B. Pirenne

Gravitational lensing statistics based on a large sample of highly luminous quasars

Ve present here some recent results on gravitational lensing statistics as applied to a sample of 470 highly luminous quasars. These objecb were clirectly imaged, either from the ground (ESO, CFH) under optimal seeing co.rditio.rr, or using the Hubble Space Telescope. u/e have derived values for the effectiveness pruri.t. F of galaxies, modeled by mearn o[ singular isothemral spheres, to produce macro-lensed imagss of distant quasars, and upper lirnits on the densiry parameter C); ol compact objects with masses - 1010 1012 Mo. Adopting Ho = 50 krn/sec/Mpc, C)o = 1 and  : 0, we find that rt the 99.70/o confidence level, 0.005 < F < 0.527 and that Cl1< 0.02. Comparing the efIiciencies of ground-based and space instruments used to search for gravitational lens systems u-o! highly luninous quasars, we conclude that for the near frrture, ground-based direct imaging ciraracterized by igood dynamicil range srill consrirutes the best observational strategy. Replacement 9f the WTpC I camera UV VnpC II will of course improve rnuch the efficiency of HST to constrain Cl1 This is particularly true in rhe rrass range 108 - 1010 Mo. A more detailed account of this work will be reported soon in the Astronor.nical Joumal.

Ground-Based and HST Direct Imaging of HLQS

In the context of our studies on gravitational lensing effects among Highly Luminous Quasars (HLQs), we are presently compiling at STScI an archive of direct CCD frames for more than 1000 bright quasars observed with HST and ground-based telescopes. This archive will soon become publicly accessible through the Internet. On the basis of these observations, we are pursuing in a systematic way the analysis (subtraction of numerical PSFs and/or deconvolution) of the HLQ images in order to detect multiple QSO images and/or nearby foreground galaxies at very small angular separations. Residual images corresponding to several new possible multiply imaged HLQs are presented here. From the observed number and image configuration of gravitational lens candidates identified in this large sample of HLQs, it is possible to infer realistic values for parameters characterizing the galaxy deflectors, the number counts of quasars, etc. (cf. Claeskens et al. 1995ab), and also to set interesting constraints on the cosmological density of compact objects in the mass range 1O10 – 1012 M ⊙.

A search for gravitational lenses using sky survey plate scans

We used scans of sky survey plates available at STScl to look for the presence of multiple-image QSOs which could be due to gravitational lensing. We extracted about 1500 fields, each having 5 arcmin in size, around QSOs brighter than m v = 18. No lenses have been found so far. We found several cases of galaxies close in projection (within a few arcsec) from QSOs.

Optical observations of gravitational lenses

We report here on some of the major developments achieved on optical observations of gravitational lenses since the last conference devoted to “Gravitational Lensing” (Toulouse, September 1989). After describing the main surveys and photometric monitorings which are being carried out in the form of systematic searches for gravitational lensing effects, we review the optical observations published during the last two years for selected individual cases (essentially multiply imaged quasars and micro-lensed candidates. Preliminary results of lensing statistics applied to a sample of more than 200 highly luminous quasars are then presented both for the effectiveness parameter F of galaxies and for the density parameter ΩL of compact lenses to produce macro-lensed images of distant quasars.