Model independent means of catagorizing X-ray binaries. I: Colour-Colour-Intensity Diagrams

TL;DR

This paper introduces a model-independent method using 3D colour-colour-intensity diagrams to classify X-ray binaries and distinguish their central objects, such as neutron stars and black holes, based on their intrinsic properties.

Contribution

The study presents a novel classification approach that clusters X-ray binaries in a 3D diagram, enabling identification of system types and jet-producing states without relying on dynamical mass measurements.

Findings

Four distinct object categories occupy separate regions in the CCI diagram.

The method can identify X-ray pulsars without known periods.

It suggests physical factors influencing source locations in the CCI space.

Abstract

The diverse behaviors 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 four categories of object (classic black holes, GRS1915-like black holes, pulsars, and non-pulsing neutron stars) occupy distinct regions in a 3-dimensional 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 sub-clustering within a category and may prove useful for other classes of objects, such as cataclysmic variables and active galactic nuclei.