Orbital motion effects in astrometric microlensing
  We investigate lens orbital motion in astrometric microlensing and its
detectability. In microlensing events, the light centroid shift in the source
trajectory (the astrometric trajectory) falls off much more slowly than the
light amplification as the source distance from the lens position increases. As
a result, perturbations developed with time such as lens orbital motion can
make considerable deviations in astrometric trajectories. The rotation of the
source trajectory due to lens orbital motion produces a more detectable
astrometric deviation because the astrometric cross-section is much larger than
the photometric one. Among binary microlensing events with detectable
astrometric trajectories, those with stellar-mass black holes have most likely
detectable astrometric signatures of orbital motion. Detecting lens orbital
motion in their astrometric trajectories helps to discover further secondary
components around the primary even without any photometric binarity signature
as well as resolve close/wide degeneracy. For these binary microlensing events,
we evaluate the efficiency of detecting orbital motion in astrometric
trajectories and photometric light curves by performing Monte Carlo simulation.
We conclude that astrometric efficiency is 87.3 per cent whereas the
photometric efficiency is 48.2 per cent.
