Change in intensity of the blackbody spectrum with kT_rad = 10 keV under different temperatures of thermal electron gas calculated by use of Eqs. (39) and (40) as a function of dimensionless frequency X = hν/(kT_rad), with scaling . kT_rad and kT_e are labeled in the figures. The results obtained under the extended Kompaneets equation and the classical Kompaneets equation are shown as blue and red lines, respectively.