Global characteristics of the first IBIS/ISGRI catalogue sources: unveiling a murky episode of binary star evolution

INTEGRAL is the first gamma-ray astronomy mission with a sufficient sensitivity and angular resolution combination appropriate to the detection and identification of considerable numbers of gamma-ray emitting sources. The large field of view (~30° zero response FWHM) enables INTEGRAL to survey the galactic plane on a regular (~weekly) basis as part of the core programme. The first source catalogue, based on the 1st year of core programme data (~5 Ms) has been completed and published (Bird et al. 2004, ApJ, 607, L33). It contained 123 γ-ray sources (24 HMXB, 54 LMXB, 28 “unknown”, plus 17 others) – sufficient numbers for a reasonable statistical analysis of their global properties. These were located to a positional accuracy of typically 0.72 arcmin. The detection of previously unknown γ-ray emitting sources generally exhibiting high intrinsic absorption, which do not have readily identifiable counterparts at other wavelengths, is intriguing. The substantial fraction (roughly 20% of the total number) of unclassified γ-ray sources suggests they must constitute a significant family of objects. In this paper we review the global characteristics of the known galactic sources as well as the unclassified objects with the twin aims of investigating how the unclassified set may fit into stellar evolution and improving our understanding of known X-ray binary systems through the non-thermal γ-ray channel. In the context of the known systems we are very conscious that they constitute a γ-ray selected set, and may exhibit subtle generic differences to the rest of the class. We present Log(N)–Log(S) distributions, angular distributions, and for systems with reliable distance estimates the spatial distributions within the Galaxy and luminosity functions. For the unknown sources, this statistical analysis has shown that they are most likely to be HMXBs containing a highly magnetised neutron star. The lack of X-ray counterparts for these sources indicates a high degree of intrinsic obscuration.