A Search for Ultrarapid Microvariability in the Seyfert Galaxy NGC 7469 with the Hubble Space Telescope

The Faint Object Spectrograph on the Hubble Space Telescope (HST ) was used to obtain spectro- photometric observations of the Seyfert 1 galaxy NGC 7469. The spectra were obtained with a 90 s time resolution in order to resolve the fastest expected variability and to search for rapid characteristic time- scales (e.g., quasi-periodic oscillations or breaks in the power spectrum). The sensitivity of the obser- vations is such that a D1% spectral variation, or a D0.1% photometric variation, are detectable. The observations were taken through a square aperture and do not su†er contamination from the host 0A.86 galaxy and nearby bright starburst ring. During the 11.5 hr spanned by the observations, a 4% rise in the 1315 continuum was seen, with a corresponding smaller rise at longer wavelengths. Variations on Ae timescales of hours or less are of very low amplitude, at or below the Poisson noise levelEthe rms amplitude of the Nuctuations is D1% in the UV and 0.2% in the G160L zero-order photometric bandpass. No periodicities or characteristic timescales are present, nor do we see any evidence for delays between variations at di†erent wavelengths. The HST observations were made during an intense moni- toring campaign using IUE, RXT E, and ground-based observatories, and a comparison with the results of those observations is presented. The 4% rise seen in the HST data agrees well with the IUE light curve. The very small amplitude, rapid Nuctuations are roughly consistent with a power-law extrapo- lation of the power spectrum of the much longer timescale IUE observations. We were unable to deter- mine the amount of correlated variability between the HST and RXT E light curves because the amplitude of the Nuctuations are extremely small at these very rapid timescales. Subject headings: accretion, accretion disks E galaxies: individual (NGC 7469) E galaxies: photometry E galaxies: Seyfert E galaxies: starburst E ultraviolet: galaxies

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