FRB coherent emission from decay of Alfvén waves

Abstract

We present a model for FRBs where a large amplitude Alfvén wave packet is launched by a disturbance near the surface of a magnetar, and a substantial fraction of the wave energy is converted to coherent radio waves at a distance of a few tens of neutron star radii. The wave amplitude at the magnetar surface should be about 10 11 G in order to produce a FRB of isotropic luminosity 10 44 erg s −1 . An electric current along the static magnetic field is required by Alfvén waves with non-zero component of transverse wave-vector. The current is supplied by counter-streaming electron-positron pairs, which have to move at nearly the speed of light at larger radii as the plasma density decreases with distance from the magnetar surface. The counter-streaming pairs are subject to two-stream instability which leads to formation of particle bunches of size of order c/ω p ; where ω p is plasma frequency. A strong electric field develops along the static magnetic field when the wave packet arrives at a radius where electron-positron density is insufficient to supply the current required by the wave. The electric field accelerates particle bunches along the curved magnetic field lines, and that produces the coherent FRB radiation. We provide a number of predictions of this model.

Emission Mechanisms of Fast Radio Bursts

“…It was assumed [35] that at relativistic shocks, synchrotron maser instability develops. Numerical simulations of relativistic, magnetized shocks [36][37][38][39][40][41][42][43] revealed strong electromagnetic precursors with the characteristic frequencies compatible with the estimate (21).…”

“…Since then, the popular view is that bunches of charged particles are somehow formed in the magnetosphere, and that pulsar radio emission could be attributed to the curvature emission of these bunches. Recently, the same model was applied to FRBs [12][13][14][15][16][17][18][19][20][21][22][23][24].…”

“…Please note that the frequencies (21) are in fact given in the downstream frame. In the upstream frame, they are Lorentz-boosted so that the frequencies of the emitted waves are well above the local plasma and Larmor frequencies.…”

Scintillation Can Explain the Spectral Structure of the Bright Radio Burst from SGR 1935+2154

“…The dissipation is predicted to result in the acceleration of charge bunches with characteristic scale corresponding to the plasma-oscillation scale, resulting in a radio burst. A strength of this model is that the characteristic burst energies and frequencies naturally result from typical magnetar characteristics (Kumar & Bošnjak 2020). In general, "close-in" models of emission in the magnetosphere cannot produce the large transverse motion (or separation) of 8.3×10 7 m that we infer in our scintillation model, because this is similar to the radius of the light cylinder.…”

“…We conclude in Section 4 with a discussion of how future observations of SGR 1935+2154 may be able to test our interpretation of the spectral structure as scintillation. Kumar & Bošnjak (2020) to account for the joint observation of a hard X-ray burst from SGR 1935+2154 together with the radio burst. In this model, Alfvén waves are launched along field lines near the magnetic poles by a crustal disturbance, and dissipate upon charge starvation at heights of tens of r NS (or hundreds of kilometers).…”