The FRB-SGR Connection

TL;DR

The paper links FRB 200428 with SGR 1935+2154, proposing a model where coherent curvature radiation from electron bunches explains the observed phenomena and their relation to SGR luminosity.

Contribution

It introduces a Lorentz factor-based model for FRB emission from SGRs, explaining why some SGRs produce FRBs while others do not, and clarifies the role of electron dynamics and scattering.

Findings

FRB 200428 is associated with SGR 1935+2154.

High Lorentz factors are required for radiating bunches.

Luminosity determines whether SGRs emit FRBs.

Abstract

The discovery that the Galactic SGR 1935$+$2154 emitted FRB 200428 simultaneous with a gamma-ray flare demonstrated the common source and association of these phenomena. If FRB radio emission is the result of coherent curvature radiation, the net charge of the radiating "bunches" or waves may be inferred from the radiated fields, independent of the mechanism by which the bunches are produced. A statistical argument indicates that the radiating bunches must have a Lorentz factor $\gtrapprox 10$. The observed radiation frequencies indicate that their phase velocity (pattern speed) corresponds to Lorentz factors $\gtrapprox 100$. Coulomb repulsion implies that the electrons making up these bunches may have yet larger Lorentz factors, limited by their incoherent curvature radiation. These electrons also Compton scatter the soft gamma-rays of the SGR. In FRB 200428 the power radiated coherently at radio frequencies exceeded that of Compton scattering, but in more luminous SGR outbursts Compton scattering dominates, precluding the acceleration of energetic electrons. This explains the absence of a FRB associated with the giant 27 December 2004 outburst of SGR 1806$-$20. SGR with luminosity $\gtrsim 10^{42}$ ergs/s do not emit FRB, while those of lesser luminosity can do so. "Superbursts" like FRB 200428 are produced when narrowly collimated FRB are aligned with the line of sight; they are unusual, but not rare, and "cosmological" FRB may be superbursts.