The extreme ultraviolet spectrum of the BL Lacertae object PKS 2155-304

Arieh Königl, John F. Kartje, Stuart Bowyer, Steven M. Kahn, Chorng Yuan Hwang

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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 α = 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 (≲0.1c), 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.

Original languageEnglish
Pages (from-to)598-601
Number of pages4
JournalAstrophysical Journal
Issue number2
StatePublished - 20 Jun 1995


  • BL Lacertae objects: individual (PKS 2155-304)
  • Galaxies: jets
  • Ultraviolet: galaxies
  • X-rays: galaxies


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