title: Compact Stellar Mergers: The Origin and Electromagnetic Counterparts of Gravitational Waves
Creators: name: Fernandez, J
Publisher: name: Liverpool John Moores University
Year: 2021

description: The detection of gravitational waves (GWs) originating from a black hole (BH) binary merger in 2015 by LIGO marked the beginning of the age of GW astronomy. Another defining moment came in 2017 with the joint detection of GWs and their electromagnetic (EM) counterpart, beginning with a short gamma ray burst (sGRB), from a neutron star merger. For GW astronomy to reach its scientific potential accurate models of the binary GW inspiral are needed. In addition, an understanding of the possible formation channels of merging compact binaries (including how the formation history is encoded into GW observables) and of the possible EM counterparts to GWs (such as sGRBs, kilonovae and GRB afterglows in the case of NS mergers) is required. This thesis is dedicated to these two aspects of GW astrophysics. In part I we discuss a new formation channel of compact binaries: tidal encounters with a massive BH at galactic centres or potentially in dense star clusters. First we discuss simple cases where initially circular binaries are injected towards a massive BH. The analysis is later extended to initially eccentric systems (corresponding to different BH binary origins). These tidal encounters can disrupt binaries, but do not always lead their break-up. Since surviving binaries tend to become hard and eccentric, this process can produce BH mergers in principle. For initially circular binaries, we show that the gravitational wave (GW) merger times become shorter by a factor of more than 10^2 (10^5 ) in 10% (1%) of the surviving cases. This reduction is primarily due to the growth in binary’s eccentricity at the tidal encounter. We obtain the effective spin distribution of the survivors. It is found that binary orientations can flip in the opposite direction at the tidal encounter. For the survivors with large merger time reduction factors, the effective spin distribution is found to be rather flat. The merger rate due to the tidal encounter channel is estimated to be ∼ 0.6Gpc −3 yr −1 . This estimate is not especially sensitive to whether initially circular or initially eccentric binaries are considered. In part II we study merger jet radio images. These images recently proved to be essential in breaking the degeneracy between different ejecta models for the afterglow of the neutron star (NS) merger event GW170817. The properties of synthetic radio images are characterized in detail by using semi-analytic models of laterally spreading GRB jets, and compared to the case of collimated jets. The image centroid evolution is obtained, and we find that this feature initially moves away from the explosion point in the sky with apparent superlumianal velocity, following the principal jet. After reaching a maximum displacement its motion is reversed. This behavior is in line with that observed in full hydrodynamics simulations. We then explicitly show images can be used to break intrinsic degeneracies in afterglow light curves, and in particular how they can be used to determine the viewing angle θ_obs , or more precisely ∆θ = θ_obs − θ_c , where θ_c is the jet core opening-angle. Two methods for determining ∆θ are contrasted: the direct comparison of two images and the point emitter approximation for apparent superluminal motion, which states that for the apparent velocity at peak time β_app ∼ 1/∆θ. By considering five different jets with identical light curves at their peak time and which roughly agree with GW170817 afterglow radio data (ν = 3 GHz), the viewing angle for this event is estimated to be θ_obs ∼ 0.32 rad.