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Accurate classification of 29 objects detected in the 39 month Palermo Swift /BAT hard X-ray catalogue

2012; EDP Sciences; Volume: 545; Linguagem: Inglês

10.1051/0004-6361/201219192

1432-0746

P. Parisi, N. Masetti, E. Jiménez‐Bailón, V. Chavushyan, E. Palazzi, R. Landi, A. Malizia, L. Bassani, A. Bazzano, A. J. Bird, P. A. Charles, Gaspar Galaz, E. Mason, V. A. McBride, D. Minniti, L. Morelli, F. Schiavone, P. Ubertini,

Gamma-ray bursts and supernovae

Through an optical campaign performed at four telescopes located in the northern and the southern hemispheres, plus archival data from two on-line sky surveys, we obtained optical spectroscopy for 29 counterparts of unclassified or poorly studied hard X-ray emitting objects detected with Swift /Burst Alert Telescope (BAT) and listed in the 39 month Palermo catalogue. All these objects also have observations taken with Swift /X-ray Telescope (XRT) or XMM-European Photon Imaging Camera (EPIC) which not only allow us to pinpoint their optical counterpart, but also study their X-ray spectral properties (column density, power law photon index, and F2−10 keV flux). We find that 28 sources in our sample are active galactic nuclei (AGNs); 7 are classified as type 1, while 21 are of type 2; the remaining object is a Galactic cataclysmic variable. Among our type 1 AGNs, we find 5 objects of intermediate Seyfert type (1.2−1.9) and one narrow-line Seyfert 1 galaxy; for 4 out of 7 sources, we are able to estimate the central black hole mass. Three of the type 2 AGNs of our sample display optical features typical of low-ionization nuclear emission-line regions (LINER) and one is a likely Compton thick AGN. All galaxies classified in this work are relatively nearby objects since their redshifts lie in the range 0.008−0.075; the only Galactic object found lies at an estimated distance of 90 pc. We also investigate the optical versus X-ray emission ratio of the galaxies of our sample to test the AGN unified model. For these galaxies, we also compare the X-ray absorption (caused by gas) with the optical reddening (caused by dust): we find that for most of our sources, specifically those of type 1.9−2.0 the former is higher than the latter confirming early results of Maiolino and collaborators; this is possibly due to the properties of dust in the circumnuclear obscuring torus of the AGN.