Fast radio bursts as synchrotron maser emission from decelerating relativistic blast waves

Fast radio bursts (FRBs) can arise from synchrotron maser emission at ultrarelativistic magnetized shocks, such as produced by flare ejecta from young magnetars. We combine particle-in-cell simulation results for the maser emission with the dynamics of self-similar shock deceleration, as commonly applied to gamma-ray bursts (GRBs), to explore the implications for FRBs. The upstream environment is a mildly relativistic baryon-loaded shell released following a previous flare, motivated by the high electron-ion injection rate M ∼ 10¹⁹-10²¹ g s⁻¹ needed to power the persistent radio nebula coincident with the repeating burster FRB 121102 and its high rotation measure. The radio fluence peaks once the optical depth ahead of the shock to induced Compton scattering τ_c 3. Given intervals between major ion ejection events T ∼ 10⁵ s similar to the occurrence rate of the most powerful bursts from FRB 121102, we demonstrate the production of ∼0.1-10 GHz FRBs with isotropic radiated energies ∼10³⁷-10⁴⁰ erg and durations ∼0.1-10 ms for flare energies E ∼ 10⁴³-10⁴⁵ erg. Deceleration of the blast wave, and increasing transparency of the upstream medium, generates temporal decay of the peak frequency, similar to the observed downward frequency drift seen in FRB 121102 and FRB 180814.J0422+73. The delay T 10⁵ s between major ion-injection events needed to clear sufficiently low densities around the engine for FRB emission could explain prolonged 'dark periods' and clustered burst arrival times. Thermal electrons heated at the shock generate a short-lived 1 ms (1 s) synchrotron transient at gamma-ray (X-ray) energies, analogous to a scaled-down GRB afterglow.