Differentiating black hole from neutron star in X-ray binaries can be done by investigating their curvature and spectra. Similarly, as black hole binaries move through different accretion states, their X-ray spectra change dramatically at > 10keV, much more so than at lower energies.
NuSTAR's great increase in sensitivity at > 10keV over the past missions has provided the opportunity to constrain the nature of the resolved sources and to investigate accretion states of black hole binary populations in nearby galaxies for the first time.
The color-intensity and color-color diagrams are powerful tools for diagnosing point sources which are too faint for spectral analysis. (Yukita et al. 2016)