Unabsorbed Seyfert 2 galaxies: the case of 'naked' AGN

Abstract

Hawkins reported on a class of 'naked' active galactic nuclei (AGN) characterized by strong amplitude optical brightness variability and the complete absence of broad emission lines in the optical spectrum. The variability suggests that the nucleus is seen directly, however the absence of broad lines contradicts the simple formulation of Unified Models for AGN. We present the results of quasi-simultaneous spectroscopic observations with XMM-Newton and New Technology Telescope (NTT) (La Silla) of two 'naked' AGN. We confirm the 'naked' nature of Q2131-427 for which no broad emission line components have been detected in the optical spectrum and its X-ray spectrum shows no signs of intrinsic absorption. The optical and X-ray mismatch in this source cannot be ascribed to a high nuclear dust-to-gas ratio and a Compton-thick nature is ruled out on the basis of the high FX/F [O iii] ratio. The broad-line region (BLR) may be completely absent in this source, possibly as a consequence of its low Eddington ratio. On the other hand, the optical spectrum of Q2130-431 shows Hα and Hβ broad emission line components, revealing the presence of a BLR. A mild X-ray absorption is expected in intermediate type 1.8 Seyfert galaxies like Q2130-431, however we put a very low upper limit on the column density (<2 × 1020 cm-2), also the low Balmer decrement suggests that the BLR itself does not suffer from reddening. We propose that in this object the BLR is intrinsically weak, making a case of 'true' intermediate Seyfert galaxy. We also report on the X-ray detection of the Abell 3783 galaxy cluster in the XMM-Newton field of view of the Q2131-427 observation.