Blast Waves from Magnetar Flares and Fast Radio Bursts

Magnetars younger than one century are expected to be hyperactive: besides winds powered by rotation, they can generate frequent magnetic flares, which launch powerful blast waves into the wind. The resulting magnetized shocks act as masers producing bright radio emission. This theoretical picture predicts radio bursts with the following properties. (1) GHz radio emission occurs at radii $r\sim {10}^{14}cm$ and lasts $\lesssim 1$ ms in the 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 radio bursts; periodicity might appear on submillisecond timescales if the magnetar rotates with a ∼1 s period. However, the observed burst structure is likely changed by lensing effects during propagation through the host galaxy. (6) The magnetar bursts should repeat, with rare ultrastrong events (possibly up to ∼10⁴³ erg in radio waves) or more frequent weak bursts. (7) When a repeating magnetic flare strikes the wind bubble in the tail of a previous flare, the radio burst turns into a bright optical burst lasting $\lesssim 1s$. Locations of hyperactive magnetars in their host galaxies depend on how they form: magnetars created in supernova explosions will trace star formation regions, and magnetars formed in mergers of compact objects will be offset. The merger magnetars are expected to be particularly hyperactive.