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Antonella Fruscione (1), Stuart Bowyer (1,2), Arieh Königl (3), and Steven M. Kahn (4)
1) Center for EUV Astrophysics, 2150 Kittredge Street,
University of California, Berkeley, CA 94720
2) Astronomy
Department, University of California, Berkeley, CA 94720
3) Department of Astronomy and Astrophysics, University of Chicago,
5640 S. Ellis Ave., Chicago, IL 60637
4) Departments of Physics and
Astronomy, and Space Sciences Laboratory, University of California,
Berkeley, CA 94720
We present the extreme ultraviolet (75-110 Å) spectrum of the BL Lac object PKS_2155-304, the first spectrum of an extragalactic source obtained with the Extreme Ultraviolet Explorer (EUVE). The spectrum shows a generally smooth continuum, which can be modeled by a single power law plus interstellar absorption, and possibly an absorption feature at ~80 Å. The best fit to the data suggests that the EUV spectrum can be interpreted as a simple extrapolation of the X-ray continuum, with an energy index alpha ~ 1.6; however, shallower or steeper power laws with indices between 0.4 and 2.7 cannot be ruled out by the existing EUV data alone. The data provide strong constraints on the interstellar neutral H and He along the line of sight. Using a column density of 1.36E+20 cm-2 for the galactic neutral hydrogen along the PKS 2155-304 line of sight, the neutral helium column density is constrained to be 9-10% of the hydrogen amount.
Subject headings: BL Lacertae objects - galaxies: individual: PKS 2155-304 - ultraviolet: galaxies - X-rays: galaxies - ISM: abundances
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