On the nature of unabsorbed Seyfert 2 galaxies

We present an analysis of six 12 μm selected Seyfert 2 galaxies that have been reported to be unabsorbed in the X-ray. By comparing the luminosities of these galaxies in the mid-infrared (12 μm), optical ([O iii]) and hard X-ray (2–10 keV), we show that they are all underluminous in the 2–10 keV X-ray band. Four of the objects exhibit X-ray spectra indicative of a hard excess, consistent with a heavily obscured X-ray component and hence a hidden nucleus. In these objects, the softer X-rays may be dominated by a strong soft scattered continuum or contamination from the host galaxy, which is responsible for the unabsorbed X-ray spectra observed, and accounts for the anomalously low 2–10 keV X-ray luminosity. We confirm this assertion in NGC 4501 with a Chandra observation, which shows hard X-ray emission coincident with the nucleus, consistent with heavy absorption, and a number of contaminating softer sources which account for the bulk of the softer emission. We point out that such ‘Compton thick’ sources need not necessarily present iron Kα emission of high equivalent width. An example in our sample is IRASF 01475–0740, which we know must host an obscured active galactic nucleus (AGN) as it hosts a hidden broad line region (BLR) seen in scattered light. The X-ray spectrum is nonetheless relatively unobscured and the iron Kα line only moderate in strength (∼160 eV). These observations can be reconciled if the hidden nuclear emission is dominated by transmitted, rather than reflected X-rays, which can then be weak compared to the soft scattered light or galactic emission even at 6.4 keV. Despite these considerations, we conclude that two sources, NGC 3147 and 3660, may intrinsically lack a BLR, confirming the recent results of Bianchi et al. in the case of NGC 3147. Neither X-ray spectrum shows signs of hidden hard emission and both sources exhibit X-ray variability leading us to believe that we are viewing the nucleus directly.