THE EXTREME-ULTRAVIOLET SPECTRUM OF THE BL LACERTAE OBJECT PKS-2155-304

We carried out two spectroscopic observations of the BL Lacertae object PKS 2155 - 304 with the Extreme Ultraviolet Explorer during 1993 June (similar to 111 ks) and July (similar to 157 ks). The source was detected in the similar to 75-110 Angstrom range during both epochs, but the two spectra differ in detail, and the flux has increased by similar to 60% between the two observations. A power-law fit to the data yields an energy spectral index alpha approximate to 3-4 for the measured Galactic H I column density and likely choices of the He I and He II abundances: such values are inconsistent with the soft X-ray spectral index of 1.65 measured by the ROSAT Position Sensitive Proportional Counter, which approximately corresponds also to the observed flux ratio of EUV to X-ray. Fitting a power law with alpha = 1.65 to the EUV data implies strong absorption at the source between similar to 75 and similar to 85 Angstrom. We argue that this absorption is not due to continuum opacity and demonstrate that it can be attributed, instead, to a superposition of Doppler-smeared absorption lines originating in high-velocity (less than or similar to 0.1c), radially localized, broad emission line-type clouds of total column density similar to 5 x 10(20) cm(-2) that are ionized by the beamed continuum of the associated relativistic jet. We identify the lines as mostly L- and M-shell transitions of Mg, Ne, and Fe. The same model also implies a pronounced O VII K alpha X-ray absorption feature at roughly the same energy as the feature detected in 1990 by BBXRT, which provides strong support for the apparent association of this object with a galaxy at z = 0.116. We suggest that the higher energy X-ray absorption feature detected in 1980 by the Einstein Objective Grating Spectrometer and identified with a broadened O VII Ly alpha line might have originated in outflowing clouds that had a higher abundance of O VIII than of O VIII, possibly because they crossed our line of sight closer to the continuum source.