| Abstract
| - We analyse continuum and emission-line variability in NGC 4151 based on IUE spectra taken with nearly complete 1-d sampling over a 35-d interval during 1991 November-December. The 1320-Å continuum undertook a bumpy exponential decline by a factor ∼3 over 25 d, and then made a dramatic recovery in ∼2 d near the end of the campaign. The same pattern of variations occurred in all continuum bands with an amplitude that decreases with wavelength out to 3000 Å. Cross-correlations reveal emission-line variations lagging behind the continuum variations. The red wing, core and blue wing of the C iv emission line lag by 1.8, 4.4 and 3.2 d respectively. Maximum entropy fits to continuum light curves and line profile variations yield more detailed velocity-delay maps of the C iv and He ii responses at delays from 0 to 20 d. The line maps generally decline from a peak at 0 delay, implying high-velocity gas inside 1 light-day. The velocity-delay structure of C iv is asymmetric, with a strong red wing at τ ≲ 2 d, and a faint blue wing at τ ≲ 10 d. This suggests inflow in the high-velocity inner regions, but might also be interpreted as outflow combined with an inward anisotropy. The C iv velocity range decreases with delay in a way that suggests virial motions around a mass of ∼107 M⊙.
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