On the relation of optical obscuration and X-ray absorption in Seyfert galaxies

L. Burtscher, R. I. Davies, J. Graciá-Carpio, M. J. Koss, M. Y. Lin, D. Lutz, P. Nandra, H. Netzer, G. Orban De Xivry, C. Ricci, D. J. Rosario, S. Veilleux, A. Contursi, R. Genzel, A. Schnorr-Müller, A. Sternberg, E. Sturm, L. J. Tacconi

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The optical classification of a Seyfert galaxy and whether it is considered X-ray absorbed are often used interchangeably. There are many borderline cases, however, and also numerous examples where the optical and X-ray classifications appear to be in disagreement. In this article we revisit the relation between optical obscuration and X-ray absorption in active galactic nuclei (AGNs). We make use of our "dust colour" method to derive the optical obscuration AV, and consistently estimated X-ray absorbing columns using 0.3-150 keV spectral energy distributions. We also take into account the variable nature of the neutral gas column NH and derive the Seyfert subclasses of all our objects in a consistent way. We show in a sample of 25 local, hard-X-ray detected Seyfert galaxies (log LX/(erg/s) ≈ 41.5-43.5) that there can actually be a good agreement between optical and X-ray classification. If Seyfert types 1.8 and 1.9 are considered unobscured, the threshold between X-ray unabsorbed and absorbed should be chosen at a column NH = 1022.3 cm-2 to be consistent with the optical classification. We find that NH is related to AV and that the NH/AV ratio is approximately Galactic or higher in all sources, as indicated previously. However, in several objects we also see that deviations from the Galactic ratio are only due to a variable X-ray column, showing that (1) deviations from the Galactic NH/AV can be simply explained by dust-free neutral gas within the broad-line region in some sources; that (2) the dust properties in AGNs can be similar to Galactic dust and that (3) the dust colour method is a robust way to estimate the optical extinction towards the sublimation radius in all but the most obscured AGNs.

Original languageEnglish
Article numberA28
JournalAstronomy and Astrophysics
StatePublished - 1 Feb 2016


  • Dust, extinction
  • Galaxies: Seyfert
  • Galaxies: active
  • Galaxies: nuclei


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