Multiwavelength Observations of Short-Timescale Variability in NGC 4151. I. Ultraviolet Observations

We present the results of an intensive ultraviolet monitoring campaign on the\nSeyfert 1 galaxy NGC 4151, as part of an effort to study its short time-scale\nvariability over a broad range in wavelength. The nucleus of NGC 4151 was\nobserved continuously with the {\\it International Ultraviolet Explorer} (IUE)\nfor 9.3 days, yielding a pair of LWP and SWP spectra every $\\sim$70 minutes,\nand during four-hour periods for 4 days prior to and 5 days after the\ncontinuous monitoring period. The sampling frequency of the observations is an\norder of magnitude higher than that of any previous UV monitoring campaign on a\nSeyfert galaxy.\n The continuum fluxes in bands from 1275 \\AA\\ to 2688 \\AA\\ went through four\nsignificant and well-defined ``events'' of duration 2 -- 3 days during the\ncontinuous monitoring period. We find that the amplitudes of the continuum\nvariations decrease with increasing wavelength, which extends a general trend\nfor this and other Seyfert galaxies to smaller time scales (i.e., a few days).\nThe continuum variations in all of the UV bands are {\\it simultaneous} to\nwithin an accuracy of about 0.15 days, providing a strict constraint on\ncontinuum models. The emission-line light curves show only one major event\nduring the continuous monitoring (a slow rise followed by a shallow dip), and\ndo not correlate well with continuum light curves over the (short) duration of\nthe campaign, because the time scale for continuum variations is apparently\nsmaller than the response times of the emission lines.\n