Double-peaked Pulse Profile of FRB 200428: Synchrotron Maser Emission from Magnetized Shocks Encountering a Density Jump

Very recently a fast radio burst (FRB) 200428 associated with a strong X-ray burst from the Galactic magnetar SGR 1935+2154 was detected; this provides direct evidence supporting the magnetar progenitor models of FRBs. Assuming that the FRB radiation mechanism is synchrotron maser emission from magnetized shocks, we develop a specific scenario by introducing a density-jump structure of upstream medium, thus making the double-peaked character of FRB 200428 a natural outcome. The luminosity and emission frequency of two pulses can be well explained in this scenario. Furthermore, we find that the synchrotron emission of shock-accelerated electrons is in the X-ray band, and therefore can be responsible for at least a portion of observed X-ray fluence. With the proper upgrade, in the future this density-jump scenario could be applied to FRBs with multiple peaks.