MULTIWAVELENGTH OBSERVATIONS OF SHORT-TIMESCALE VARIABILITY IN NGC-4151 .1. ULTRAVIOLET OBSERVATIONS

We present the results of an intensive ultraviolet monitoring campaign on the Seyfert 1 galaxy NGC 4151, as part of an effort to study its s hort-timescale variability over a broad range in wavelength. The nucle us of NGC 4151 was observed continuously with the International Ultrav iolet Explorer for 9.3 days, yielding a pair of LWP and SWP spectra ev ery similar to 70 minutes, and during 4 hr periods for 4 days prior to and 5 days after the continuous-monitoring period. The sampling frequ ency of the observations is an order of magnitude higher than that of any previous UV monitoring campaign on a Seyfert galaxy. The continuum fluxes in bands from 1275 to 2688 Angstrom went through four signific ant and well-defined ''events'' of duration 2-3 days during the contin uous-monitoring period. We find that the amplitudes of the continuum v ariations decrease with increasing wavelength, which extends a general trend for this and other Seyfert galaxies to smaller timescales (i.e. , a few days). The continuum variations in all the UV bands are simult aneous to within an accuracy of similar to 0.15 days, providing a stri ct constraint on continuum models. The emission-line light curves show only one major event during the continuous monitoring (a slow rise fo llowed by a shallow dip) and do not correlate well with continuum ligh t curves over the short duration of the campaign, because the timescal e for continuum variations is apparently smaller than the response tim es of the emission lines.