School of Physics - Theses
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ItemMassive Black Holes( 2023-08)Black holes are one of the most fundamental astrophysical objects in our universe. In this thesis I look at massive black holes (MBH) with masses $10^{4}-10^{10}$ times that of our sun. In particular, I investigate how their gravitational influence distorts photon trajectories and describe how this can be used to study MBH. This phenomena, known as gravitational lensing, results in changes in shape and brightness of the images of the source as seen by a distant observer. The most striking manifestation of gravitational lensing is multiple images, known as \emph{strong} gravitational lensing. Strong gravitational lensing also results in the magnification of one or more of the images above that which would have been observed in the absence of deflecting matter. The number of cosmological black holes (MBH that do not belong to a galaxy core) is not well constrained. Gravitational lens statistics is one of the few ways to probe their number density. The fraction of sources experiencing strong gravitational lensing (multiple-image formation) is proportional to the number density of gravitational lenses which are able to form such images. GRBs are short bursts of $\gamma$-rays which signify the birth of a stellar mass black hole. Gravitational lensing of time-series data (light-curves) manifests as repetition of the primary signal as a lensed ``echo''. I describe the Bayesian parameter estimation and model selection software \pygrb{} which I wrote for this thesis. I use \pygrb{} to analyse GRB lens candidates from the Burst And Transient Source Experiment (BATSE) GRB catalogue to determine how similar the putative GRB lensed echo images are. I find one convincing candidate -- GRB~950830 -- which passes all our tests for statistical self-similarity. I conclude that GRB~950830 was gravitationally lensed by a $(1+z_l)M_l\approx\unit[5.5\times 10^4]{\msun}$ intermediate mass black hole (IMBH). Furthermore, based on the occurrence rate of this lensing event, I am able to estimate that the density of IMBH in the universe is $n_\textsc{imbh}=\unit[6.7^{+14.0}_{-4.8}\times10^{3}]{Mpc^{-3}}$. I also study the merger of black holes, looking at the recoiling quasar E1821+643 (E1821 hereafter). E1821 has a mass of $\mbh \sim \unit[2.6\times10^9]{\msun}$ and is moving with a line-of-sight velocity $v_\text{los}\approx \unit[2,070\pm50]{\kms}$ relative to its host galaxy. I use Bayesian inference to infer that E1821+643 was likely formed from a binary black hole system with masses of $m_1\sim 1.9^{+0.5}_{-0.4}\times \unit[10^9]{M_\odot}$, $m_2\sim 8.1^{+3.9}_{-3.2} \times \unit[10^8]{M_\odot}$ (90\% credible intervals). Given our model, the black holes in this binary were likely to be spinning rapidly with dimensionless spin magnitudes of ${\chi}_1 = 0.87^{+0.11}_{-0.26}$, ${\chi}_2 = 0.77^{+0.19}_{-0.37}$. I find that E1821+643 is likely to be rapidly rotating with dimensionless spin ${\chi} = 0.92\pm0.04$. Recoiling black holes are one method to populate the universe with massive black holes, however, these are expected to be rare. Massive black holes carry with them a tight cluster of stars and stellar remnants. These stars will pass through the optical caustic(s) of the black hole occasionally, which may lead to observable brightening of the star. Magnifications of greater than one million can easily be achieved, which I term ``Gargantuan Magnification Events'' (GMEs). I estimate the rate at which this lensing occurs, including the distribution of magnifications and event durations. I consider GMEs of pulsars in orbit of MBH as a possible generating mechanism for Fast Radio Bursts (FRBs). I find that pulsar GMEs are able to account for $0.1-1\%$ of the total FRB rate as observed by the Canadian Hydrogen Intensity Mapping Experiment Fast Radio Burst (CHIME/FRB) radio observatory. These seemingly unrelated problems all tied together in the end. This thesis is a study of black holes, their interaction with light and matter, and how they evolve through cosmic time. Many lifetimes of work have gone into generating the theory behind the sentence just prior. I hope that my contributions embellish these theories.
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ItemNature of quasar disk-wind( 2019)The brightest persistent astrophysical sources in the universe are quasars, a group of active galactic nuclei (AGN) that appear star-like and radiate across all wavelengths. The emitted radiation is believed to be powered by a supermassive black hole at the core of a galaxy. Matter that falls into the black hole is being fed onto the accretion disk, heating up the disk in the process due to friction. A wind emanating from the accretion disk, or a disk-wind, appears ubiquitous in these objects and acts as one effective way to generate the spectral lines observed in the quasar's spectrum. The broad spectral lines, originating from the broad line region (BLR), show diverse properties, specifically in velocity shift, line width, and degree of asymmetry. Yet, the exact structure of the BLR has remained perplexing due to its small size, which means it is unresolved even with the current astronomical instrumentation. Thus, simulations are important. By developing a model of the BLR, an informative analysis of the line profiles allows us to explore some of the key questions about the BLR, emphasising the shape of spectral lines, the disk-wind BLR, and the orientation. We simulate line profile modelling using a simple kinematical disk-wind model of the BLR with radiative transfer in the high velocity limit. The model provides a framework to explore the characteristics of the emission line profile induced by the different geometries and kinematics of the BLR, including the opening angle of the wind and the geometry of the line emitting region. The effect of orientation in these systems is also examined. As a first step, we use the model to simulate a narrow outflowing disk-wind, which has been described in the literature. The primary objective is to determine whether the observed emission line properties are consistent with a narrow wind scenario. We find that the line profiles are more blueshifted for a narrow polar wind model as opposed to intermediate and equatorial models. When viewing at pole-on angles, the simulated emission lines show a narrower line width, which is asymmetric and more blueshifted than that viewed edge-on. The blueward shift of the line profile increases as the line-of-sight and wind intersect. The model is also able to recover a shorter time delay in the red or blue side of the line profiles, consistent with observational evidence in reverberation mapping studies. The second part of the thesis considers the properties of broad absorption line quasars (BALQs). These objects are rare and often display a blueward absorption trough relative to the emission line. One interpretation of the velocity offsets is the unification based on orientation, whereby a BAL is viewed within a constrained narrow wind angle. In order to test whether the BALQs and non-BALQs can be distinguished by their emission features, we conduct statistical tests and machine learning on the two populations. We find that their continuum and emission features are qualitatively similar, which contradicts the narrow disk-wind model in the geometric unification. Therefore, we propose a model of the disk-wind comprising a wide wind opening angle with multiple dense radial streams, where the BAL is detected when the line-of-sight crosses these streams. These findings have lead us to the discovery of a novel orientation indicator of quasars in the ultraviolet-optical regime. We propose a simple yet robust angle-of-viewing probe using the correlation between the velocity shifts and line widths. Our idea is shown to be qualitatively consistent with other orientation proxies. We also perform a wide angle disk-wind simulation and successfully retrieve the predicted correlation with inclination. In addition, we extend our model to estimate the bias in the virial black hole mass due to the scale factor f, which is related to the unknown nature of the BLR. Using a wide disk-wind configuration, we retrieve the f factors for a range of inclination angle. The f factor shows significant dependence with orientation, characterisation of the line width, and location of the emission region in the wind. Therefore, using a constant f value biases the estimation of the mass of the black hole.