Predicted X-ray flare luminosities arising from Compton upscattering of disk and flare synchrotron photons as a function of the line-of-sight bulk velocity of the IR-emitting flare source: IR flare photons by thermal electrons in the disk (green), submillimeter photons from the disk by the nonthermal electrons in the IR flare source (cyan), and SSC in the IR flare source (purple). The gray line indicates the peak X-ray luminosity detected by NuSTAR: this can only be achieved if the IR-emitting electrons are moving away from the observer and toward the disk with bulk speed v ∼ 0.7c. The thickness of the Channel I and II curves shows the flare source size dependence (see text for further explanation).