School of Physics - Theses
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ItemSpin-down signatures of young neutron stars( 2022)The spin down of neutron stars has been invoked to explain a wide variety of electromagnetic and gravitational-wave signals. This thesis explores two different signals associated with the spin down of neutron stars, one electromagnetic signal and one gravitational-wave signal. Binary neutron star coalescences, confirmed as the progenitor of at least some short Gamma-ray bursts (sGRBs) in 2017, are predicted to form either a black hole or a highly magnetized neutron star. Up to 20% of sGRBs observed by the Neil Gehrels Swift telescope display prolonged X-ray emission, sometimes called a ``canonical'' afterglow, consisting of three phases: an initial power-law luminosity decay; a 'plateau', lasting between 10 s and 105 s, during which the X-ray luminosity is approximately constant; and a final power-law decay. Previous authors have noted that the evolution of the canonical light curve is broadly consistent with the expected spin-down luminosity of a neutron star. Key ideas from analytic, one-zone models of plerions (also called pulsar wind nebulae) can be used to model the evolution of a synchtrotron nebula fuelled by the the spin-down luminosity of a neutron star formed in an sGRB. An analytic expression for time-dependent, spatially-averaged electron energy distribution in the nebula is found and used to calculate the light curve and the point-in-time spectra. The light curves predicted by the plerionic model are consistent with the shape and luminosity of the X-ray light curves and reproduce the observed correlation between plateau duration and luminosity (i.e. brighter plateaux end sooner). Furthermore, Bayesian parameter estimation comparing the point-in-time spectra to time-averaged spectra of six Swift sGRBs with canonical X-ray afterglows and of known redshift allows estimation of the parameters of the neutron-star central engine, including its poloidal field strength Bp and its rotation period P0 at birth, and injection parameters within the shock, including the energy range of the relativistic electrons and their power-law index. All six sGRBs favour a neutron star with Bp ~ 1011 T and P0 ~ s, consistent with the prediction the neutron star should be highly magnetized and rapidly spinning. We also apply the point-in-time spectra to four time-averaged spectra taken at four separate epochs in the X-ray afterglow of GRB130603B and infer the evolution of the magnetic field in the synchrotron bubble B. We find the evolution of B is slower than the expected evolution of the far-field limit of the stellar magnetic field. Rotating, non-axisymmetric neutron stars spin down via the emission of continuous gravitational waves which may be detectable by current terrestrial interferometers such as the advanced Laser Interferometric Gravitational-wave Observatory (LIGO) and advanced Virgo. Young core-collapse supernova remnants are likely hosts of young neutron stars and are common targets for wide-band directed searches for continuous gravitational waves targeting non-pulsating neutron stars. In this work, we present the results for two searches for continuous waves from neutron stars in young supernova remnants using a hidden Markov model (HMM). The HMM tracking scheme models the frequency evolution as a random walk with secular spin down and remains sensitive in the presence of stochastic spin wandering similar to that observed in pulsar timing observations. A search targeting twelve neutron stars in young supernova remnants in the second observing run (O2) of advanced LIGO using an HMM tracking scheme identifies 1012 potential candidates, 18 of which survive a series of standard vetoes. Further assessment of the 18 survivors based on their dependence on sky position and Doppler modulation confirms they are all consistent with terrestrial noise. A second search, conducted with the the LIGO-Virgo-KAGRA (LVK) collaboration, targets fifteen neutron stars in young supernova remnants in the first half of the third observing run (O3a) of advanced LIGO and advanced Virgo using three search pipelines, including an HMM tracking scheme, and reports no candidates consistent with an astrophysical origin after a rigorous veto and follow-up process. The HMM tracking scheme sets the first 95% confidence limits on gravitational-wave strain, h095%, for these targets with a random-walk signal model, reaching a sensitivity of h095% = 2.64 x 10-25 at 172 Hz for G353.6-0.7. The constraints on h095% are converted to upper limits on neutron-star ellipticity below 10^-5 above 150 Hz and constrain the maximum amplitude of internal r-mode oscillations below 10^-3 above 150 Hz.
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ItemGravitational lensing and clustering of galaxies in the epoch of reionization( 2015)The epoch of reionization marks the period where neutral hydrogen in the intergalactic medium was ionized by high energy photons emitted by the first stars and galaxies. Observations of galaxies during this period aim to uncover the types of stars and galaxies that were responsible for producing the ionizing flux to complete reionization within one billion years after the Big Bang (by z~6), and study the formation of the first galaxies in the Universe. These galaxies are observed in the near-infrared (NIR) today, and so require space-based observatories with sensitive NIR cameras such as Wide Field Camera 3 on the Hubble Space Telescope (HST). Considerable effort has been dedicated to ultradeep observations with HST in order to identify galaxies in the epoch of reionization. Surveys such as the eXtreme Deep Field (XDF), Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS), Early Release Science (ERS) and the Brightest of Reionizing Galaxies (BoRG) have now discovered hundreds of these galaxies. This thesis makes use of these observations to address two important topics: how gravitational lensing affects the observations of such galaxies, and what their spatial distribution can tell us about the underlying dark matter distribution at this early time. Photons emitted by the first galaxies traverse most of the Universe’s history before reaching our telescopes. As such, they are subject to gravitational lensing by foreground galaxies along the line of sight. Gravitational lensing can result in the magnification of high redshift galaxies, skewing their observed luminosities. Because bright galaxies are significantly rarer than their fainter counterparts, the chance of gravitational magnification for observationally bright galaxies is significantly enhanced. This effect is known as magnification bias. We use the largest samples of Lyman-break galaxy (LBG) candidates observed in the first 1:5 billion years after the Big Bang (46.5. The clustering signal at z~7 is detected at >4 sigma, and corresponds to a real-space correlation length of r_0 = 6.7 +0.9/-1.0 cMpc, a galaxy bias of b = 8.6 +/- 0:9, and dark matter haloes of mass M = 10^(11:3+0:2/-0.3) M_sun . We reassess the clustering of LBGs at z=4–6 and find a trend of increasing bias from z=3.8 (b~3.0) to z=7:2 (b~8.6). We use these measurements to infer the fraction of dark matter haloes hosting UV-bright galaxies, and find that values near unity are preferred at z=7.2, which may be explained by the shorter halo assembly time at high redshift.
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ItemBlack holes and higher dimensions( 2007)Higher dimensional black holes are studied in the extra large dimensions scenario. Bulk fermion quasi-normal modes and bulk fermion Hawking emission is calculated. It is found that bulk emission dominates brane emission for d > 5. To address the Planck phase an effective field theory is investigated. Lepton family number violating processes are elucidated and the corrections to the muon magnetic moment from these channels are calculated. Bounds are placed on the couplings of the theory. A discrete symmetry between quarks and leptons, and left- right- chirality fields, is orbifolded in 5 dimensions. Using split fermions a one generational standard model extension is found. An investigation of entanglement in black holes and accelerated motion is presented. It is found that in a certain system the acceleration between two spinors enhances the rate of their disentanglement.