Cartoon representing the “q” diagram for XRBs, adapted from Fender et al. (2004) and Fender & Belloni (2012), which depicts the progression of an XRB outburst through the different spectral states. An XRB starts an outburst in the low-hard state, progresses to the hard state through intermediate states to the high-soft state, then down to the soft state, and then back to the low-hard state to begin again (described in more detail in Sect. 1).