THE CHROMATIC EFFECT IN GRAVITATIONAL MICROLENSING OF STARS

A decrease in the amplification factor of a gravitational lens with in creasing angular distance from its optical axis and the wavelength dep endence of stellarlimb darkening give rise to the chromatic effect in observations of microlensing of stars. This effect usually manifests i tself in reddening of starlight. The color variations in the standard broadband UBV system have been estimated for various microlensing cond itions. The chromatic effect is only marginal, and actual variations i n color indices do not exceed 0.(m)015. However, it can be enhanced by a special choice of those spectral bands of the detector that provide the maximum difference in limb darkening. An important feature of the chromatic effect is that it depends only on the ratio of the angle be tween the lens and the true direction toward the star to the star's an gular radius. A comparison of the observed color indices and flux ampl ification factors with the calculated values makes it possible to esti mate the angular size of the Einstein ring of a gravitational lens.