Thermodynamics of 2D string theory

We calculate the free energy, energy and entropy in the matrix quantum mechanical formulation of 2D string theory in a background strongly perturbed by tachyons with the imaginary minkowskian momentum +/-i/R ("Sine-Liouville" theory). The system shows a thermodynamical behaviour corresponding to the temperature T = 1/(2piR). We show that the microscopically calculated energy of the system satisfies the usual thermodynamical relations and leads to a non-zero entropy.