2004ApJ...607...84D

We observed four active galactic nuclei (AGNs) (the type 1 Seyfert systems 3C 249.1, NGC 6814, and Mrk 205, and the BL Lac object 3C 371) using the High Speed Photometer on the Hubble Space Telescope (HST) to search for short-timescale microvariability in the UV. Continuous observations of ∼3000 s duration were obtained for each system on several consecutive HST orbits using a 1 s sample time in a 1400-3000 Å bandpass. No photometric variability greater than 0.3% (0.003 mag) was detected in any AGN on timescales shorter than 1500 s. The distribution of photon arrival times observed from each source was consistent with Poisson statistics. Because of HST optical problems, the limit on photometric variability at longer timescales is less precise. These results restrict the masses of black holes as the central engines of AGNs and the diskoseismic oscillations of any accretion disk around such a black hole.