Detailed observations of NGC 4151 with the IUE. I. Low dispersion data up to 1979 january.

Low resolution ultraviolet spectroscopic monitoring of NGC 4151, the brightest Type 1 Seyfert galaxy, with the IUE from February, 1978 to January, 1979, is presented. Data acquisition and reduction are described. Observations were made at seven different epochs. Optimum exposure times, when the object is bright, were found to be 25-30 min and 50-60 min in large and small apertures respectively. When the object was faint or when a better signal-to-noise ratio in the continuum was required, a large-aperture exposure of 40-50 min was added. After the wavelength scale was determined for each spectrum, data obtained on the same day were averaged and regions affected by saturation, fiducials, geocoronal Lyman alpha emission and particle events were excluded from the mean spectra. Fifteen absorption features (most of which arise in the nucleus of NGC 4151) are identified and their equivalent widths measured. Velocity variations in the absorption region are indicated by changing V/R ratios in the overall C IV feature. The Si IV lines are unsaturated and must therefore be broad. The strong emission lines include those seen in quasars. The intensities of the high-ionization emission lines are variable, as are the linewidths of C IV.