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Recently, a fast radio burst (FRB)-like event is found to be associated witha Galactic magnetar, SGR 1935+2154, accompanied by an X-ray burst. We find thisradio burst challenges the typical emission mechanisms involving magnetars,which includes coherent curvature radiation from charged bunches, fastmagnetosonic (FMS) wave, synchrotron maser from shocks, and the pulsar-likemechanism for low-twist magnetars. More specifically, we find that (1) theX-rays are most-likely to be produced inside the magnetosphere. (2) For thecoherent curvature radiation from the decay of Alfv\'en wave, it will generallypredict a duration ($\lesssim0.1$ ms) smaller than observations, because of thestrong twists of magnetic field lines and the internal damping of Alfv\'enwaves. (3) The FMS wave model predicts a very low emission frequency $\nu_{\rmp}\sim0.03$ MHz $\ll$ GHz, unless it is produced inside the magnetosphere. Butthe absorption effect of the magnetospheric FMS wave model remains to bestudied. (4) The synchrotron maser model is challenged, because observationsshow that the peaks in both X-ray and radio light curves are with the sametemporal separation $\Delta t_{\rm FRB}=\Delta t_\gamma\approx0.03$ s, while itwould predict $\Delta t_{\rm FRB}\ll\Delta t_\gamma$. (5) It seems to bedifficult to directly apply the low-twist pulsar-like mechanism to flaringmagnetars, as magnetar activity can significantly deform the magnetosphere. (6)We suggested four possibilities to study the general properties of FRBs forfuture observations, especially the possibility of identifying quasi-periodicoscillations with period $\sim1-10$ ms in double/multiple-peaked FRBs.

The radio/X-ray burst from SGR 1935+2154: radiation mechanisms and the possible QPOs