Radiatively-driven shock waves in quasar envelopes

The authors study the response of a quasar envelope to periodic disturbances occurring below the surface, using a simple form for the density-dependent radiative acceleration. Quasi-steady winds flow, with strong shock waves propagating outwards as the gas absorbs momentum from the quasar radiation field. The compressed gas behind a shock has high speed in the quasar frame and so can yield broad emission lines. A short wave approximation method yields self-consistent solutions in which strong shocks forming near the surface decelerate outwards and steadily weaken, but does not describe rigorously solutions with shock-trains that simultaneously accelerate and strengthen. Micro-physical limitations on the parameters of the macro-solutions are discussed. Difficulties emerge in attempting to account for the broadest observed emission lines, suggesting that radiation driving plays an auxiliary rather than a dominant role in the dynamics of quasar envelopes.