We study the conditions required for the production of the synchrotron maser emission downstream of a relativistic shock. We show that for weakly magnetized shocks, synchrotron maser emission can be generated at frequencies significantly exceeding the relativistic gyrofrequency. This high-frequency maser emission seems to be the most suitable for interpreting peculiar GHz radio sources. To illustrate this, we consider a magnetar flare model for FRBs. Our analysis shows that the maser emission is radiated away from the central magnetar, which guarantees a short duration of bursts independently of the shock wave radius. If FRBs are produced by the high-frequency maser emission then one can significantly relax the requirements for several key parameters: the magnetic field strength at the production site, luminosity of the flare, and the production site bulk Lorentz factor. To check the feasibility of this model, we study the statistical relation between powerful magnetar flares and the rate of FRBs. The expected ratio is derived by convoluting the redshift-dependent magnetar density with their flare luminosity function above the energy limit determined by the FRB detection threshold. We obtain that only a small fraction, (sim10^{-5}), of powerful magnetar flares trigger FRBs. This ratio agrees surprisingly well with our estimates: we obtained that (10%) of magnetars should be in the evolutionary phase suitable for the production of FRBs, and only (10^{-4}) of all flares are expected to be weakly magnetized, which is a necessary condition for the high-frequency maser emission.
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