Active Galactic Nuclei in the Extreme Ultraviolet

Extract

The extreme ultraviolet (EUV) wavelength interval (commonly defined to constitute the range 68 — 912 Å, or 13.6 — 182 eV) hes between the intensively studied FUV (912 - 3000 Å) and X-ray (1 - 68 Å) spectral regimes, but until recently has not been subjected to systematic investigations. This lack of information has been particularly acute in the case of active galactic nuclei (AGNs), where the very ability to carry out successful EUV observations has been seriously questioned. This has stemmed from the fact that, for a Galactic H I column density N_G(H I) = 1 × 10²⁰ cm⁻², the interstellar transmission factor is 0.42 at 60 Å, but a factor of 10 smaller at 100 Å. In turn, there are no known regions of the interstellar medium (ISM) that have [N_G(H I)/10²⁰ cm⁻²] < 0.6 (resp., 0.8) for positive (resp., negative) latitudes (e.g., Heiles 1991).

There are, however, several reasons why EUV observations of AGNs could provide important information about the nature of these objects. In the case of radio-quiet QSOs, the peak power is known to be emitted in the EUV range (several hundred A). Furthermore, in these sources there is evidence for a distinct soft X-ray component (representing a “soft excess” over an extrapolation of the ISM-absorbed hard X-ray component), whose properties are at present controversial. In particular, the first results of ROSAT PSPC observations of a complete sample of optically selected QSOs (Laor et al. 1994) have pointed to a spectral index αx = 1.50 ± 0.40 in the 0.2 − 2 keV range that flattens by Δαx ≈ 0.5 above ∽ 2 keV. This study has concluded that steep-αx sources are characterized by a week hard component. A similar distribution of αx has been inferred for soft X-ray-bright QSOs and Seyfert 1 galaxies observed in the ROSAT all-sky survey. On the other hand, previous observations (enumerated in Laor et al. 1994) by the Einstein and EXOSAT satellites yielded different soft X-ray spectral slopes and led to an alternative explanation of the steep-αχ objects. These questions could in principle be clarified by EUV measurements, which would then also help determine the soft X-ray emission mechanism.