PhD Defence: David O´Neill

Abstract: 

Compact binaries of black holes and neutron stars are prime gravitational-wave sources, and growing evidence suggests that many evolve within gaseous environments. I will present recent work modelling gas–binary interactions from the linear to the fully non-linear regime.

Using linear response theory, we compute dynamical friction on bodies in elliptical and hyperbolic Keplerian orbits. Gas systematically drives binaries toward more eccentric, often supersonic trajectories: the semi-major axis always shrinks, while eccentricity may grow or decay depending on the Mach number. Orbital energy is therefore always dissipated, while angular-momentum exchange can change sign, showing that the force is not purely frictional.

To probe the non-linear regime near merger, we simulate supermassive black hole binaries embedded in retrograde circumbinary disks. We find disk–binary decoupling at separations of ∼33–3 gravitational radii (viscosity-dependent) and identify quasi-periodic instabilities that may produce high-frequency electromagnetic signatures and rapid post-merger rebrightening.

Together, these results provide a unified picture of how gas can shape the full inspiral of compact binaries—from wide separations to the final gravitational-wave–dominated coalescence—and highlight potential electromagnetic counterparts to massive black-hole mergers.

Date/Time: 26.11., 11:30h

Location: Auditorium A, Blegdamsvej 17