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We present a flux variability study of simultaneous RXTE and EUVE observations of the highly variable Seyfert galaxy NGC 4051. We find a strong correlation between variability in the EUV and medium-energy X-ray bands, indicating that both are sampling the same power-law continuum. The lag between the two bands is less than 20 ks and, depending on model assumptions, may be <1 ks. We examine the consequences of such a small lag in the context of simple Comptonization models for the production of the power-law continuum. A lag of <1 ks implies that the size of the Comptonizing region is less than 20 Schwarzschild radii for a black hole of mass >10[sup 6] M[sub ⊙]. [ABSTRACT FROM AUTHOR]

Analyzes archival Exosat observations of the bright Seyfert galaxy NGC5548 and finds in the power spectrum a broad peak corresponding to quasi-periodic oscillations (QPO) with period of approximately 500 s. NGC6814; Active galactic nuclei (AGN); Summary of observations and model fits; Power spectra of various observations of NGC5548 arranged in order of source count-rate, together with model fits; Dependence of QPO properties on source intensity; Further details.