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

Recently, a fast radio burst (FRB)-like event is found to be associated with a Galactic magnetar, SGR 1935+2154, accompanied by an X-ray burst. We find this radio burst challenges the typical emission mechanisms involving magnetars, which includes coherent curvature radiation from charged bunches, fast magnetosonic (FMS) wave, synchrotron maser from shocks, and the pulsar-like mechanism for low-twist magnetars. More specifically, we find that (1) the X-rays are most-likely to be produced inside the magnetosphere. (2) For the coherent curvature radiation from the decay of Alfvén wave, it will generally predict a duration ($\lesssim0.1$ ms) smaller than observations, because of the strong twists of magnetic field lines and the internal damping of Alfvén waves. (3) The FMS wave model predicts a very low emission frequency $ν_{\rm p}\sim0.03$ MHz $\ll$ GHz, unless it is produced inside the magnetosphere. But the absorption effect of the magnetospheric FMS wave model remains to be studied. (4) The synchrotron maser model is challenged, because observations show that the peaks in both X-ray and radio light curves are with the same temporal separation $Δt_{\rm FRB}=Δt_γ\approx0.03$ s, while it would predict $Δt_{\rm FRB}\llΔt_γ$. (5) It seems to be difficult to directly apply the low-twist pulsar-like mechanism to flaring magnetars, as magnetar activity can significantly deform the magnetosphere. (6) We suggested four possibilities to study the general properties of FRBs for future observations, especially the possibility of identifying quasi-periodic oscillations with period $\sim1-10$ ms in double/multiple-peaked FRBs.