A parallax effect due to gravitational micro-lensing

Abstract

The astrophysical importance of gravitational micro-lensing due to stars in the deflecting galaxy is now well-known^1–7. Each macro-image (caused by the corresponding smoothed-out galaxy) can be split up into several micro-images with typical angular distances measured in micro-arcseconds. Here we discuss a parallax effect due to gravitational micro-lensing (star disturbances). We show that such an effect would most favourably be observed during high amplification events when a compact source (quasar) crosses a critical curve. During such an event even a relatively small displacement (≳0.lAU) of the observer may lead to measurable differences in the image brightness. We discuss this parallax effect with special emphasis on the determination of (1) the size of the quasar and (2) the relative transversal velocities involved. It should be possible to observe such a parallax-effect with a relatively small telescope (10–20 cm) included in an interplanetary mission. In extreme cases, annual oscillations in the light curves of lensed quasars can be observed from the Earth, but in most cases these oscillations can be neglected.