# Misaligned snowplough effect and the electromagnetic counterpart to black hole binary mergers

## TL;DR

This study investigates how misaligned gas discs around supermassive black hole binaries influence accretion rates and potential electromagnetic signals during mergers, revealing that small inclination angles can cause significant luminosity increases.

## Contribution

It extends previous models by analyzing the impact of disc-binary misalignment on accretion dynamics using 3D SPH simulations, highlighting the persistent snowplough effect at large angles.

## Key findings

- Small inclination discs (<10°) can produce super-Eddington luminosities.
- Discs inclined at 20-30° show moderate accretion rate increases.
- No accretion peak observed for discs at 180° inclination.

## Abstract

We estimate the accretion rates produced when a circumprimary gas disc is pushed into the primary supermassive black hole (SMBH) by the tidal force of the decaying secondary during a SMBH merger. Using the 3D Smoothed Particle Hydrodynamics (SPH) code {\sc phantom}, we extend previous investigations of co-planar discs to the case where the disc and binary orbital planes are misaligned. We consider a geometrically thin disc with inclination angles varying from 1 to 180 degrees and a binary with mass ratio $q$=$10^{-3}$. We find that discs with small inclination angles (< 10 degrees) produce an increase in luminosity exceeding the Eddington rate. By contrast, discs with inclinations between 20 and 30 degrees show a less pronounced rise in the accretion rate, whilst discs inclined by 180 degrees show no peak in the mass accretion rate. While previous analytic work predicted that the effective tidal torque drops with increasing inclination angle, we show that the misaligned snowplough effect remains important even for angles larger than the disc aspect ratio. The rise in the accretion rate produced by discs inclined at small angles to the binary orbit can produce an electromagnetic counterpart to the gravitational wave signal emitted from final stages of the binary orbital decay.