Tidal Encounters of Close White Dwarf Binaries with Spinning Black Holes

When a stellar binary encounters a spinning black hole (BH), interesting phenomena might result due to the mutual interaction between the binary spin, orbital angular momentum (OAM), and the BH 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 (CM) 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 BH, 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 CM leading to equatorial and off-equatorial orbits, as also various initial inclinations between the binary’s initial spin angular momentum and its initial OAM. We find that the effects of BH spin manifest 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 BH spin emerges in distinct ways for different initial configurations of the binary’s spin alignment. We establish that, within the ambit of the Hills mechanism, in certain cases, the fallback rate may show a three-hump structure, due to interactions between the tidal debris of the individual stars.