The Optical Gravitational Lensing Experiment Monitoring of Qso 2237+0305 *

We present results from 2 years of monitoring of Huchra's lens (QSO 2237+0305) with the 1.3 m Warsaw telescope on Las Campanas, Chile. Photometry in the V band was done using a newly developed method for image subtraction. Reliable subtraction without Fourier division removes all complexities associated with the presence of a bright lensing galaxy. With positions of lensed images adopted from HST measurements it is relatively easy to fit the variable part of the flux in this system, as opposed to modeling of the underlying galaxy. For the first time we observed smooth light variation over a period of a few months, which can be naturally attributed to microlensing. We also describe automated software capable of real time analysis of the images of QSO 2237+0305. It is expected that starting from the next observing season in 1999 an alert system will be implemented for high amplification events (HAE) in this object. Time sampling and photometric accuracy achieved should be sufficient for early detection of caustic crossings. Subject headings: gravitational lensing: observations – photometry – objects: individual: QSO 2237+0305 1. INTRODUCTION Optical Gravitational Lensing Experiment (OGLE) is a long term project focusing on detection and monitoring of microlensing events. During second phase of the experiment data are collected with the dedicated 1.3 m Warsaw telescope at the Las Campanas Observatory, Chile (Udal-ski, Kubiak and Szyma´nski 1997). Due to the intrinsi-cally small probability of the microlensing phenomenon, the main targets are the densest stellar fields, namely the Galactic Bulge and Magellanic Clouds (e.g. Paczy´nski 1996). Some telescope time, however, was also allocated to observations of other objects in which gravitational lensing is present, e.g. distant quasars lensed by foreground galaxies. Given good seeing at the Las Campanas Observatory and the amount of available telescope time, the OGLE-II survey can provide good time coverage of the variability in a few selected multiply imaged lenses. Currently two such systems are monitored by OGLE: QSO 2237+0305 and HE 1104-1805. Objects of this kind are particularly interesting for cosmology through determination of time delays. QSO 2237+0305 is a unique system and consists of a high-redshift quasar at z = 1.695 quadruply lensed by a relatively nearby galaxy at z = 0.039 (Huchra et al. 1985). Because light rays of the four images pass through the bulge of the foreground galaxy, the optical depth to lensing on individual stars could be as large as 0.4–0.9 depending …