2025 Theses Doctoral

Study of the Physics and Detectability of Compact Black Hole Binaries using Multi-messenger Observations

Xin, Chengcheng

This dissertation investigates the detectability and multimessenger signatures of two types of black hole binaries: compact supermassive black hole binaries (SMBHBs) driven by gravitational wave radiation, and the tidal disruptions of a star by a stellar-mass black hole (sBH) in close and low-eccentricity encounters.

In this dissertation, we first discuss the detectability of compact SMBHBs in time-domain surveys as periodic quasars, and in gravitational wave (GW) missions as low-frequency GWs. Using a series of analytical and statistical approaches, we estimate that up to ∼ 100 million quasars can be observed by the upcoming Legacy Survey of Space and Time (LSST), among which a small fraction can be associated with compact SMBHBs driven by GW-inspiral. We then demonstrate once their GWs are detected by the Laser Interferometer Space Atenna (scheduled to launch in mid-2030s), then their EM counterparts can be uniquely identified in LSST using the historical frequency evolution and other key information from GWs. Finally, we show that chirping compact SMBHBs are identifiable using a Bayesian method on their EM lightcurves alone.

We then investigate the close and low-eccentricity encounters between sBHs and stars, causing tidal disruption events; an interaction frequently expected in dense star clusters and the disks of active galactic nuclei. We perform a suite of hydrodynamical simulations of tidal disruption of a main-sequence star by a sBH. We study a range of binary mass ratios, semi-major axes and eccentricities. We demonstrate that under certain circumstances, the mass of the star is slowly stripped away by the sBH – we dub this a “tidal-peeling event” (TPE). We discover that some TPEs can be a new form of luminous galactic transients; TPEs exhibit significant accretion rates that can sometimes lead to semi-periodic luminosity fluctuations and powerful outflows.