Coherent emission in pulsars, magnetars and Fast Radio Bursts

Maxim Lyutikov (Purdue University)

February 26, 2021

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Abstract: We develop a model of the generation of coherent radio emission in the Crab pulsar, magnetars and Fast Radio Bursts (FRBs). Emission is produced by a reconnection-generated beam of particles via a Free Electron Laser (FEL) mechanism, operating in a weakly-turbulent, guide-field dominated plasma; the radio emission is not rotationally but reconnection powered. Alfvenic turbulence creates both the FEL wiggler and, via a ponderomotive force, charge bunches in the beam that Compton scatter the wiggler field coherently. The model is both robust to the underlying plasma parameters and succeeds in reproducing a number of subtle observed features: (i) emission frequencies depend mostly on the properties of turbulence and the Lorentz factor of the reconnection generated beam, omega∼gamma_0^2(ck_w) – the emission frequency is independent of the absolute value of the underlying magnetic field. (ii) The model explains both broadband emission and the presence of emission stripes, including multiple stripes observed in the High Frequency Interpulse of the Crab pulsar. (iii) The model reproduces correlated polarization properties: presence of narrow emission bands in the spectrum favors linear polarization, while broadband emission can have arbitrary polarization; this matches the FRB phenomenology. (iv) The mechanism is robust to the momentum spread of the particle beam. (v) The model even reproduces brightness temperatures observed in FRB and Crab GPs (∼10^40 K and ∼10^33 K correspondingly). Presence of several distinct harmonic wigglers may further enhance the efficiency of the FEL due to large amplitude beat oscillations and the creation of longer living density patterns. We also discuss a model of wigglers as non-linear force-free Alfven solitons, limited both in the transverse and longitudinal directions.