Astronomy and Astrophysics – Astronomy
Scientific paper
Aug 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995agm....11p..72k&link_type=abstract
AG Abstract Services, vol. 11, Abstracts of Contributed Talks and Posters presented at the Annual Scientific Meeting of the Astr
Astronomy and Astrophysics
Astronomy
Warm Absorbers, Emission Lines, Photoionization Modelling, X-Ray Observations, Individual Agn: Ngc 4051
Scientific paper
The absorption edges seen in the X-ray spectra of several Seyfert 1 galaxies have been interpreted as originating in warm material along the line of sight to the active nucleus. The geometrical and physical properties of these warm absorbers are still poorly constrained. They might be located well within the BLR, tracing the conditions very near the central black hole, or further outwards, e.g. in the NLR. Here we discuss the multi-wavelength emission and absorption behaviour of such warm material and present a ROSAT observation of the warm absorber in NGC 4051. For this purpose we have calculated warm absorber models using the code Cloudy (Ferland 1994), assuming that the absorption arises in material photoionized by the central continuum source and including the effects of emission and reflection. The predicted emission line spectrum is dominated by high-ionization lines, some of which are given in Tab. 1 [not included]. We particularly note the possible warm-absorber contribution to some of the `usual' BLR-lines. Several lines, e.g. OVI λ1035, are exceptionally strengthened for densities larger than log nH >= 11 arguing for a lower density of the warm gas. The absolute line intensities also depend on the column density and covering factor of the warm material and thus provide further constraints on its properties. The absorption spectrum is characterized by absorption edges in the X-ray regime (e.g. Halpern 1984). We note that for low ionization parameters (log U of the order -1) one-component power-law fits to the absorbed X-ray spectrum would result in a power-law index Gamma effectively flatter than the intrinsic one. High ionization parameters and column densities lead to deep oxygen and iron absorption edges and a recovering even beyond the ASCA sensitivity region. The X-ray spectrum of the Seyfert 1 galaxy NGC 4051 shows evidence for warm absorption, as was first suggested by Fiore et al. (1992). We discuss a pointed ROSAT observation of this object. Detailed warm-absorber modelling indeed significantly improves the fit as compared to simple power law models with a single additional absorption edge. However, the non-absorbed intrinsic power-law index Γ ≈ -2.3 is still steeper than the Seyfert-1 typical value of -1.9. An additional soft excess is present. The lighcurve of NGC 4051 shows the source to be variable by about a factor of 6 within a day.
Fink Henner
Komossa Stefanie
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