IUE observations of X-ray selected Seyfert galaxies.

Abstract

Results are presented for 1150-3150 A spectrophotometry of six type I Seyfert galaxies - NGC 5548, Markarian 79, Markarian 279, Markarian 506, Markarian 376, and 3C 390.3. It is found that the continuous flux distribution can be represented by a single power law in four of these objects, with spectral indices between + 0.2 and - 3.1, while NGC 5548 exhibits a variable hard UV excess. Indirect evidence is found for continuum variability in NGC 5548 over three days, while in 3C 390.3 the emission line strength can vary in six days. The emission line profiles in NGC 5548, Mkn 79, and 3C 390.3 are determined to vary over several months. A weak correlation is noted between the continuum luminosity and the spectral index, which indicates a common synchrotron origin for the power-law component in the UV continuum. However, no evidence of any correlation between UV and X-ray properties is found. This Seyfert sample does not conform to the Baldwin relationship between luminosity and C IV equivalent width, alghough the emission line properties do seem to correlate with the spectral index, which suggests that the UV power law is responsible for photoionization and heating of the broad-line region in the far UV.