The Synchrotron Boiler

The steady energy distribution of particles emitting and absorbing synchrotron radiation in compact sources is discussed, under the assumption of constant injection of energetic particles. It is shown that synchrotron reabsorption is an important source of heating for low-energy electrons. Since they efficiently exchange energy with the self-absorbed radiation field, their steady distribution is quasi-thermal, up to the Lorentz factor γ_t, related to the synchrotron self-absorption frequency. The usual distribution, found by balancing injection and cooling rates, is appropriate above γ_t.