The plasma suppression effect can be ignored in realistic FRB models invoking bunched coherent radio emission

TL;DR

This paper argues that plasma suppression effects are negligible in realistic fast radio burst models that involve bunched coherent emission, due to the physical conditions required for such emission mechanisms.

Contribution

It clarifies that plasma suppression factors derived for pulsars do not apply to FRBs and shows that electric fields in FRB regions negate plasma suppression effects.

Findings

Plasma suppression is insignificant in FRB models with bunched emission.

High electric fields in FRB regions offset plasma suppression effects.

Plasma density in FRB emission regions is generally too low for suppression to matter.

Abstract

One widely discussed mechanism to produce highly coherent radio emission of fast radio bursts (FRBs) is coherent emission by bunches, either via curvature radiation or inverse Compton scattering (ICS). It has been suggested that the plasma oscillation effect can significantly suppress coherent emission power by bunches. We examine this criticism in this paper. The suppression factor formalism was derived within the context of radio pulsars in which radio waves are in the low-amplitude, linear regime and cannot directly be applied to the large-amplitude, non-linear regime relevant for FRBs. Even if one applies this linear treatment, plasma suppression is not important for two physical reasons. First, for an efficient radiation mechanism such as ICS, the required plasma density is not high so that a high-density plasma may not exist. Second, both bunched coherent mechanisms demand that a large global parallel electric field ($E_\parallel$) must exist in the emission region in order to continuously inject energy to the bunches to power an FRB. In order to produce typical FRB duration via coherent curvature or ICS radiation, a parallel electric field must be present to balance the acceleration and radiation back-reaction. The plasma suppression factor should be modified with the existence of $E_\parallel$. We show that the correction factor for curvature radiation, $f_{\rm cur}$, increases with $E_\parallel$ and becomes 1 when $E_\parallel$ reaches the radiation-reaction-limited regime. We conclude that the plasma suppression effect can be ignored for realistic FRB emission models invoking bunched coherent radio emission.