Tidal disruption events from supermassive black hole binaries

TL;DR

This paper studies how supermassive black hole binaries influence tidal disruption events, revealing significant variability in debris morphology and accretion rates, with implications for detecting such events in galactic nuclei.

Contribution

It provides the first large-scale statistical analysis of stellar disruptions by SMBH binaries and detailed hydrodynamic simulations showing altered debris evolution and accretion behaviors.

Findings

Binary SMBHs cause significant variance in disruption energy and angular momentum.

Debris morphology and accretion rates are greatly affected by the second black hole.

Accretion curves show power at binary orbital timescales, with no strict periodicity.

Abstract

We investigate the pre-disruption gravitational dynamics and post-disruption hydrodynamics of the tidal disruption of stars by supermassive black hole (SMBH) binaries. We focus on binaries with relatively low mass primaries ($10^6M_{\odot}$), moderate mass ratios, and separations with reasonably long gravitational wave inspiral times (tens of Myr). First, we generate a large ensemble (between 1 and 10 million) of restricted three-body integrations to quantify the statistical properties of tidal disruptions by circular SMBH binaries of initially-unbound stars. Compared to the reference case of a disruption by a single SMBH, the binary potential induces significant variance into the specific energy and angular momentum of the star at the point of disruption. Second, we use Newtonian numerical hydrodynamics to study the detailed evolution of the fallback debris from 120 disruptions randomly selected from the three-body ensemble (excluding only the most deeply penetrating encounters). We find that the overall morphology of the debris is greatly altered by the presence of the second black hole, and the accretion rate histories display a wide range of behaviors, including order of magnitude dips and excesses relative to control simulations that include only one black hole. Complex evolution persists, in some cases, for many orbital periods of the binary. We find evidence for power in the accretion curves on timescales related to the binary orbital period, though there is no exact periodicity. We discuss our results in the context of future wide-field surveys, and comment on the prospects of identifying and characterizing the subset of events occurring in nuclei with binary SMBHs.