Temperature and Entropy of a Quantum Black Hole and Conformal Anomaly

Attention is paid to the fact that temperature of a classical black hole can be derived from the extremality condition of its free energy with respect to variation of the mass of a hole. For a quantum Schwarzschild black hole evaporating massless particles the same condition is shown to result in the following one-loop temperature $T=(8\pi M)^{-1} (1+\sigma (8\pi M^2)^{-1})$ and entropy $S = 4\pi M^2 - \sigma\log M$ expressed in terms of the effective mass $M$ of a hole together with its radiation and the integral of the conformal anomaly $\sigma$ that depends on the field species. Thus, in the given case quantum corrections to $T$ and $S$ turn out to be completely provided by the anomaly. When it is absent ($\sigma=0$), which happens in a number of supersymmetric models, the one-loop expressions of $T$ and $S$ preserve the classical form. On the other hand, if the anomaly is negative ($\sigma<0$) an evaporating quantum hole seems to cease to heat up when its mass reaches the Planck scales.