Pair Separation in Parallel Electric Field in Magnetar Magnetosphere and Narrow Spectra of Fast Radio Bursts

Abstract

When the magnetosphere of a magnetar is perturbed by crustal deformation, an electric field E ∥ parallel to the magnetic field line would appear via Alfvén waves in the charge starvation region. The electron–positron pair bunches will be generated via two-stream instability in the magnetosphere, and these pairs will undergo charge separation in the E ∥ and in the meantime emit coherent curvature radiation. Following the approach of Yang & Zhang, we find that the superposed curvature radiation becomes narrower due to charge separation, with the width of spectrum depending on the separation between the electron and positron clumps. This mechanism can interpret the narrow spectra of fast radio bursts (FRBs), in particular, the spectrum of Galactic FRB 200428 recently detected in association with a hard X-ray burst from the Galactic magnetar SGR J1935+2154.

The physical mechanisms of fast radio bursts

“…Since the observed FRB duration is much longer than the bunch disperse timescale, this model requires a continuous generation of sparks throughout the duration of each FRB. The application of this mechanism to FRBs has been discussed by various authors 79,95,96,111,112,125 . One critical requirement is that there should be a strong electric field parallel to the local magnetic field (E ) in the FRB emission region 95 .…”

“…200428, by Occam's Razor 79,125,139 . A GRB-like model interprets the observed X-rays as the superposition of magnetospheric and shock emissions, with the latter component consistent with the relatively hard spectra during the two lightcurve peaks 138 .…”

Emission Mechanisms of Fast Radio Bursts

“…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].…”

“…The plasma in the pulse moves with the Lorentz factor (34) with respect to the wind. In a highly magnetized medium, the shock propagates with respect to the downstream plasma with the fast magnetosonic Lorentz factor √ σ, Equation (24). Therefore, in the lab frame, the shock Lorentz factor is Γ shock = 2Γσ 1/2 wind .…”

Upper Field Strength Limit of Fast Radio Bursts

“…For the models employing solitary neutron stars, possible FRB sources are located within or out of magnetospheres surrounding neutron stars. In the magnetosphere, the radiation mechanisms have plasma maser emissions from relativistic plasmas or plasma instabilities [14], and curvature radiation of charged bunches [15][16][17][18]. Outside of magnetosphere, relativistic shocks driven by outflows from neutron stars may also induce FRBs [19][20][21][22].…”