Asymptotic formulas for the magnification of a gravitational lens system near a fold caustic
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An approximate formula for the magnification of a point source near a fold caustic obtained in the first linear caustic approximation is widely used in the theory of gravitational lens systems. Here, this formula is refined to include the post-linear terms that have been found both for a point source and for an extended Gaussian source in the absence of continuous matter on the line of sight. The formulas are reduced to a form containing three additional parameters; the derivation of nontrivial corrections requires including the expansion terms in the lens equation up to the fourth order. The modified formula for an extended source is used to analyze strong microlensing events in the gravitational lens system Q2237+0305 (the Einstein Cross). For such an event on the light curve of image C (1999, OGLE data), the corrections found are statistically significant.
Key wordsgravitational lenses microlensing Q2237+0305 Einstein Cross
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