X-ray variability of quasars

Fifty-one previously known quasars observed with the Einstein Observatory have been analyzed for intensity variations on time scales ranging from a few hundred seconds to 18 months. While none of them has shown variability on a very short time scale, four of the quasars have been seen to vary on a time scale of about 1 day. As about 30 objects could be analyzed on this time scale, it is concluded that 1 day appears to be a relatively common time scale for variability. On longer time scales (6-18 months), nine out of 12 quasars which could be analyzed show variability. However, in contrast to results previously reported for BL Lac objects, no variation in quasar luminosity larger than a factor of 2 has been observed.