Expected luminosity of the 2.2 MeV line, L_2.2 MeV, as a function of the total accretion luminosity L of an XRP. Both axes are scaled by the critical accretion luminosity L_crit. At low mass accretion rates and luminosity, the flux in the 2.2 MeV line is proportional to the total accretion luminosity. At high mass accretion rates, however, the radiative force decelerates the accretion flow above the NS surface, which results in a sharp drop in luminosity in the 2.2 MeV line.