Line shifts in accretion disks - the case of Fe K$\alpha$
  Here we present a short overview and main results of our investigations of
several effects which can induce shifts in the broad Fe K$\alpha$ line emitted
from relativistic accretion disks around single and binary supermassive black
holes. We used numerical simulations based on ray-tracing method in the Kerr
metric to study the role of classical Doppler shift, special relativistic
transverse Doppler shift and Doppler beaming, general relativistic
gravitational redshift, and perturbations of the disk emissivity in the
formation of the observed Fe K$\alpha$ line profiles. Besides, we also
investigated whether the observed line profiles from the binary systems of
supermassive black holes could be affected by the Doppler shifts due to
dynamics of such systems. The presented results demonstrate that all these
effects could have a significant influence on the observed profiles of the
broad Fe K$\alpha$ line emitted from relativistic accretion disks around single
and binary supermassive black holes.
