Hardness-intensity diagram (HID) for Swift J1644+57 using the Swift observations (2011–2012) during spectral evolution from the high state to the low state. In the vertical axis, the hardness ratio (HR) is a ratio of the source counts in the two energy bands: the hard (1.5–10 keV) and soft (0.3–1.5 keV). HR decreases with a source brightness in the 0.3–10 keV range (horizontal axis). For clarity, we plot only one point with error bars (in the bottom right corner) to demonstrate typical uncertainties for the count rate and HR.