profile, for a typical, spherically symmetric red supergiant stellar wind, and an axisymmetric torus profile, inspired by the observation of a dense, dusty torus of the circumstellar material around the red supergiant, WOH G64 (Ohnaka et al., 2008). Radially assembled Hierarchical Equal Area isoLatitude Pixelization (HEALPix) shells were used to set-up the initial density and velocity profiles for the ejecta, which is marked by a flat inner core and a steeply declining outer edge. The Weighted Voronoi Tessellation code was used to produce the 1/r 2 and axisymmetric torus density distributions. We showed that the growth of Richtmyer-Meshkov instabilities in the 2D and 3D 1/r 2 profiles are visible as early as 20 yrs into the evolution of the remnant and become increasingly unstable up to 100 yr. While 2D simulations of 1/r 2 profiles show the presence of the Richtmyer-Meshkov instabilities in the hot shell of a contact discontinuity, in 3D we see large bubbles and filamentary structure of the instabilities. Our results for the numerical approaches to simulating the systems for the 1/r 2 density cases were broadly consistent with previous studies in the literature where stationary grids were used. Two scenarios with different torus-cavity density contrasts were considered in which we found that the instability rolls along the half-opening angle takes ∼ 40 yr to develop in the axisymmetric torus with smooth density drop, whereas the axisymmetric torus with steep density drop does not develop any instability rolls up to the end of the simulation. We concluded with a discussion of the implications of our models for the morphology of supernova remnants and their expected levels of multi-wavelength emission.
- ItemOpen AccessMultiwavelength studies of classical novae(2018) Aydi, Elias; Mohamed, Shazrene; Whitelock, PatriciaClassical novae are transient astronomical events resulting from an eruption on the surface of an accreting white dwarf in interacting binary stars, namely cataclysmic variables. These eruptions and their progenitors are ideal astronomical events/objects to study a variety of interesting topics, such as mass-transfer mechanisms, binary evolution, and thermonuclear and radiative emission processes. Novae share some general observational characteristics during their eruption phases, yet each nova is new and can show unique light-curve and spectral developments. Therefore, carrying out detailed multiwavelength studies of individual novae is important to provide an holistic picture of the eruption development with the ultimate aim of a general understanding of the physical mechanisms involved in nova eruptions, as well as the structure and evolutionary condition of their progenitors. This dissertation presents detailed multiwavelength studies of three nova events, namely V5852 Sgr, SMCN 2016-10, and V407 Lup, as a series of chapters (papers). Our data were collected using several ground-based and space telescopes including: high- and mediumresolution optical spectroscopy from SALT, SOAR, and FLOYDS, optical and near-infrared photometry from IRSF, OGLE and SMARTS, UV and X-ray data from Swift, Chandra, and XMM-Newton. In the second chapter, we present an optical and infrared study of the unusual nova V5852 Sgr. This nova is a moderately fast nova showing features of the Fe II spectroscopic type. The light-curve development is unique, showing a combination of several light-curve classes. Estimates of the distance and measurements of the line velocities suggest that the nova might be associated with the Sagittarius stream. If so, it would be the first nova to be discovered in a dwarf spheroidal galaxy. The third chapter presents a multiwavelength study of the very fast nova SMCN 2016-10a. The fast light-curve development suggests that the system hosts a massive white dwarf (& 1.25 M), in good agreement with the high temperature of the super-soft X-ray emission and the turn-on/turn-off time of the super-soft state. At the distance of the Small Magellanic Cloud our measurements suggest that SMCN 2016-10a is the brightest nova in the Small Magellanic Cloud and one of the brightest novae on record, with an absolute maximum magnitude of ∼ −10.5 in the V -band. Chapter four is a multiwavelength study of nova V407 Lup. With a light-curve decline time t2 ≤ 2.9 d, this is one of the fastest known examples and the white dwarf is possibly more massive than 1.25 M. Our set of optical, UV, and X-ray data suggest that this system is an intermediate-polar cataclysmic variable based essentially on the presence of two periodicities (3.57 h and 565 s) in the light-curves. These periodicities are attributed to the orbital period of the binary and rotational period of the white dwarf. The late optical spectra (taken from day 165 post-eruption) show narrow and moving lines of He II and O VI, possibly associated with accretion regions within the binary system. This, along with the X-ray light-curves and spectra, suggest that the accretion probably resumed around 168 days post-eruption. These studies are modest, yet essential steps in the quest for a better understanding of nova eruptions. They also demonstrate the importance of multiwavelength follow-up of novae for constraining the physical parameters of the eruption, the ejecta, and the properties of the progenitor. In the era of large all-sky surveys, such as the All-Sky Automated Survey for Supernovae and eventually the Large Synoptic Survey Telescope which have been and are ex- pected to find a large number of optical transients and classical novae, similar multiwavelength follow-up will play a crucial role for initially identifying these transient events and further understanding their physical behaviour.
- ItemOpen AccessOptical and gamma-ray study of gamma-ray binaries(2018) Monageng, Itumeleng; Mcbride, Vanessa; Böttcher, Markus; Mohamed, ShazreneGamma-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.
