Variability of optically selected high redshift quasars

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Abstract
We compare the 1981 and 1950 magnitudes of 64 optically selected quasars from the University of Michigan survey. Our sample includes all possible candidates in a 84 square degree field, and is probably complete for strong line m ≲ 18 mag quasars. The method used is a combination of measuring image diameters on the POSS prints and Iris photometry, and the error associated with it is about 0.2 mag. 39 per cent of the quasars in our sample have varied by more than 0.45 mag between 1950 and 1981. The incidence of variability is almost identical in two redshift subgroups with z > 1.7 and z < 1.7, and there is no variability–redshift dependence. Our sample doubles the number of known z ≽ 2 variables, and includes the highest red-shift (z = 2.92) variable found yet. Optically selected quasars of all redshift seem to be similar to the radio selected ones in their light variation, and earlier claims to the contrary are probably due to selection effects.

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