The confrontation of the shock-powered synchrotron maser model with the Galactic FRB 200428

The recent discovery of a fast radio burst (FRB 200428) from the Galactic magnetar SGR 1935+2154 robustly indicated that FRB phenomena can sometimes be produced by magnetars, although it is uncertain whether the cosmological FRBs can share the same origin with this Galactic event. The association of FRB 200428 with an X-ray burst (XRB) further offers important implications for the physical processes responsible for the FRB phenomena. By assuming that the XRB emission is produced in the magnetosphere of the magnetar, we investigate the possibility of that the FRB emission is produced by the synchrotron maser (SM) mechanism, which is powered by a shock due to the collision of an $e^{\pm}$ ejecta with a baryonic cloud. It is found that this shock-powered SM model can in principle account for the FRB 200428 observations, if the collision just occurred on the line of sight and the ejecta lunched by magnetar bursts can have appropriate ingredients and structures. To be specific, a burst ejecta should consist of an ultra-relativistic and extremely highly collimated $e^{\pm}$ component and a sub-relativistic and wide-spreading baryonic component. The cloud blocking the $e^{\pm}$ ejecta is just a remnant of a previous baryonic ejecta. Meanwhile, as a result of the synchrotron emission of the shocked material, an intense millisecond X-ray pulse is predicted to overlap the magnetosphere XRB emission, which in principle provides a way to test the model. Additionally, the peak frequency of the SM radiation is constrained to be about a few hundred MHz and the radiation efficiency is around $10^{-4}$.