Tidal encounters of close white dwarf binaries with spinning black holes

TL;DR

This study investigates how the spin of a black hole affects tidal interactions with a close white dwarf binary during a near-parabolic encounter, revealing spin-dependent dynamics and observable signatures.

Contribution

It introduces a combined three-body and hydrodynamics simulation approach to analyze the impact of black hole spin on binary tidal encounters, including diverse initial configurations.

Findings

Black hole spin influences tidal dynamics and fallback rates.

Initial inclination affects observable tidal signatures.

Possible three-hump fallback rate structure identified.

Abstract

When a stellar binary encounters a spinning black hole, interesting phenomena might result due to the mutual interaction between the binary spin, orbital angular momentum and the black hole spin. Here we consider such encounters between an intermediate mass spinning black hole and a close identical white dwarf binary system whose center of mass follows a parabolic trajectory. After studying a corresponding three-body problem in the point particle approximation, we perform a suite of smoothed particle hydrodynamics based numerical simulations of such scenarios. For this, we integrate the geodesic equations for the spinning black hole, while considering the hydrodynamics and the self and mutual gravitational interactions of the stars in a Newtonian approximation, an approach justified by the choice of parameters in the theory. We consider various initial configurations of the binary center of mass leading to equatorial and off-equatorial orbits, as also various initial inclinations between the binary's initial spin angular momentum and its initial orbital angular momentum. We find that the effects of black hole spin manifest clearly in the tidal dynamics of the binary components, while the observables of tidal encounters such as mass fallback rates are strongly dependent on the initial inclination angle. We show that the influence of the black hole spin emerges in distinct ways for different initial configurations of the binary's spin alignment. We establish that within the ambits of the Hills mechanism, in certain cases, the fallback rate may show a three-hump structure, due to interactions between tidal debris of the individual stars.