UC Berkeley

Compact objects can produce a variety of distinct observable signatures due to their extreme properties. In particular, mergers and encounters involving compact objects can produce electromagnetic (EM) transients that probe these systems and inform broader astrophysical questions. This dissertation focuses on two categories of fast and bright electromagnetic transient: kilonovae (kNe) produced by black hole - neutron star (BH-NS) and neutron star - neutron star (NS-NS) mergers, and tidal disruption events (TDEs) produced when a star is tidally disrupted by a supermassive black hole (SMBH). The introductory chapter contains a brief summary of the basic physics of these transients. We then present Monte Carlo radiative transfer (MCRT) simulations that calculate the EM emission from aspherical kilonovae. Multidimensional kilonova models can aid efforts to detect and characterize these events. The second chapter presents two axisymmetric geometric models of aspherical ejecta: an ellipsoid and a torus. These generic models capture the simplest global deviations from sphericity. The third chapter examines the kilonova signatures from the tidal dynamical ejecta of a BH-NS merger. We study the hydrodynamic evolution of the ejecta and calculate the bolometric and broadband light curves. In the fourth chapter we take an interlude and consider a potential EM signature from a different category of compact object transient: the accretion-induced collapse (AIC) of white dwarfs. The MCRT calculation in this work is based on a now-outdated ejecta model, but provides a benchmark for other potential AIC signals. We close by studying the signatures from some atypical TDEs, which might exhibit extreme properties that uniquely probe SMBHs. The fifth chapter examines the new regime of ultra-deep TDEs, in which a star is disrupted on a highly penetrating encounter with a low-mass SMBH and the disrupted debris strongly self-intersects at first pericenter passage. We determine the encounter properties, then predict a novel observable and estimate the detection rate. In the sixth chapter we present a parameter study of the statistics and signatures of TDEs from binary SMBHs.