Stellar Stripping and Disruption in Disks around Supermassive Black Hole Binaries: Repeating nuclear transients prior to LISA events

TL;DR

This paper proposes that stars trapped near supermassive black hole binaries can be tidally stripped, producing repeating X-ray flares that serve as potential electromagnetic signatures of SMBH mergers detectable by LISA.

Contribution

It introduces a new model where stellar stripping in circumbinary disks leads to observable repeating transients prior to SMBH mergers.

Findings

Stripped stars produce Eddington-level X-ray flares lasting decades.

Flares repeat on hours-to-days timescales, matching some observed phenomena.

The process is most effective for SMBHBs with masses between 10^4 and 10^6 solar masses.

Abstract

If supermassive black hole binaries (SMBHBs) are driven together by gas disks in galactic nuclei, then a surrounding nuclear star cluster or in-situ star-formation should deliver stars to the disk plane. Migration through the circumbinary disk will quickly bring stars to the edge of a low-density cavity cleared by the binary, where the stellar orbit becomes trapped and locked with the binary decay. Here we explore the scenario where the trapped stellar orbit decays with the binary until the binary tidally strips the star in a runaway process. For Sun-like stars, this occurs preferentially for $10^4-10^6 M_{\odot}$ SMBHBs, as the SMBHB enters the LISA band. We estimate that the runaway stripping process will generate Eddington-level X-ray flares repeating on hours-to-days timescales and lasting for decades. The flaring timescales and energetics of these circumbinary-disk tidal-disruption events (CBD-TDEs) match well with the recently discovered Quasi-Periodic Eruptions. However, the inferred rates of the two phenomena are in tension, unless low-mass SMBHB mergers are more common than expected. For less-dense stars, stripping begins earlier in the SMBHB inspiral, has longer repetition times, lasts longer, is dimmer, and can occur for more massive SMBHBs. Whether CBD-TDEs are a known or a yet-undiscovered class of repeating nuclear transients, they could provide a new probe of the elusive SMBH mergers in low mass / dwarf galaxies, which lie in the sweet-spot of the LISA sensitivity.