Active galaxy-X-ray sources are well known to be variable on timescales of days to years (Ricketts, et al. 1977, Mushotzky, et al. 1979, Lawrence 1979). Here we present some new data from the Einstein Observatory which shows that these sources also vary in less than one day, on timescales of hours. Large luminosity changes in such short times promise to allow the investigation of the physics of such sources in several new ways. We shall give some examples of how this can be begun. This is only a preliminary report of this work. A full account will be given elsewhere.
The Einstein Observatory (Giacconi, et al. 1979) is capable of exploring this regime of variability because its imaging capability gives it two unique advantages. Firstly, the background rate in one beam area is negligible so that intensity determinations are limited only by Poisson counting statistics. Secondly, the background counts in the remainder of the field can be integrated to give a simultaneous monitor with the same instrument of cosmic ray and background X-ray events. Thus, any peculiarities in detector behaviour, telemetry or software can be traced and separated from real source variations. This is a significant advantage and gives us a great deal more confidence in our results. Many sources do not show variability. For example, in our data, Cen A is constant to ˜ 2% over 1 day.