Far-Ultraviolet Observations of NGC 4151 during the [ITAL]ORFEUS-SPAS II[/ITAL] Mission

We observed the Seyfert 1 galaxy NGC 4151 on eleven occasions at 1-2 dayintervals using the Berkeley spectrometer during the ORFEUS-SPAS II mission in1996 November. The mean spectrum covers 912-1220 A at ~0.3 A resolution with atotal exposure of 15,658 seconds. The mean flux at 1000 A was 4.7e-13erg/cm^2/s/A. We identify the neutral hydrogen absorption with a number ofcomponents that correspond to the velocity distribution of \ion{H}{1} seen inour own Galaxy as well as features identified in the CIV 1549 absorptionprofile by Weymann et al. The main component of neutral hydrogen in NGC 4151has a total column density of log N_HI = 18.7 +/- 1.5 cm^{-2} for a Dopplerparameter b=250 +/- 50 km/s, and it covers 84 +/- 6% of the source. This isconsistent with previous results obtained with the Hopkins UltravioletTelescope. Other intrinsic far-UV absorption features are not resolved, but theCIII* 1176 absorption line has a significantly higher blueshift relative to NGC4151 than the CIII 977 resonance line. This implies that the highest velocityregion of the outflowing gas has the highest density. Variations in theequivalent width of the CIII* 1176 absorption line anticorrelate with continuumvariations on timescales of days. For an ionization timescale <1 day, we set anupper limit of 25 pc on the distance of the absorbing gas from the centralsource. The OVI 1034 and HeII 1085 emission lines also vary on timescales of1-2 days, but their response to the continuum variations is complex. For somecontinuum variations they show no response, while for others the response isinstantaneous to the limit of our sampling interval.Comment: 4 pages, 2 PostScript figures, uses emulateapj.sty, apjfonts.sty. To appear in the Astrophysical Journal (Letters) special issue for ORFEU