Seminar by Emanuele Sobacchi
Zoom - January 16, 2025 - 15.00
WEBINAR LINK
Title: Fast radio bursts as strong electromagnetic waves.
Abstract: Understanding the propagation of electromagnetic waves in plasmas is important for astrophysics. The discovery of fast radio bursts makes it crucial to consider the regime in which electrons oscillate with relativistic velocities in the field of the wave. In this regime, the wave strength parameter is a >> 1 (where a is defined as the peak transverse component of the electron four-velocity in units of the speed of light). The propagation of electromagnetic waves in plasmas is governed by Maxwell's and two-fluid equations. These equations contain nonlinear terms. In the nonrelativistic limit a << 1, nonlinear terms can be treated as a small perturbation by making an expansion in powers of a, making it possible to study the wave propagation analytically. In this talk, I will discuss the propagation of strong (a >> 1) electromagnetic waves in tenuous plasmas, where the wave frequency, omega, is much larger than the plasma frequency, omega_P. I will show that in pair plasmas nonlinear effects are weak for a << omega/omega_P. In electron-proton plasmas a more restrictive condition must be satisfied, namely a << 1/(omega_P*tau), where tau is the duration of the radiation pulse. These results have important implications for the modeling of fast radio bursts. I will argue that millisecond duration bursts with a smooth profile must be produced in a proton-free environment, where nonlinear effects are weaker. On the other hand, propagation through an electron-proton plasma near the source can imprint a sub-microsecond variability on the burst profile. Finally, I will discuss constraints for theoretical models of fast radio burst emission.