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

¹ Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, Gießenbachstr., 85741 Garching, Germany
e-mail: burtscher@mpe.mpg.de
² Institute for Astronomy, Department of Physics, ETH Zürich, Wolfgang-Pauli-Strasse 27, 8093 Zürich, Switzerland
³ School of Physics and Astronomy, Tel Aviv University, 69978 Tel Aviv, Israel
⁴ Instituto de Astrofísica, Facultad de Física, Pontificia Universidad Católica de Chile, 306, Santiago 22, Chile
⁵ Department of Astronomy, University of Maryland, College Park, MD 20742, USA
⁶ Joint Space-Science Institute, University of Maryland, College Park, MD 20742, USA
⁷ Raymond and Beverly Sackler School of Physics & Astronomy, Tel Aviv University, 69978 Ramat Aviv, Israel

Received: 16 October 2015
Accepted: 16 November 2015

Abstract

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 A_V, 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 N_H 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 L_X/ (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 N_H = 10^22.3 cm^-2 to be consistent with the optical classification. We find that N_H is related to A_V and that the N_H/A_V 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 N_H/A_V 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.