# Constraining a black hole companion for M87* through imaging by the Event Horizon Telescope

## TL;DR

This paper explores the potential to detect or rule out binary black hole companions to M87* using long-term imaging with the Event Horizon Telescope, constraining possible companion parameters and the presence of an accretion disk.

## Contribution

It demonstrates that high-precision, long-term imaging can effectively constrain the existence and properties of binary companions to M87* and rules out close companions based on accretion disk stability.

## Key findings

- Long-term monitoring can detect binary companions with specific parameters.
- A binary companion with a separation of about a milliparsec is excluded due to accretion disk disruption.
- Large parameter space for potential companions can be constrained with current observational capabilities.

## Abstract

The Event Horizon Telescope (EHT), a global very long baseline interferometric array observing at a wavelength of 1.3 mm, detected the first image of the M87 supermassive black hole (SMBH). M87 is a giant elliptical galaxy at the center of Virgo cluster, which is expected to have formed through merging of cluster galaxies. Consequently M87* hosted mergers of black holes through dynamical friction and could have one or multiple binary companions with a low mass ratio at large separations. We show that a long-term monitoring of the M87 SMBH image over $\sim$1 year with absolute positional accuracy of 1$\approx\mu as$ could detect such binary companions and exclude a large parameter space in semi major axis ($a_0$) and mass ratio ($q$), which is currently not constrained. Moreover, the presence of the accretion disk around M87* excludes a binary companion with $a_0\approx$ of order a mili parsec, as otherwise the accretion disk would have been tidally disrupted.