ESO & NOT photometric monitoring of the Cloverleaf quasar ^*,**

¹ Institute of Mathematical and Physical Sciences, University of Tromsø, N-9037 Tromsø, Norway
² Institut d'Astrophysique, Université de Liège, Belgium
³ Stockholm Observatory, S-13336 Saltsjöbaden, Sweden
⁴ Hamburger Sternwarte, Gojenbergsweg 112, D-21029, Germany
⁵ Institute of Theoretical Astrophysics, University of Oslo, Box 1029 Blindern, N-0315 Oslo, Norway
⁶ Kapteyn Astronomical Institute, Rijksuniversiteit, Landleven 12, Postbus 800, 9700 AV Groningen, The Netherlands
⁷ Astron. Obs. Box 515, S-751 20 Uppsala, Sweden
⁸ Institute for Astronomy, 2680 Woodlawn Drive, Honolulu HI 96822, U.S.A.
⁹ Helsinki University Observatory, Tahtitorninmaki, SF-00014 Helsinki, Finland
¹⁰ NORDITA, Blegdamsvej 17, DK-2100 Copenhagen, Denmark
¹¹ Copenhagen University Observatory, Juliane Maries Vej 30, DK-2100 Copenhagen, Denmark
¹² Institute of Physics and Astronomy, University of Aarhus, DK-8000 Aarhus C, Denmark

Received: 18 November 1996
Accepted: 18 March 1997

Abstract

The Cloverleaf quasar, H1413+117, has been photometrically monitored at ESO (La Silla, Chile) and with the NOT (La Palma, Spain) during the period 1987–1994. All good quality CCD frames have been successfully analysed using two independent methods (i.e. an automatic image decomposition technique and an interactive CLEAN algorithm). The photometric results from the two methods are found to be very similar, and they show that the four lensed QSO images vary significantly in brightness (by up to 0.45 mag), nearly in parallel. The lightcurve of the D component presents some slight departures from the general trend which are very likely caused by micro-lensing effects. Upper limits, at the 99% confidence level, of 150 days on the absolute value for the time delays between the photometric lightcurves of this quadruply imaged variable QSO, are derived. This is unfortunately too large to constrain the lens model but there is little doubt that a better sampling of the lightcurves should allow to accurately derive these time delays. Pending a direct detection of the lensing galaxy (position and redshift), this system thus constitutes another good candidate for a direct and independent determination of the Hubble parameter.