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We observed NGC 4151 on six separate occasions at intervals of one to three days using the Hopkins Ultraviolet Telescope during the flight of Astro-2 aboard the space shuttle Endeavour in March 1995. The far-ultraviolet spectra cover the spectral range from the interstellar cutoff at 912 \AA\ to 1840 \AA\ with a spectral resolution of 2--4 \AA. Integrations of 300--1300 s yielded spectra with mean signal-to-noise ratios of 12--25 per \AA; the mean spectrum has a S/N of $\sim$30. Slight variations are apparent among the six recorded spectra, particularly in the absorption line strengths, but comparison of the mean spectrum to that obtained during the flight of Astro-1 in December 1990 shows profound differences. The continuum, $\rm 5.0 \times 10^{-13}~erg~cm^{-2}~s^{-1}~\AA^{-1}$ at 1450 \AA, is five times brighter, nearly the brightest UV flux ever observed for NGC~4151. The far-UV transparency near the Lyman limit has changed from optically thick to optically thin. We infer a neutral hydrogen column of $\rm \sim 3 \times 10^{17}~cm^{-2}$ broadened by a Doppler parameter of $\rm \sim 250~\rm km~s^{-1}$. All high-ionization absorption lines have strengthened considerably --- S~{\sc VI} $\lambda\lambda 933,945$, C~{\sc III} $\lambda\lambda 977$, O~{\sc VI} $\lambda\lambda 1032,1038$, N~{\sc V} $\lambda\lambda 1239,1243$, Si~{\sc IV} $\lambda\lambda 1394,1403$, and C~{\sc IV} $\lambda\lambda 1548,1551$. The Si~{\sc IV} doublet is observed at its optically thin limit. The high velocity of the absorbing neutral hydrogen, the change in transparency from Astro-1 to Astro-2, and the high ionization lines suggest a high temperature (few $\times 10^6$ K), photoionized absorbing medium, possibly related to the ``warm" absorbers seen with moderate resolution X-ray telescopes.
This work was supported by NASA contract NAS 5-27000 to the Johns Hopkins University.
