A survey for medium-redshift (2.2<z<3.0) optically variable QSOs

P. Veron, M.R.S. Hawkins

We describe a survey for variable QSOs carried out for 15 years with the UK Schnidt telescope, using IIIaJ plates exposed behind a Schott GG395 filter. Objects brighter than B=21.0 on a reference plate and displaying a peak to peak amplitude of variability larger than DB=0.35 mag are selected. Plates in U,V,R and I were also obtained. We show that about 20% of all QSOs are missed becaused of overlapping images, but that about 95% of UVX QSOs (z<2.2) are variable. A smaller fraction of non UVX QSOs are variable; it is demonstrated that this is not a redshift effect, but rather a luminosity effect. We show than about 20% of the low luminosity (MB>-25.5) QSOs have an amplitude DB>1.0 mag, while no high luminosity QSOs have such a large amplitude. We give a list of 107 QSOs with z>2.20, 71 of them constituting an homogeneous sample of QSOs brighter than B=21.0, in the redshift range 2.2