Effect of the sourse ellipticity on light curves in extragalactic gravitational lens systems

Abstract

In extragalactic gravitational lens system (GLS) several macro-images of a remote source (quasar) are observed. Gravitational microlensing leads to uncorrelated brightness variations of the macro-images. These variations can be separated from proper brightness changes of the source, which are repeated in each image with some relative delay. In most papers the gravitational microlensing effects in extragalactic GLS typically deal with centrally symmetric sources, though the real form of the source can be more complicated. In this paper we consider statistical signals of the source ellipticity in light curves of microlensed objects, which arise as a result of observations of many high amplification events. As a first step, we consider a qualitative version of the problem, when an elliptic source with Gaussian brightness distribution is crossed by a number of simple (fold) caustics with different orientations. Also we assume that the caustic network is isotropic, that is all orientations of the caustics and directions of their velocities are equally possible. We use the linear fold caustic approximation for the amplification of a small point source, when two critical images appear/disappear after the caustic crossing. For a rough estimate, the strength and velocity of the caustic are fixed by certain characteristic values. We estimate the averaged light curve after many caustic crossing events and compare maxima of brightness on the light curves for different eccentricities of the source. The ellipticity signal in the light curves is noticeable for a considerable oblateness of the source. Though we expect that the effect will be smeared out after taking into account the distribution of the caustic velocities and caustic strengths. Possible complications of the problem are indicated due to interplay of effects due to velocity distribution of stars in the lensing galaxy and bulk motion of the lensing galaxy as a whole.