Unveiling the nature of INTEGRAL objects through optical spectroscopy

IV. A study of six new hard X-ray sources

¹ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica di Bologna, via Gobetti 101, 40129 Bologna, Italy (formerly IASF/CNR, Bologna) e-mail: masetti@iasfbo.inaf.it
² INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica di Roma, via Fosso del Cavaliere 100, 00133 Rome, Italy (formerly IASF/CNR, Rome)
³ School of Physics & Astronomy, University of Southampton, Southampton, Hampshire, SO17 1BJ, UK
⁴ School of Physical Sciences, Dublin City University, Glasnevin, Dublin 9, Republic of Ireland
⁵ Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
⁶ INTEGRAL Science Data Centre, Chemin d'Ecogia 16, 1290 Versoix, Switzerland

Received: 28 February 2006
Accepted: 21 April 2006

Abstract

We present further results from our onging optical spectrophotometric campaign at the Astronomical Observatory of Bologna in Loiano (Italy) on unidentified hard X-ray sources detected by INTEGRAL. We observed spectroscopically the putative optical counterparts of the INTEGRAL sources IGR J00234+6141, IGR J01583+6713, IGR J06074+2205, IGR J13091+1137 and IGR J20286+2544. We find that the first two are Galactic objects, namely a Cataclysmic Variable at a distance  pc and a Be/X transient High-Mass X-ray Binary (HMXB) located at ~6.4 kpc, respectively, whereas the last one is identified with MCG +04-48-002, a Starburst/H ii galaxy at redshift hiding a Seyfert 2 nucleus. We identify IGR J13091+1137 as the (likely Seyfert 2 type) active nucleus of galaxy NGC 4992, which we classify as an X-ray Bright, Optically Normal Galaxy; this is the first example of this type of object to be detected by INTEGRAL, and one of the closest of this class. We moreover confirm the possible Be/X nature of IGR J06074+2205, and we estimate it to be at a distance of ~1 kpc. We also reexamine the spectrum of the  elliptical radio galaxy PKS 0352-686, the possible counterpart of the INTEGRAL source IGR J03532-6829, and we find that it is a BL Lac. Physical parameters for these sources are also evaluated by discussing our findings in the context of the available multiwavelength information. These identifications further stress the importance of INTEGRAL in the study of the hard X-ray spectrum of Active Galactic Nuclei, HMXBs and Cataclysmic Variables.