An absorbed view of a new class of INTEGRAL sources

The European γ-ray observatory INTEGRAL has found a group of hard X-ray sources which are highly absorbed, i.e., with column densities higher than about 1023 cm−2. Here I give an overview of this class of INTEGRAL sources. The X-ray, as well as the optical/IR, properties of these sources and their location in the sky suggest that they belong to the class of high-mass Xray binaries, some of them possibly long-period X-ray pulsars. The donors in these binaries are most probably giant or supergiant stars. I suggest that the soft X-ray spectrum below ∼5 keV of IGR J16318−4848, as well as in several other X-ray binaries (e.g., XTE J0421+56), can be described by emission from a compact object which is strongly absorbed by a partionally ionised dense envelope.