X-ray-infrared colour–colour diagrams. Hardness ratio versus (H − KS) for all X-ray sources having a 2MASS counterpart with a probability higher than 90%. Active coronae are represented by filled circles where colour indicates the spectral type, extragalactic objects with blue filled squares. Giant stars are overplotted as stars, pre-main sequence stars are highlighted by an open square and X-ray binaries (CVs, HMXBs) are plotted as a filled star in red. Lines are the expected colours for stars of different ages, main sequence and evolved binaries, and accreting objects (see legend and text). We only plot sources with errors lower than 0.3 in X-ray and infrared colours.
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