- ItemOpen AccessStellar Halos: modelling formation in the L-Galaxies 2020 semi-analytic model(2020) Murphy, Geoff; Yates, Robert; Cunnama, Daniel; Mohamed, ShazreneA study was carried out to determine how well the L-Galaxies 2020 semi-analytic model simulates the stellar halos of galaxies and the intracluster stellar (ICS) components of galaxy clusters. Two galaxy disruption models were tested, namely instantaneous disruption and gradual disruption. Furthermore, two stellar halo profiles were applied to the simulation results: a power-law profile with slope γ = −3.5 and a Navarro-Frenk-White (NFW) profile. In the latter case, the stellar halo stars follow the distribution of the galaxy's dark matter. It was found that a combination of an NFW profile and gradual disruption provided the best results across the widest range of literature data, namely measurements of stellar halo mass, total stellar mass, stellar mass fractions, and stellar halo iron abundances. Gradual disruption of satellite galaxies also resulted in the central galaxies having more massive stellar halos in comparison to instantaneous disruption. Additional stellar halo formation mechanisms, such as in-situ star formation, were not needed, as the stellar halo masses seen in observations can be obtained in L-Galaxies by considering only tidal disruption of infalling satellite galaxies. The number of high mass accretions into the halos of Milky Way-mass galaxies in the gradual disruption model agreed well with simulation literature. It was found that while central galaxies can induce many disruptions of satellite galaxies (over a thousand in some cases), the majority of the Milky Way-sized stellar halos in L-Galaxies are formed by the disruption of one to fourteen satellite galaxies, in good agreement with simulation literature. A population of galaxies with unexpectedly low stellar halo iron abundances was found. These were determined to be a result of disruptions of high mass, low metallicity satellite galaxies. Furthermore, rather than iron or oxygen, carbon was found to be the dominant element produced by stellar halo stars for the majority of redshifts in most high mass central and satellite galaxies, due mainly to asymptotic giant branch stars. The relative contribution of stellar halo stars was found to be minor, however, with circumgalactic medium enrichment from halo stars in comparison to outflows from galactic stars being on average . 1%. For clusters with virial masses exceeding 1.6 × 1014M, the brightest central galaxy and ICS (BCG+ICS) stars contained 42.44% of the total cluster stellar iron content, while the fraction MICS MBCG+MICS was found to be 82.50%, both results being in good agreement with observation.
- ItemOpen AccessStellar occultations by bodies in the outer solar system at the South African Astronomical Observatory(2018) Genade, Anja; Sickafoose, Amanda A; Mohamed, ShazreneInformation on the origin and evolutionary processes of the Solar System is harbored by primitive bodies called trans-Neptunian objects (TNOs). Their preserved state is due to these bodies having orbits at and beyond that of Neptune, as this specific area is considered to be the least thermally modified in the Solar System and could contain a large population of primordial remnants. These archaic remnants not only provide us with information on our infant Solar System, but also improve our understanding of extrasolar planetary formation processes. Stellar occultations by TNOs enable the determination of sizes and shapes with kilometric accuracy (confining albedos, leading us to compositions and densities), the detection of atmospheres down to pressures of a few nanobars, as well as an investigation of the immediate vicinity of the target body (indicating the presence of rings, satellites, jets, comas). These TNOs roam the icy outskirts of our Solar System which, together with their small sizes, make them faint and their stellar occultations short-lived. With a duration ranging from a couple of seconds for small bodies up to a few hundreds of seconds for the larger TNOs, specific imaging cameras combined with Global Positioning devices and optical telescopes, ensure the fast-cadence capture of these events with microsecond timing accuracy. This high-time resolution observing in combination with the relative speed of the star and the occulting body ensures accurate results with resolutions down to the kilometriclevel, which overthrows the resolvability of ground-based telescopes and rivals that of space probes. Here, we present work done by the South African Astronomical Observatory (SAAO) stellar occultation observing program during the period of July 2016 - July 2017. Specifically, we describe the telescopes, instruments and data analysis pipelines that are used for the SAAO stellar occultation program. The stellar occultation results are obtained from the slightly modified, data reducing SHOC pipeline as originally developed by Dr. Marissa Kotze. The SHOC pipeline laid the groundwork for two additional pipelines to be developed and therefore the MORIS instrument on the 3-m IRTF as well as the FLI autoguider cameras mounted on multiple 1-m LCO telescopes are included to provide many opportunities to observe the predicted stellar occultations. These pipelines include reduction features, correct and accurate derivation of timing information, optimization of the signal-to-noise ratio (SNR) through aperture corrected photometry and most important, provide the user with light curve plots of the point sources. The light curves are normalized and individually analyzed for any signs of a positive stellar occultation detection while testing the effects of reduction on the SNR. This is followed by checking the statistical distribution of the data as well as determining a few values for the line-of-sight optical depth. Instrumental deadtimes for the LCO guider cameras are calculated to effectively determine and use time allocated through proposals. Finally, a single positive stellar occultation by Orcus was observed on 7 March 2017 from two separate sites. Here, chord length calculations as well as timing offsets are calculated from the normalized light curves which led to a possible detection of both Orcus and Orcus’s satellite, Vanth. An in depth discussion is provided to justify this reasoning. This thesis serves to characterize and consolidate the now well-established program of stellar occultation observations at the SAAO.