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

¹ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica di Bologna, via Gobetti 101, 40129 Bologna Italy
e-mail: pietro.parisi@iaps.inaf.it
² Instituto de Astronomía, Universidad Nacional Autónoma de México, Apartado Postal 70-264, 04510 México D.F., Mexico
³ Instituto Nacional de Astrofísica, Óptica y Electrónica, Apartado Postal 51-216, 72000 Puebla, Mexico
⁴ INAF – Istituto di Astrofisica e Planetologia Spaziali di Roma, via del Fosso del Cavaliere 100, 00133 Roma, Italy
⁵ Physics & Astronomy, University of Southampton, Southampton, Hampshire, SO171 BJ, UK
⁶ Departamento de Astronomía y Astrofísica, Pontificia Universidad Católica de Chile, Casilla 306, Santiago 22, Chile
⁷ European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile
⁸ Specola Vaticana, 00120 Città del Vaticano, Vatican
⁹ Dipartimento di Astronomia, Università di Padova, Vicolo dell’Osservatorio 3, 35122 Padua, Italy
¹⁰ South African Astronomical Observatory, PO Box 9, 7935 Observatory, South Africa
¹¹ University of Cape Town, Private Bag X3, 7701 Rondebosch, South Africa

Received: 8 March 2012
Accepted: 15 June 2012

Abstract

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.