Brightness distribution of Vela X-1 throughout its orbit as measured with Swift/BAT (15–50 keV, top panel) and MAXI (2–20 keV, bottom panel). In comparison, 1 Crab corresponds to values ∼0.22 for Swift/BAT and ∼4 for MAXI. We used the full history for each instrument, binned to a resolution of 1 d. The mean orbital flux profile is superimposed in black. The probability for a measurement to fall into the respective histogram bin is color-coded according to the color scales on the right. The profile is repeated once for clarity. For details, see text.