FAR-ULTRAVIOLET OBSERVATIONS OF NGC4151 DURING THE ORFEUS-SPAS-II MISSION

We observed the Seyfert 1 galaxy NGC 4151 on 11 occasions at 1-2 day i ntervals using the Berkeley spectrometer during the ORFEUS-SPAS II mis sion in 1996 November. The mean spectrum covers 912-1220 Angstrom at s imilar to 0.3 Angstrom resolution with a total exposure of 15,658 s. T he mean flux at 1000 Angstrom was 4.7 x 10(-13) ergs cm(-2) s(-1) Angs trom(-1). We identify the neutral hydrogen absorption with a number of components that correspond to the velocity distribution of H I seen i n our own Galaxy as well as features identified in the C IV lambda 154 9 absorption profile by Weymann et al. The main component of neutral h ydrogen in NGC 4151 has a total column density of log N-HI = 18.7 +/- 1.5 cm(-2) for a Doppler parameter b = 250 +/- 50 km s(-1), and it cov ers 84% +/- 6% of the source. This is consistent with previous results obtained with the Hopkins Ultraviolet Telescope. Other intrinsic far- UV absorption features are not resolved, but the C III lambda 1176 ab sorption line has a significantly higher blueshift relative to NGC 415 1 than the C III lambda 977 resonance line. This implies that the high est velocity region of the outflowing gas has the highest density. Var iations in the equivalent width of the C III lambda 1176 absorption l ine anticorrelate with continuum variations on timescales of days. I;o r an ionization timescale of less than 1 day, we set an upper limit of 25 pc on the distance of the absorbing gas from the central source. T he O VI lambda 1034 and He II lambda 1085 emission lines also vary on timescales of 1-2 days, but their response to the continuum variations is complex. For some continuum variations they show no response, whil e for others the response is instantaneous to the limit of our samplin g interval.