Constraints on the engines of fast radio bursts

We model the sample of fast radio bursts (FRBs), including the newly discovered CHIME repeaters, using the decelerating synchrotron maser blast wave model of Metzger, Margalit & Sironi (2019), which built on earlier work by Lyubarsky (2014), Beloborodov (2017). This model postulates that FRBs are precursor radiation from ultrarelativistic magnetized shocks generated as flare ejecta from a central engine collides with an effectively stationary external medium. Downward drifting of the burst frequency structure naturally arises from the deceleration of the blast wave coupled with the dependence of the maser spectral energy distribution, and induced Compton scattering depth, on the upstream medium. The data are consistent with FRBs being produced by flares of energy E_flare ∼ 10⁴³-10⁴⁶(f_ξ/10^-3)^-4/5 erg, where f_ξ is the maser efficiency, and minimum bulk Lorentz factors Γ ≈ 10²-10³, which generate the observed FRBs at shock radii r_sh ∼ 10¹²-10¹³ cm. We infer upstream densities n_ext(r_sh) ∼ 10²-10⁴ cm^-3 and radial profiles n_ext ∝ r^-k showing a range of slopes k ≈ [ - 2, 1] (which are seen to evolve between bursts), both broadly consistent with the upstream medium being the inner edge of an ion-loaded shell released by a recent energetic flare. The burst time-scales, energetics, rates, and external medium properties are consistent with repeating FRBs arising from young, hyperactive flaring magnetars, but the methodology presented is generally applicable to any central engine which injects energy impulsively into a dense magnetized medium. Several uncertainties and variations of the model regarding the composition and magnetization of the upstream medium, and the effects of the strong electric field of the FRB wave (strength parameter a ≫ 1) on the upstream medium and its scattering properties, are discussed. One-dimensional particle-in-cell simulations of magnetized shocks into a pair plasma are presented which demonstrate that high maser efficiency can be preserved, even in the limit a ≫ 1 in which the FRB wave accelerates the upstream electrons to ultrarelativistic speeds.