Rapid multiwavelength astrophysics: on variability in black hole low-mass x-ray binaries

This work presents state-of-the-art research on Black Hole Low Mass X-ray Binaries, carried out using rapid multiwavelength timing, and motivated by the wish to understand the structure and nature of these sources. I show the results of observations from strictly simultaneous observations - primarily at optical and X-ray wavelengths - probing at timescales up to a resolution of almost 300 Hz, with the goal of better understanding the processes that give rise to this rapid variability. Firstly, I present my investigations into the faint and enigmatic Swift J1357.2-0933, which shows puzzling optical dips. I find that the source is bluer during these dips, in contradiction to conventional dust occultation theories; as such, I put forward a geometric- and jet- focused model of a perturbation in the disc obscuring a red inner region, such as an extended, ‘smeared-out’ jet region. Secondly, I present six epochs on the superbright MAXI J1820+070 over its initial hard state. I show an evolution of the variability over these epochs, with a hot flow component growing in relative significance and a jet component declining in strength (which I posit could be due to a changing coronal geometry), as well as the effects of a Quasi-Periodic Oscillation, which all have significance on the evaluation of future sources. I also show a consistent optical lag on the order of ∼100 ms, which I find to be consistent with a jet model in the source; I then show a fascinating wavelength dependence to this lag by up to 20 ms between the shortest (us) and longest (zs) optical bands, which could be some of the first ever evidence of stratification at the base of a compact jet. Thirdly, I discuss a more theoretical and mathematical underpinning to this research. I present software that I have written that has the ability to simulate multiwavelength observations based on a series of observational and source parameters, including Fourier descriptions of its variability. I show how this code can be used in observation planning, investigating the effects of various parameters such as noise level, target brightness, and telescope quality, and also how it can be used to model real sources and investigate the components that give rise to observed variability. Finally, I provide a summary of my work in the field, and what these results mean in context. I end by presenting a salient selection of remaining questions in the field, and possible avenues that future research may take