Escape of Fast Radio Bursts from magnetars' magnetospheres

Abstract

We discuss dissipative processes occurring during production and escape of Fast Radio Bursts (FRBs) from magnetars' magnetospheres, the presumed loci of FRBs. High magnetic fields are required in the emission region, both to account for the overall energetics of FRBs, and in order to suppress ``normal'' (non-coherent) radiative losses of radio emitting particles; this limits the emission radii to $\leq {\rm few} \times 10 R_{NS}$. Radiative losses by particles in the strong FRB pulse may occur in the outer regions of the magnetosphere for longer rotation periods, $P\geq 1$ second. These losses are suppressed by several effects: (i) the ponderomotive pre-acceleration of background plasma along the direction of wave propagation (losses reduced approximately as $\gamma_\parallel^{3}$: smaller frequency, $ \propto \gamma_\parallel^2$ in power, and times scales stretched, $ \propto \gamma_\parallel$); this acceleration is non-dissipative and is reversed on the declining part of the pulse; (ii) Landau-Pomeranchuk-Migdal effects (long radiation formation length and ensuing destructive interference of scattered waves). In some cases an FRB pulse may be dissipated on external perturbations (e.g., an incoming pulse of Alfven waves): this may produce a pulse of UV/soft X-rays, a swan song of an FRB, possibly detectable by Chandra.