Many researchers investigate the spectral evolution of BHBs using a hardness-intensity diagram (HID), which is a plot of X-ray intensity versus a “hardness ratio” (), i.e., the ratio of detector counts in two energy bands (e.g., van der Klis 2005; Homan et al. 2001; Belloni 2004). This diagram is widely used in tracking the behavior of accreting neutron stars. Compared to the spectral-fitting approach described above, the HID approach has the advantage that it is model independent and the disadvantage that it is difficult to relate the results to physical quantities. Interpretations of variations in the HID depend on the particular energy bands chosen to define in a given study. If both bands are above ∼5 keV, then the value effectively tracks the slope of the PL component (i.e., lower is a steeper PL). Softer energy bands admit a mixture of thermal and nonthermal components, and interpretations are then more complicated.
Source: Remillard & McClintock 2006 ┃ X-Ray Properties… ⚬𓂃
(echo:: @ ᯤ)