A Search for Ultra-Rapid Microvariability in the Seyfert
Galaxy NGC 7469 with HST
Welsh, W.F., Peterson, B.M., Koratkar, A.P., and Korista, K.T. 1998, ApJ, in press
The Faint Object Spectrograph on the Hubble Space Telescope was used to obtain spectrophotometric observations of the Seyfert 1 galaxy NGC 7469. The spectra were obtained with a 90s time resolution in order to resolve the fastest expected variability and to search for rapid characteristic timescales (e.g., quasi-periodic oscillations or breaks in the power spectrum). The sensitivity of the observations is such that a ~1% spectral variation or a `0.1% photometric variation are detectable. The observations were taken through a 0.86" square aperture and do not suffer contamination from the host galaxy and nearby bright starburst ring.
During the 11.5 hrs spanned by the observations, a 4% rise in the 1315 Å continuum was seen, with a corresponding smaller rise at longer wavelengths. Variations on timescales of hours or less are of very low amplitude, at or below the Poisson noise level - the rms amplitude of the fluctuations is ~1% 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 different wavelengths.
The HST observations were made during an intense monitoring campaign using the IUE, RXTE, 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 fluctuations are roughly consistent with a power-law extrapolation of the power spectrum of the much longer timescale IUE observations. We were unable to determine the amount of correlated variability between the HST and RXTE light curves because the amplitude of the fluctuations are extremely small at these very rapid time scales.