The Extreme Ultraviolet Spectrum of the BL Lacertae Object PKS 2155-304

Details The Extreme Ultraviolet Spectrum of the BL Lacertae Object PKS 2155-304 The Extreme Ultraviolet Spectrum of the BL Lacertae Object PKS 2155-304

Jun 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995apj...446..598k&link_type=abstract

Astrophysical Journal v.446, p.598

Astronomy and Astrophysics

Astronomy

19

Galaxies: Bl Lacertae Objects: Individual Alphanumeric: Pks 2155-304, Galaxies: Jets, Ultraviolet: Galaxies, X-Rays: Galaxies

Scientific paper

We carried out two spectroscopic observations of the BL Lacertae object PKS 2155 - 304 with the Extreme Ultraviolet Explorer during 1993 June (˜111 ks) and July (˜157 ks). 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 α ≍ 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 ˜75 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, broad emission line-type clouds of total column density ˜5 × 1020 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 Kα 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 VIII Lyα 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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