Emission peak of the KERRBB spectrum in the same position for different values of the black hole mass M, accretion rate Ṁ and spin a which shows that there is a family of solutions. Consider spectrum A (thick solid blue line): if M increases at constant Ṁ and spin, spectrum B is obtained. Now, if the spin increases, but M and Ṁ are constant, spectrum C is obtained because a larger spin corresponds to a larger efficiency. Finally, if Ṁ decreases, spectrum D is obtained, which is almost exactly equal to the initial spectrum A (they differ slightly in the high-frequency, exponential part).