Unveiling the nature of three INTEGRAL sources through optical spectroscopy ⋆

The results of an optical spectroscopy campaign performed at the Astronomical Observatory of Bologna in Loiano (Italy) on three hard X–ray sources detected by INTEGRAL (IGR J17303−0601, IGR J18027−1455 and IGR J21247+5058) are presented. These data have allowed a determination of the nature for two of them, with IGR J17303−0601 being a low mass X–ray binary in the Galaxy and IGR J18027−1455 a background Type 1 Seyfert galaxy at redshift z = 0.035. IGR J21247+5058, instead, has a quite puzzling spectroscopic appearance, with a broad, redshifted H α complex superimposed onto a 'normal' F/G-type Galactic star continuum: these features, together with the spatially coincident extended radio emission, might suggest a chance alignment between a relatively nearby star and a background radio galaxy. These results underline the still non-negligible importance of smaller telescopes in modern astrophysics. 1. Introduction One main objective of the INTEGRAL mission is a regular survey of the Galactic Plane complemented by a deep exposure of the Galactic Centre. This makes use of the unique imaging capability of IBIS (Ubertini et al. 2003) which allows the detection of sources at the mCrab level with an angular resolution of 12 ′ and a typical point source localization accuracy of 2-3 ′ : this has made it possible, for the first time, to resolve crowded regions such as the Galactic Centre and the spiral arms, and to discover many new hard X–ray objects. In its first year, IBIS has detected 123 sources between 20 and 100 keV (Bird et al. 2004): within this sample of hard X-ray emitting objects, 53 low mass and 23 high mass X-ray binaries, 5 Active Galactic Nuclei (AGN) and a handful of other objects such as pulsars, cataclysmic variables and a dwarf nova are found. The remaining objects (28, or about 23% of the sample) have no obvious conterparts at other wavelengths and therefore cannot yet be associated with any known class of high-energy emitting objects. Only for a tiny fraction of these sources have follow-up observations at X–ray energies as well as in the optical/near-infrared wavebands been carried out so far (Rodriguez 2004). Although the cross-correlation with catalogues or surveys at other wavelengths (especially soft X–rays, optical and radio) is of invaluable help in pinpointing the puta-tive optical candidates, only accurate optical spectroscopy can confirm the association and reveal the nature of the object.