Color plots: flux maps for the geometry of Fig. 1. The additional white panels show the corresponding energy-dependent phase shifts as calculated with respect to the full energy pulse profile using cross-correlation as described in detail and applied to observational data by Ferrigno et al. (2011). Left: static plasma. Right: downward bulk velocity of v = 0.4c. The values of B, kT_e, and τ as well as the flux scaling are the same as in Fig. 3 (see Sect. 3.1).