The AGN nature of 11 out of 12 Swift/rxte unidentified sources through optical and X-ray spectroscopy

The Swift Burst Alert Telescope (BAT) is performing a high Galactic latitude survey in the 14-195 keV band at a flux limit of ∼10^-11 ergs cm^-2 s^-1, leading to the discovery of new high-energy sources, most of which have not yet been properly classified. A similar work has also been performed with the RXTE slew survey, leading to the discovery of 68 sources detected above 8 keV, many of which are still unclassified. Follow-up observations with the Swift X-ray Telescope (XRT) provide, for many of these objects, source localization with a positional accuracy of a few arcseconds, thus allowing the search for optical counterparts to be more efficient and reliable. We present the results of optical/X-ray follow-up studies of 11 Swift BAT detections and one AGN detected in the RXTE Slew Survey, aimed at identifying their longer wavelength counterparts and at assessing their nature. These data allowed, for the first time, the optical classification of eight objects and a distance determination for three of them. For another object, a more refined optical classification than that available in the literature is also provided. For the remaining sources, optical spectroscopy provides a characterization of the source near in time to the X-ray measurement. The sample consists of six Seyfert 2 galaxies, five Seyferts of intermediate type 1.2-1.8, and one object of Galactic nature - an intermediate polar (i.e., magnetic) cataclysmic variable. Out of the 11 AGNs, 8 (∼70%), including 2 Seyferts of type 1.2 and 1.5, are absorbed with N_H > 10²² cm ^-2. Up to 3 objects could be Compton thick (i.e., N_H > 1.5 × 10²⁴ cm^-2), but only in one case (Swift J0609.1 - 8636) does all the observational evidence strongly suggest this possibility. The present data demonstrate the capability of coordinated hard X-ray and optical observations to discover absorbed AGNs.