Hills and holes in the microlensing light curve due to plasma
  environment around gravitational lens
  In this paper, we investigate the influence of the plasma surrounding the
gravitational lens on the effect of microlensing. In presence of plasma around
the lens, the deflection angle is determined by both the gravitational field of
the lens and the chromatic refraction in the inhomogeneous plasma. We calculate
microlensing light curves numerically for point-mass lens surrounded by
power-law density distribution of plasma. A variety of possible curves is
revealed, depending on the plasma density and frequency of observations. In the
case of significant influence of plasma, the shape of microlensing light curve
is strongly deformed in comparison with vacuum case. If the refractive
deflection is large enough to compensate or to overcome the gravitational
deflection, microlensing images can completely disappear for the observer. In
this case, the remarkable effect occurs: formation of a 'hole' instead of a
'hill' in the center of microlensing light curve. Observational prospects of
'hill-hole' effect in different microlensing scenarios are discussed.
