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