Detailed observations of NGC 4151 with the IUE - III. Variability of the strong emission lines from 1978 February to 1980 May.

Observations of the variability of the three strong ultraviolet emission lines in the Seyfert galaxy NGC 4151 (C IV, C III] and Mg II) are used to study the structure of the broad line region and the nuclear energy source of this active galaxy. Data on the line profile variations further permit the kinematic mapping of the broad line region through the response of the line profiles to the sudden continuum increase during 1979 May. From the widths and variability time-scales of the three emission lines the authors deduce values of the quantity v²r, which is proportional to a mass. The three independent estimates converge on 0.5 - 1×10⁸ M_sun;. It is shown that the behaviour is compatible with the black hole/accretion disc model of Lynden-Bell (1969).