The Importance of Compact-Object Spin in Extreme and Intermediate-Mass Ratio Inspirals

Abstract

Gravitational waves can be produced when a compact object (stellar-mass black hole, neutron star, or white dwarf) spirals towards a massive black hole. For moderate mass ratios of 10-2 - 10-4, these events are known as intermediate mass ratio inspirals (IMRis), whereas for mass ratios below 10-4 they are called extreme mass ratio inspirals (EMRis). These events will be important sources for the proposed gravitational-wave detector LISA (Laser Interferometer Space Antenna) and will provide a precise test of general relativity in the unexplored regime of strong gravitational fields. To detect these gravitational waves and reliably measure the parameters of the binary producing them, highly accurate models of gravitational waveforms are needed. The spin of the orbiting compact object (CO) introduces forces that affect the orbit and waveform, but these effects are often ignored. The goal of this thesis is to determine under what circumstances compact-object spin will significantly alter the waveform as measured by LISA. To do this, a post-Newtonian waveform model will be used to explore CO spin effects. It is concluded that neglecting CO spin does not effect the detectability of EMRis or IMRis. Parameter measurement errors introduced by neglecting CO spin are typically small unless the binary system happens to be particularly close. Constraining the value of compact object spin is unlikely but may be possible for a nearby IMRI. Within the approximations used in this study, it appears CO spin is marginally important, but that conclusion may change if a more realistic waveform model is used in future work.