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Blast Waves from Magnetar Flares and Fast Radio Bursts

Magnetars younger than one century are expected to be hyper active. Besides winds powered by rotation they generate frequent magnetic flares, which launch powerful blast waves into the wind. These internal shocks act as masers producing fast (millisecond) radio bursts (FRBs) with the following properties. (1) GHz radio emission occurs at radii $r\sim 10^{14}$ cm and lasts $\lesssim 1$ ms in observer's time. (2) Induced scattering in the surrounding wind does not suppress the radio burst. (3) The emission has linear polarization set by the magnetar rotation axis. (4) The emission drifts to lower frequencies during the burst, and its duration broadens at lower frequencies. (5) Blast waves in inhomogeneous winds may emit variable bursts; periodicity might appear on sub-ms timescales if the magnetar rotates with $\sim 1$ s period. However, the observed FRB structure is likely changed by lensing effects during propagation through the host galaxy. (6) The FRBs from magnetars are expected to repeat, with rare strong bursts (up to $\sim 10^{43}$ erg) or more frequent weak bursts. (7) When a repeating flare strikes the wind bubble in the tail of a previous flare, the FRB turns into a bright optical flash. Its luminosity may approach that of a supernova Ia and last seconds. The rate of these optical flashes in the universe is much lower than the FRB rate, however it may exceed the supernova rate. Locations of hyper-active magnetars in their host galaxies depend on how they form: magnetars created in supernovae explosions will trace star formation regions, and magnetars formed in mergers of compact objects will be offset. The merger magnetars are expected to be most energetic and particularly hyper-active.