Unveiling the nature of INTEGRAL objects through optical spectroscopy . II . The nature of four unidentified sources ⋆

We present the results of our optical spectrophotometric campaign ongoing at the Astronomical Observatory of Bologna in Loiano (Italy) on hard X-ray sources detected by INTEGRAL. We observed spec-troscopically the putative optical counterparts of four more INTEGRAL sources, IGR J12391−1610, IGR J18406−0539, 2E 1853.7+1534 and IGR J19473+4452. These data have allowed us to determine their nature, finding that IGR J12391−1610 (=LEDA 170194) and IGR J19473+4452 are Seyfert 2 galaxies at redshifts z = 0.036 and z = 0.053, respectively, IGR J18406−0539 (=SS 406) is a Be massive X-ray binary located at ∼1.1 kpc from Earth, and 2E 1853.7+1534 is a Type 1 Seyfert galaxy with z = 0.084. Physical parameters for these objects are also evaluated by collecting and discussing the available multiwavelength information. The determination of the extragalactic nature of a substantial fraction of sources inside the INTEGRAL surveys underlines the importance of hard X-ray observations for the study of background Active Galactic Nuclei located beyond the 'Zone of Avoidance' of the Galactic Plane.