Unveiling the nature of INTEGRAL objects through optical spectroscopy^⋆

IX. Twenty two more identifications, and a glance into the far hard X-ray Universe

¹ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica di Bologna, via Gobetti 101, 40129 Bologna, Italy
e-mail: masetti@iasfbo.inaf.it
² Dipartimento di Astronomia, Università di Bologna, via Ranzani 1, 40127 Bologna, Italy
³ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica di Roma, via Fosso del Cavaliere 100, 00133 Rome, Italy
⁴ Instituto de Astronomía, Universidad Nacional Autónoma de Mexico, Apartado Postal 70-264, 04510 Mexico D.F., Mexico
⁵ Instituto Nacional de Astrofísica, Óptica y Electrónica, Apartado Postal 51-216, 72000 Puebla, Mexico
⁶ Physics and Astronomy, University of Southampton, Southampton, Hampshire, SO17 1BJ, UK
⁷ Departamento de Astronomía y Astrofísica, Pontificia Universidad Católica de Chile, Casilla 306, Santiago 22, Chile
⁸ Specola Vaticana, 00120 Città del Vaticano, Italy
⁹ Department of Astrophysical Sciences, University of Princeton, Princeton, NJ 08544-1001, USA
¹⁰ Dipartimento di Astronomia, Università di Padova, Vicolo dell’Osservatorio 3, 35122 Padua, Italy
¹¹ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padua, Italy

Received: 1 December 2011
Accepted: 29 December 2011

Abstract

Since its launch in October 2002, the INTEGRAL satellite has revolutionized our knowledge of the hard X-ray sky thanks to its unprecedented imaging capabilities and source detection positional accuracy above 20 keV. Nevertheless, many of the newly-detected sources in the INTEGRAL sky surveys are of unknown nature. The combined use of available information at longer wavelengths (mainly soft X-rays and radio) and of optical spectroscopy on the putative counterparts of these new hard X-ray objects allows us to pinpoint their exact nature. Continuing our long-standing program that has been running since 2004, and using 6 different telescopes of various sizes together with data from an online spectroscopic survey, here we report the classification through optical spectroscopy of 22 more unidentified or poorly studied high-energy sources detected with the IBIS instrument onboard INTEGRAL. We found that 16 of them are active galactic nuclei (AGNs), while the remaining 6 objects are within our Galaxy. Among the identified extragalactic sources, the large majority (14) is made up of type 1 AGNs (i.e. with broad emission lines); of these, 6 lie at redshift larger than 0.5 and one (IGR J12319−0749) has z = 3.12, which makes it the second farthest object detected in the INTEGRAL surveys up to now. The remaining AGNs are of type 2 (that is, with narrow emission lines only), and one of the two cases is confirmed as a pair of interacting Seyfert 2 galaxies. The Galactic objects are identified as two cataclysmic variables, one high-mass X-ray binary, one symbiotic binary and two chromospherically active stars, possibly of RS CVn type. The main physical parameters of these hard X-ray sources were also determined using the multiwavelength information available in the literature. We thus still find that AGNs are the most abundant population among hard X-ray objects identified through optical spectroscopy. Moreover, we note that the higher sensitivity of the more recent INTEGRAL surveys is now enabling the detection of high-redshift AGNs, thus allowing the exploration of the most distant hard X-ray emitting sources and possibly of the most extreme blazars.