Model independent means of categorizing X-ray binaries – I. Colour–colour–intensity diagrams

The diverse behaviours displayed by X-ray binaries make it difficult to determine the nature of the underlying compact objects. In particular, identification of systems containing black holes is currently considered robust only if a dynamical mass is obtained. We explore a model-independent means of identifying the central bodies – neutron stars or black holes – of accreting binary systems. We find that four categories of object (classic black holes, GRS 1915-like black holes, pulsars and non-pulsing neutron stars) occupy distinct regions in a 3D colour–colour–intensity (CCI) diagram. Assuming that this clustering effect is due to intrinsic properties of the sources (such as mass-accretion rate, binary separation, mass ratio, magnetic field strength, etc.), we suggest possible physical effects that drive each object to its specific location in the CCI phase space. We also suggest a surface in this space which separates systems that produce jets from those which do not, and demonstrate the use of CCI for identifying X-ray pulsars where a period has not been established. This method can also be used to study subclustering within a category and may prove useful for other classes of objects, such as cataclysmic variables and active galactic nuclei.