Nonlinear Propagation of Intense Electromagnetic Waves in Quasar and Pulsar Plasmas

Abstract

We present the results of analytical as well as numerical investigations of radiation-plasma interaction instabilities in astrophysical plasmas. We consider the stimulated Raman and Compton scattering in the continuum emission of quasars. There are three ways in which an electromagnetic wave can undergo scattering in a plasma : (i) when the scattering of radiation occurs by a single electron, it is called Compton scattering; (ii) if it occurs by a longitudinal electron plasma mode, it is called Stimulated Raman Scattering (SRS), and (iii) if it occurs by a highly damped electron plasma mode, it is called Stimulated Compton Scattering. The non-thermal continuum of quasars is believed to be produced through the combined action of synchrotron and inverse Compton processes, which are essentialy single particle processes. We have shown as an example that the complete spectrum of 3C 273 can be reproduced by suitably combining Stimulated Compton Scattering and SRS (Gangadhara and Krishan 1992). The differential contributions of these stimulated scattering processes under different values of the plasma parameters are also calculated. The coherent plasma process such as parametric decay instability in a homogeneous and unmagnetized plasma, cause anomalous absorption of intense electromagnetic radiation under specific conditions of energy and momentum conservation and thus cause anomalous heating of the plasma. The maximum plasma temperatures reached are functions of luminosity of the radio radiation and plasma parameters. It is believed that these processes may be taking place in many astrophysical objects. These processes can also contribute towards the absorption of 21-cm radiation, which is otherwise mostly attributed to neutral hydrogen regions (Krishan 1988). The change in polarization of an electromagnetic wave can occur due to SRS in a plasma. In this process an electromagnetic wave undergoes coherent scattering off an electron plasma wave. It is found that some of the observed polarization properties such as the rapid temporal variations, sense reversal, rotation of the plane of polarization and change of nature of polarization in the case of pulsars and quasars, could be accounted for through SRS. The modulational instability of a large-amplitude electromagnetic wave in an electron-positron plasma, can excite due to the effect of relativistic mass variation of the plasma particles, harmonic generation, and the non-resonant, finite frequency electrostatic density perturbations, all caused by the large-amplitude radiation field. The radiation from many strong sources such as quasars and pulsars, has been observed to vary over a host of time-scales. It is possible that extremely rapid variations in the non-thermal continuum of quasars as well as in the non-thermal radio radiation from pulsars can be accounted for by the modulational instabilities to which the radiation may be subjected during its propagation out of the emission region.