Optical to mid-infrared observations revealing the most obscured high-energy sources of the Galaxy

A new type of sources has been discovered by INTEGRAL. These sources are in the course of being unveiled by means of multi-wavelength optical, near- and mid-infrared observations. Among the high-energy binary sources, two distinct classes are appearing. The first class is constituted of intrinsically obscured high-energy sources, of which IGR J16318-4848 seems to be the archetype. The second class is populated by the so-called supergiant fast X-ray transients, with IGR J17544-2619 being the archetype. We report here on multi-wavelength observations of sources from these two classes, focusing on optical to mid-infrared observations. We show that in the case of the obscured sources IGR J16318-4848 and IGR J16195-4945, our observations suggest the presence of absorbing material (dust and/or cold gas) enshrouding the whole binary system. We then discuss the nature of these two different types of sources.