Title: Transients from the Birth and Death of Compact Objects
Description: Astrophysical compact objects --- white dwarfs (WDs), neutron stars (NSs), and stellar mass black holes (BHs) --- mark the endpoints of normal stellar evolution. Their birth is often associated with dramatic explosions known as core-collapse supernovae (SNe). Such SNe are archetypal ``transients'' --- astronomical events which produce detectable emission for only a limited period of time (measurable over human timescales). This dissertation investigates the astrophysical implications of the formation and destruction of compact objects with particular focus on the transient phenomena that may be produced in such events. Part I is devoted to the ``death'' of compact objects by their coalescence with a binary companion. Such compact object binaries are driven towards merger by the extraction of orbital energy in the form of gravitational-waves (GW), and are thus prime targets for current and future GW detectors. In the first two chapters of Part I we consider the merger of a WD with a NS companion, beginning with Chapter 2, in which we explore the nuclearly-reactive accretion flow produced in the aftermath of such mergers and the possible `SN-like' transient it may give rise to.

We continue in Chapter 3 by proposing that the late-time evolution of this post-merger accretion disk may result in terrestrial planet formation, broadly consistent with the mysterious ``pulsar planets'' observed orbiting PSR B1257+12. We shift our attention in the next couple chapters of this first part of the dissertation to binary NS mergers. In Chapter 4 we address the question of disk formation in the aftermath of the collapse of a rigidly-rotating supramassive NS, which is directly applicable to various models of gamma-ray bursts (GRBs). In Chapter 5 we utilize both GW and electromagnetic signatures of the first observed NS merger GW170817 to place new constraints on the NS equation of state. Finally, in Part II of this dissertation, we explore the connection between transient phenomena ranging from long- and ultra-long- GRBs, to energetic super-luminous SNe (SLSNe) and fast radio bursts (FRB), and relate these to the ``birth'' of a rapidly rotating highly-magnetized NS, a millisecond ``magnetar''.

In Chapter 6 we show that both jetted and thermal transients (namely a GRB and a SLSN) can be powered simultaneously by such magnetars, and explore the various observational implications of this connection. We end with Chapter 7 in which we study the photo-ionization of the medium surrounding a newly born magnetar, discussing the observational signatures related to the escape of this ionizing radiation. We additionally address the propagation of radio waves and the dispersion measure induced by such photo-ionization and apply these to show that FRBs are broadly consistent with having young magnetars as their progenitors.