Time delays as a function of frequency, expressed in minutes before the time of maximum flux density at λ = 1.57 cm (19 GHz). Colored lines indicate the model time-lag predictions for different constant outflow and inflow velocities. Orange (light gray) lines indicate outflow, blue (dark gray) lines indicate inflow. The data points, derived from the full-data FRED curve fits, suggest an outflow with a moderately relativistic velocity, clustering around γβ = 2. This figure is valid for an inclination angle of 90 degrees and is adapted from Falcke et al. (2009).