Unabsorbed Seyfert 2 galaxies: the case of “naked” AGN

Abstract

Hawkins (2004) reported on a class of "naked" 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 NTT (La Silla) of two "naked" AGN. We confirm the "naked" nature of Q 2131-427 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 F X /F [oiii] 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 Q 2130-431 shows H α and H β 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 Q 2130-431, however we put a very low upper limit on the column density (< 2 × 10 20 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 Q 2131-427 observation.

The XMM-NewtonWide Angle Survey (XWAS): the X-ray spectrum of type-1 AGN

“…Spectral analyses have shown that the optical type vs. X-ray properties for most AGN generally agree with the predictions from AGN unification models. However, in recent years, the number of cases where the predictions from AGN unification models do not match the observations has increased substantially, as significant absorption has been detected in ∼10% type-1 AGN (Cappi et al 2006;Mateos et al 2005b;Akylas et al 2004) while type-2 AGN have been detected without any signatures of X-ray absorption (Panessa et al 2009;Bianchi et al 2008;Corral et al 2005;Panessa & Bassani 2002).…”