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A.Königl (1), J.F. Kartje (1), S.Bowyer (2), S. M. Kahn (3), and C -Y. Hwang (2)
1) Department of Astronomy and Astrophysics and Enrico Fermi Institute,
University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637.
2) Center for EUV Astrophysics and Astronomy Department, University
of California, Berkeley, CA 94720.
3) Departments of Physics and Astronomy, and Space Sciences
Laboratory, University of California, Berkeley, CA 94720.
We carried out two spectroscopic observations of the BL Lacertae object PKS 2155-304 with the Extreme Ultraviolet Explorer during 1993 June (~ 111 ksec) and July ( ~ 157 ksec). The source was detected in the ~ 75 - 110 Å range during both epochs, but the two spectra differ in detail, and the flux has increased by ~ 60 % between the two observations. A power-law fit to the data yields an energy spectral index alpha \approx 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 PSPC, which approximately corresponds also to the observed EUV to X-ray flux ratio. Fitting a power law with $\alpha = 1.65$ to the EUV data implies strong absorption at the source between $\sim 75$ \AA\ and =~ 85 Å. 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 (<~0.1 c), radially localized BroadEmission Line-type clouds of total column density ~5 E+20 / cm² 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 to 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 OGS and identified with a broadened O VIII Lyman alpha line might have originated in outflowing clouds that had a higher abundance of O VIII than of O VII, possibly because they crossed our line of sight closer to the continuum source.
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