Raman scattering in high-radio-brightness astrophysical systems: application to active galactic nuclei

Under conditions of high brightness temperature, stimulated Raman scattering of incident radio waves by Langmuir waves can occur. This proces is analysed for a broad-band radio spectrum, and analytic and numerical models for special cases are presented. Two cases are identified. In strong Raman scattering, the radio brightness temperature is so large that the intensity of the Langmuir waves responsible for the scattering grows to non-linear strength and the radio waves are quickly scattered. In weak Raman scattering, the electrostatic wave intensity is determined by a balance between parametric growth and ion-electron collisional damping. In either case, back-scattering will only occur if the electron density, n, is high enough to allow Langmuir waves of short enough wavelength to propagate without Landau damping; at lower density, the radio waves will be scattered through an angle $$\sim0.2(n/10^6 \text {cm}^{-3})^{1/2}(T/10^6\text K)^{-1/2}(v/1\enspace \text {GHz})^-1$$. In the weak, back-scattering regime, the reflection length is roughly $$3\times10^{11}(T/10^6\text K)^{-3/2}(v/1\enspace\text {GHz})^{-2}(T_\text B/10^15 \text K)^-2 (T_\text B/10^15 \text K)^-2\text {cm}$$, independent of density, where T_B is the brightness temperature. It is argued that intraday variability in compact extragalactic radio sources is probably not caused by a coherent emission mechanism with brightness temperature ∼10¹⁸ K, because the plasma density within and around the source would then have to be unreasonably low. Implications for space very long baseline interferometry (VLBI) and for tracing accretion flows in active galactic nuclei are also briefly discussed. By contrast, source models where the brightness temperature $$\gtrsim 10^{14}\text K$$ are not seriously constrained by Raman scattering.