title: Extreme ultraviolet spectroscopy of PKS 2155-304.
abstract: We present the extreme ultraviolet (75-110 angstroms) spectrum of the BL Lac object PKS 2155-304, the first spectrum of an extragalactic source obtained with the Extreme Ultraviolet Explorer. 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 approximately 80 angstroms. 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 approximately 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 1.36 x 10(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.
authors.affiliation: Center for EUV Astrophysics, University of California, Berkeley 94720, USA.
authors.forename: A
authors.initials: A
authors.lastname: Fruscione
authors.forename: S
authors.initials: S
authors.lastname: Bowyer
authors.forename: A
authors.initials: A
authors.lastname: Konigl
authors.forename: S M
authors.initials: SM
authors.lastname: Kahn
mesh_terms: D001256:Astronomy / Q000295:instrumentation / Q000379:methods*; D005118:Extraterrestrial Environment; D006371:Helium; D006859:Hydrogen; D008962:Models, Theoretical; D013026:Space Flight / Q000295:instrumentation*; D018531:Spacecraft / Q000295:instrumentation*; D013057:Spectrum Analysis / Q000295:instrumentation / Q000379:methods* / Q000706:statistics & numerical data; D014466:Ultraviolet Rays; D014965:X-Rays
chemical_list: D006371:Helium; D006859:Hydrogen