Radiative Efficiencies of Continuously Powered Blast Waves

We use general arguments to show that a continuously powered radiative blast wave can behave self-similarly if the energy injection and radiation mechanisms are self-similar. In that case, the power-law indices of the blast wave evolution are set by only one of the two constituent physical mechanisms. If the luminosity of the energy source drops fast enough, the radiation mechanisms set the power-law indices; otherwise, they are set by the behavior of the energy source itself. We obtain self-similar solutions for the Newtonian and the ultrarelativistic limits. Both limits behave self-similarly if we assume that the central source supplies energy in the form of a hot wind and that the radiative mechanism is the semiradiative mechanism of Cohen, Piran, and Sari. We calculate the instantaneous radiative efficiencies for both limits and find that a relativistic blast wave has a higher efficiency than a Newtonian one. The instantaneous radiative efficiency depends strongly on the hydrodynamics and cannot be approximated by an estimate of local microscopic radiative efficiencies, since a fraction of the injected energy is deposited in shocked matter. These solutions can be used to calculate gamma-ray-burst afterglows for cases in which the energy is not supplied instantaneously.