Abstract
We present the results of a campaign of simultaneous X-ray and optical observations of `true' Type 2 Seyfert galaxies candidates, i.e. AGN without a Broad Line Region (BLR). Out of the initial sample composed by 8 sources, one object, IC1631, was found to be a misclassified starburst galaxy, another, Q2130-431, does show broad optical lines, while other two, IRAS01428-0404 and NGC4698, are very likely absorbed by Compton-thick gas along the line of sight. Therefore, these four sources are not unabsorbed Seyfert 2s as previously suggested in the literature. On the other hand, we confirm that NGC3147, NGC3660, and Q2131-427 belong to the class of true Type 2 Seyfert galaxies, since they do not show any evidence for a broad component of the optical lines nor for obscuration in their X-ray spectra. These three sources have low accretion rates ($\dot{m}= L_{bol}/L_{Edd} \la0.01$), in agreement with theoretical models which predict that the BLR disappears below a critical value of $L_{bol}/L_{Edd}$. The last source, Mrk273x, would represent an exception even of this accretion-dependent versions of the Unification Models, due to its high X-ray luminosity and accretion rate, and no evidence for obscuration. However, its optical classification as a Seyfert 2 is only based on the absence of a broad component of the H\beta, due to the lack of optical spectra encompassing the H\alpha\ band.
Highlights
The fundamental idea behind the standard Unified Model (Antonucci 1993) is that type 1 and type 2 active galactic nuclei (AGN) have no intrinsic physical differences, their classification being instead determined by the presence or absence of absorbing material along the line of sight to the object
We present the results of a campaign of simultaneous X-ray and optical observations of ‘true’ type 2 Seyfert galaxies candidates, i.e. active galactic nuclei without a broad-line region (BLR)
We confirm that NGC 3147, NGC 3660 and Q2131−427 belong to the class of true type 2 Seyfert galaxies, since they do not show any evidence for a broad component of the optical lines nor for obscuration in their X-ray spectra
Summary
The fundamental idea behind the standard Unified Model (Antonucci 1993) is that type 1 and type 2 active galactic nuclei (AGN) have no intrinsic physical differences, their classification being instead determined by the presence or absence of absorbing material along the line of sight to the object. Pappa et al 2001; Panessa & Bassani 2002; Boller et al 2003; Wolter et al 2005; Gliozzi, Sambruna & Foschini 2007; Bianchi et al 2008; Brightman & Nandra 2008; Panessa et al 2009; Shi et al 2010; Tran, Lyke & Mader 2011; Trump et al 2011), with one case in which the BLR is present but characterized by an intrinsically high Balmer decrement (Corral et al 2005) Most of these sources have the low accretion rates/luminosities required by the above-mentioned theoretical models. We report on the results of the complete systematic campaign of simultaneous X-ray and optical observations of eight true type 2 Seyfert galaxies candidates
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