PROFESSOR DIPANJAN MITRA

Collective plasma emissions in astrophysics are referred to as coherent emission, which can be observationally distinguished by their high brightness (or equivalent blackbody) temperatures. Radio emission from pulsars have brightness temperatures around 1028 K, exceeding any incoherent emission process limit, and can be classified as an extreme example of coherent emission. After five decades of pulsar discovery the problem of how collective plasma process excites the coherent radio emission still remains an unresolved problem. However, observational and phenomenological studies of pulsars have made noteworthy progress in obtaining two useful constraints : (i) the coherent radio emission is excited in a relativistic pair plasma at frequency (¿cr), which is below the local plasma frequency (¿o), i.e. ¿cr < ¿o and (ii) the radio emission is excited by curvature radiation due to acceleration of “charge bunches”. Theoretically it is challenging to form a stable charge bunch that follow the above observational constraints. In this talk we will propose a potential resolution of this problem, based on a combination of linear and non-linear plasma theory of the growth of Langmuir waves in the relativistic pair plasma, which can form such stable charge bunches. In the non-linear regime we will demonstrate the existence of relativistic charge solitons (the “charge bunches”) as solutions of non-linear Schrodinger equation (NLSE). We have included the crucial effect of non-linear Landau damping, which is inherent in the derivation of the NLSE. Using reasonable pulsar parameters we demonstrate the emergence of intense soliton-like pulses from an initial disordered state of Langmuir waves. These pulses can propagate stably over sufficiently long times and are capable of exciting coherent curvature radiation. The emission mechanism presented here has potential for wider application and can be extended to explain other astrophysical phenomenon like the Fast Radio Bursts.