Browsing by Author "Monageng, Itumeleng"

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    A spectral and timing study of MAXI J1820+070 during outburst
    (2024) Solomons, Kyle; Monageng, Itumeleng
    Black Hole X-ray binaries (BHXRBs) represent a unique class of astrophysical systems where a stellar-mass black hole accretes matter from a companion star via accretion disks. The accretion disk emits in X-ray and UV because of the radiative losses. The transfer of matter can occur through continuous donor supply or episodic capturing of material from stellar winds, leading to sudden enhancements in X-ray flux known as outbursts. The instabilities in the disk, the geometry of the inner disk, the coupling between the disk and corona, etc, also contribute to the observed variability. The exploration of BHXBs contributes significantly to our understanding of the broader astrophysical landscape. These offer a unique testing ground for theories related to accretion physics, extreme gravity, jet formation, and the evolution of binary systems. Moreover, the BHXBs play a pivotal role in shaping galactic dynamics and evolution in their neighbourhood. Among the myriad BHXBs, MAXI J1820+070 stands out as a particularly intriguing target. This low-mass X-ray binary, located in the constellation Ophiuchus, garnered attention due to its exceptional outburst in March of 2018. The intense luminosity of MAXI J1820+070, which reached a peak X-ray flux of ∼ 4 Crab, allowed for detailed multi-wavelength campaigns, facilitating comprehensive investigations into its properties and behaviour. MAXI J1820+070 provides a unique opportunity to deepen our understanding of accretion processes near black holes, enabling the refinement of theoretical models and enhancing our ability to interpret observations across the electromagnetic spectrum. This study presents a comprehensive analysis of the archival X-ray data of MAXI J1820+070 from NuSTAR, NICER, and Swift-XRT. The investigation focusses primarily on the system's spectral properties and timing characteristics. Spectral evolution and transitions between the hard and soft states are examined, with key parameters defining these states extracted. The inner disk radius was constrained to ≲ 2.6 ISCO, extending down to 1.5 ISCO before the state transition. The disk temperature steadily increases from 0.71 keV, peaking at 0.8 keV. Our simplistic spectral model prefers different inclinations at various stages of the outburst, varying from ∼ 45 to 73 degrees, probably due to the model's limitations. Furthermore, the power-law index was restricted to ∼ 1.6 and the coronal electron temperature to ∼24-38 keV. We identify the presence and evolution of quasi-periodic oscillations and quantify them through Lorentzian curve fitting. Additionally, we identified hard and soft time lags, varying in amplitude and frequency during the outburst. Together, the spectral and timing results suggest a QPO originating from the corona, with the corona contracting during the hard state and expanding during the state transition. Possible signatures of outflows are detected through absorption features between 6.9-7.3 keV.
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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.