Supermassive black hole spin-flip during the inspiral

TL;DR

This paper investigates how supermassive black hole spins can flip during inspiral due to spin-orbit interactions, explaining observed X-shaped radio galaxies and providing new formulas for final spin states.

Contribution

It introduces a statistical analysis of supermassive black hole mergers, revealing conditions under which spin-flips occur and deriving formulas for final spins during inspiral.

Findings

Spin-flip occurs in 40% of mergers with mass ratios (1/30,1/3).

No spin-flip in 40% of mergers with mass ratios (1/30,1/1000).

Spin-flip may occur during plunge in 20% of mergers with mass ratios (1/3,1).

Abstract

During post-Newtonian evolution of a compact binary, a mass ratio different from 1 provides a second small parameter, which can lead to unexpected results. We present a statistics of supermassive black hole candidates, which enables us first to derive their mass distribution, then to establish a logarithmically even probability of the mass ratios at their encounter. In the mass ratio range (1/30,1/3) of supermassive black hole mergers representing 40% of all possible cases, the combined effect of spin-orbit precession and gravitational radiation leads to a spin-flip of the dominant spin during the inspiral phase of the merger. This provides a mechanism for explaining a large set of observations on X-shaped radio galaxies. In another 40%, with mass ratios (1/30,1/1000) a spin-flip never occurs, while in the remaining 20% of mergers with mass ratios (1/3,1) it may occur during the plunge. We analyze the magnitude of the spin-flip angle occurring during the inspiral as function of the mass ratio and original relative orientation of the spin and orbital angular momentum. We also derive a formula for the final spin at the end of the inspiral in this mass ratio range.