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 ( \lesssim 0.1 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 \nu _ { p } \sim 0.03 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 \Delta t _ { FRB } = \Delta t _ { \gamma } \approx 0.03 s , while it would predict \Delta t _ { FRB } \ll \Delta t _ { \gamma } .
( 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 discussed four possibilities to study radio burst properties for future observations , especially the possibility of high-frequency ( 0.5 - 2 ms ) quasi-periodic oscillations in double/multiple-peaked FRBs .