Publikace ÚTF

Extended-source effect and chromaticity in two-point-mass microlensing

Pejcha O.; Heyrovský D.

We explore the sensitivity of two-point-mass gravitational microlensing to the extended nature of the source star,
as well as the related sensitivity to its limb darkening. We demonstrate that the sensitive region, usually considered
to be limited to a source-diameter-wide band along the caustic, is strongly expanded near cusps, most prominently
along their outer axis. In the case of multicomponent caustics, facing cusps may form a region with a non-negligible
extended-source effect spanning the gap between them. We demonstrate that for smaller sources the size of the
sensitive region extending from a cusp measured in units of source radii increases, scaling as the inverse cube
root of the radius. We study the extent of different sensitivity contours and show that for a microlensed Galactic
bulge giant the probability of encountering at least a 1% extended-source effect is higher than the probability of
caustic crossing by 40–60% when averaged over a typical range of lens-component separations, with the actual
value depending on the mass ratio of the components. We derive analytical expressions for the extended-source
effect and chromaticity for a source positioned off the caustic. These formulae are more generally applicable to any
gravitational lens with a sufficiently small source. Using exactly computed amplifications we test the often used
linear-fold caustic approximation and show that it may lead to errors on the level of a few percent even in near-ideal
caustic-crossing events. Finally, we discuss several interesting cases of observed binary and planetary microlensing
events and point out the importance of our results for the measurement of stellar limb darkening from microlensing
light curves.
journal:The Astrophysical Journal
grant:GACR 205/07/0824; Fyzikální studium objektů a procesů ve sluneční soustavě a v astro-fyzikálních systémech, výzkumný záměr MSM0021620860, 2007-2013
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