THE ULTRAVIOLET-SPECTRUM OF NGC-4151 FROM 1978 TO 1990 - GENERAL-CHARACTERISTICS AND EVOLUTION

From the examination of all the low-dispersion IUE spectra of NGC 4151 taken since the launch of IUE in 1978, including the latest data taken in 1990, we follow the evolution of this active nucleus on time scales ranging from days to 10 years. We concentrate our analysis on the campaigns of observations during which the nucleus was not continuously in a minimum state. Regarding the continuum, we find that the short variability time scales (a few days) in the optical and ultraviolet ranges rule out the possibility that the continuum intensity variations in these wavelength intervals are due to variations in the accretion rate of the possibly present accretion disk. Possible origins could be instabilities in the inner radiation-dominated part of the disk or irradiation of the disk by the variable X-ray flux. We study in detail the long- and short-term temporal behavior of the wings of the C IV and Mg II lines (\ upsilon \ > 3870 km s-1) and the variations of the C III] line. The C IV line maintains a high degree of symmetry during the decrease and increase branches of the deep minimum of 1988 November 29-1989 January 30, but, in general, there are large changes in the C IV wings intensity ratio from campaign to campaign, and different responses of the blue and red wings to the continuum variations are frequently observed during a campaign. Our preferred interpretation of these effects is the redistribution or rearrangement of the fast-moving gas on a time scale of one to a few years, probably in the form of the arrival of a new gas cloud in the inner part of the broad-line region. We speculate that the gas clouds are being detached from stars or from the accretion disk itself.