The secular tidal disruption of stars by low-mass Super Massive Black Holes secondaries in galactic nuclei

TL;DR

This paper investigates how stars orbiting an intermediate-mass black hole secondary are perturbed by a primary supermassive black hole, leading to tidal disruption events that can reveal SMBH binaries in galactic nuclei.

Contribution

It introduces a secular mechanism for TDE production in SMBH binaries, highlighting the impact of Kozai-Lidov oscillations and the conditions under which TDEs are observable.

Findings

TDEs occur mainly for SMBHs below 1.15 x 10^8 solar masses

TDE rate estimated at 10^-4 to 10^-3 per year

Stars can be ejected as hypervelocity stars

Abstract

Stars passing too close to a super massive black hole (SMBH) can produce tidal disruption events (TDEs). Since the resulting stellar debris can produce an electromagnetic flare, TDEs are believed to probe the presence of single SMBHs in galactic nuclei, which otherwise remain dark. In this paper, we show how stars orbiting an IMBH secondary are perturbed by an SMBH primary. We find that the evolution of the stellar orbits are severely affected by the primary SMBH due to secular effects and stars orbiting with high inclinations with respect to the SMBH-IMBH orbital plane end their lives as TDEs due to Kozai-Lidov oscillations, hence illuminating the secondary SMBH/IMBH. Above a critical SMBH mass of $\approx 1.15 \times 10^8$ M$_{\odot}$, no TDE can occur for typical stars in an old stellar population since the Schwarzschild radius exceeds the tidal disruption radius. Consequently, any TDEs due to such massive SMBHs will remain dark. It follows that no TDEs should be observed in galaxies with bulges more massive than $\approx 4.15\times 10^{10}$ M$_{\odot}$, unless a lower-mass secondary SMBH or IMBH is also present. The secular mechanism for producing TDEs considered here therefore offers a useful probe of SMBH-SMBH/IMBH binarity in the most massive galaxies. We further show that the TDE rate can be $\approx 10^{-4}-10^{-3}$ yr$^{-1}$, and that most TDEs occur on $\approx 0.5$ Myr. Finally, we show that stars may be ejected with velocities up to thousands of km s$^{-1}$, which could contribute to the observed population of Galactic hypervelocity stars.