Gravitational interactions of stars with supermassive black hole binaries. I. Tidal disruption events

TL;DR

This study investigates how stars are tidally disrupted by binary supermassive black holes, revealing increased disruption rates and intense fallback events, with implications for understanding galactic centers and transient phenomena.

Contribution

It provides the first detailed simulation-based analysis of tidal disruptions by binary SMBHs across a range of mass ratios and separations, highlighting enhanced disruption rates and fallback characteristics.

Findings

Disruption rate is 2-7 times higher than for isolated black holes.

Disruptions from close, equal-mass binaries can produce short, intense fallback events.

Approximately 18-40% of disruptions have rapid rise times and super-Eddington accretion rates.

Abstract

Stars approaching supermassive black holes (SMBHs) in the centers of galaxies can be torn apart by strong tidal forces. We study the physics of tidal disruption by a binary SMBH as a function of the binary mass ratio $q = M_2 / M_1$ and separation $a$, exploring a large set of points in the parameter range $q \in [0.01, 1]$ and $a/r_{t1} \in [10, 1000]$. We simulate encounters in which field stars approach the binary from the loss cone on parabolic, low angular momentum orbits. We present the rate of disruption and the orbital properties of the disrupted stars, and examine the fallback dynamics of the post-disruption debris in the "frozen-in" approximation. We conclude by calculating the time-dependent disruption rate over the lifetime of the binary. Throughout, we use a primary mass $M_1 = 10^6 M_\odot$ as our central example. We find that the tidal disruption rate is a factor of $\sim 2 - 7$ times larger than the rate for an isolated BH, and is independent of $q$ for $q \gtrsim 0.2$. In the "frozen-in" model, disruptions from close, nearly equal mass binaries can produce intense tidal fallbacks: for binaries with $q \gtrsim 0.2$ and $a/r_{t1} \sim 100$, roughly $\sim 18 - 40 \%$ of disruptions will have short rise times ($t_\textrm{rise} \sim 1 - 10$ d) and highly super-Eddington peak return rates ($\dot{M}_\textrm{peak} / \dot{M}_\textrm{Edd} \sim 2 \times 10^2 - 3 \times 10^3$).