Simultaneous X-ray and optical observations of true Type 2 Seyfert galaxies

TL;DR

This study confirms the existence of true Type 2 Seyfert galaxies with low accretion rates and no broad optical lines or X-ray obscuration, challenging some previous classifications and supporting models where the broad line region disappears at low accretion rates.

Contribution

The paper provides observational evidence for true Type 2 Seyfert galaxies with low accretion rates, clarifies misclassifications, and tests theoretical models of BLR disappearance.

Findings

NGC3147, NGC3660, and Q2131-427 are confirmed as true Type 2 Seyfert galaxies.

Some initial candidates were misclassified or absorbed, not true unabsorbed Seyfert 2s.

Low accretion rates are associated with the absence of broad lines and X-ray obscuration.

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.