Unveiling the nature of INTEGRAL objects through optical spectroscopy^*

VIII. Identification of 44 newly detected hard X-ray sources

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 Instituto de Astronomía, Universidad Nacional Autónoma de México, Apartado Postal 70-264, 04510 México D.F., México
⁴ Instituto Nacional de Astrofísica, Óptica y Electrónica, Apartado Postal 51-216, 72000 Puebla, México
⁵ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica di Roma, via Fosso del Cavaliere 100, 00133 Rome, Italy
⁶ School of Physics & Astronomy, University of Southampton, Southampton, Hampshire, SO17 1BJ, UK
⁷ South African Astronomical Observatory, PO Box 9, Observatory 7935, South Africa
⁸ Departamento de Astronomía y Astrofísica, Pontificia Universidad Católica de Chile, Casilla 306, Santiago 22, Chile
⁹ European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile
¹⁰ Specola Vaticana, V-00120 Città del Vaticano, Italy
¹¹ Dipartimento di Astronomia, Università di Padova, Vicolo dell'Osservatorio 3, 35122 Padua, Italy

Received: 23 April 2010
Accepted: 20 May 2010

Abstract

Hard X-ray surveys performed by the INTEGRAL satellite have discovered a conspicuous fraction (up to 30%) of unidentified objects among the detected sources. Here we continue our program of identification of these objects by (i) selecting probable optical candidates by means of positional cross-correlation of the INTEGRAL detections with soft X-ray, radio, and/or optical archives and (ii) performing optical spectroscopy on them. As a result, we pinpointed and identified, or more accurately characterized, 44 definite or likely counterparts of INTEGRAL sources. Among them, 32 are active galactic nuclei (AGNs; 18 with broad emission lines, 13 with narrow emission lines only, and one X-ray bright, optically normal galaxy) with redshift 0.019 < z < 0.6058, 6 cataclysmic variables (CVs), 5 high-mass X-ray binaries (2 of which in the Small Magellanic Cloud), and 1 low-mass X-ray binary. This was achieved by using 7 telescopes of various sizes and archival data from two online spectroscopic surveys. The main physical parameters of these hard X-ray sources were also determined using the multiwavelength information available in the literature. In general, AGNs are the most abundant population among hard X-ray objects, and our results confirm the tendency of finding AGNs more frequently than any other type of hard X-ray emitting object among unidentified INTEGRAL sources when optical spectroscopy is used as an identification tool. Moreover, the deeper sensitivity of the more recent INTEGRAL surveys enables one to begin detecting hard X-ray emission above 20 keV from sources such as LINER-type AGNs and non-magnetic CVs.