GECAM discovery of a peculiar magnetar X-ray burst (MXB 221120) from SGR J1935+2154 associated with a fast radio burst

Fast radio bursts (FRBs) are enigmatic cosmic transients of millisecond duration observed in the radio band. The identification of FRB-associated magnetar X-ray bursts (MXBs) from the galactic magnetar SGR J1935+2154 suggests that at least a fraction of FRBs can be produced from magnetar activity. However, the sample size of FRB-associated MXBs is still very small. Here we report a bright and peculiar FRB-associated MXB from SGR J1935+2154 detected by GECAM on November 20, 2022, dubbed MXB 221120. We find that the temporal and spectral properties of MXB 221120 exhibit distinctive features. Its light curve could generally be described by a single FRED function with the superposition of several narrow pulses. Interestingly, we identified a possible quasiperiodic oscillation feature with a center frequency of ∼18 Hz in this MXB. The time-integrated spectrum is best fit by a blackbody model with a temperature ( kT ) of 18.6 keV, rendering it the first thermal spectrum FRB-associated MXB from SGR J1935+2154. Compared to other MXBs with a single emission episode, MXB 221120 has a longer duration and a higher blackbody temperature, making it an outlier in the burst sample. These results indicate that MXB 221120 may have been produced by a special mechanism with extreme physical conditions.