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

TL;DR

This paper reports a unique, thermal-spectrum magnetar X-ray burst associated with a fast radio burst from SGR J1935+2154, featuring distinctive temporal and spectral properties, including a possible QPO at 18 Hz.

Contribution

It presents the first thermal spectrum FRB-associated magnetar X-ray burst with distinctive features, expanding understanding of magnetar burst diversity.

Findings

Detected a peculiar FRB-associated MXB with a blackbody temperature of 18.6 keV.

Identified a possible QPO feature at ~18 Hz in the burst.

Compared MXB 221120 to other bursts, noting its longer duration and higher temperature.

Abstract

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 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 both temporal and spectral properties of MXB 221120 exhibit distinctive features. Its light curve could be generally described by a single FRED function with superposition of several narrow pulses. Interestingly, we identify a possible QPO feature with center frequency of ~18 Hz in this MXB. The time-integrated spectrum is best fitted by a blackbody model with temperature (kT ) of 18.6 keV, rendering it the first thermal spectrum FRB-associated MXB from SGR J1935+2154. Compared to other MXBs with single emission episode, MXB 221120 has longer duration and higher blackbody temperature, making it an outlier in the burst sample. These results indicate that MXB 221120 may be produced by a special mechanism with extreme physical conditions.