A month in the life of NGC 4151: Velocity-delay maps of the broad-line region

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-Angstrom continuum undertook a bumpy exponential decline by a factor similar to 3 over 25 d, and then made a dramatic recovery in similar to 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 Angstrom. Cross-correlations reveal emission-line variations lagging behind the continuum variations. The red wing, core and blue wing of the CIV emission line lag by 1.8, 4.4 and 3.2d respectively, Maximum entropy fits to continuum light curves and line profile variations yield more detailed velocity-delay maps of the CIV 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 CIV is asymmetric, with a strong red wing at tau less than or similar to 2 d, and a faint blue wing at tau less than or similar to 10 d. This suggests inflow in the high-velocity inner regions, but might also be interpreted as outflow combined with an inward anisotropy, The CIV velocity range decreases with delay in a way that suggests virial motions around a mass of similar to 10(7) M..