Broadband X-ray burst spectroscopy of the fast-radio-burst-emitting Galactic magnetar

Magnetars are young, magnetically-powered neutron stars possessing the strongest magnetic fields in the Universe. Fast Radio Bursts (FRBs) are extremely intense millisecond-long radio pulses of primarily extra galactic origin, and a leading attribution for their genesis focuses on magnetars. A hallmark signature of magnetars is their emission of bright, hard X-ray bursts of sub-second duration. On April 27th 2020, the Galactic magnetar SGRJ1935+2154 emitted hundreds of X-ray bursts in a few hours. One of these temporally coincided with an FRB, the first detection of an FRB from the MilkyWay. Here we present spectral and temporal analyses of 24 X-ray bursts emitted13 hours prior to the FRB and seen simultaneously with the NASA NICER and Fermi/GBM missions in their combined energy range, 0.2 keV-30 MeV. These broadband spectra permit direct comparison with the spectrum of the FRB-associated X-ray burst (FRB-X). We demonstrate that all 24 NICER/GBM bursts are very similar temporally, albeit strikingly different spectrally, from FRB-X. The singularity of the FRB-X burst is perhaps indicative of an uncommon locale for its origin. We suggest that this event originated in quasi-polaropen or closed magnetic field lines that extend to high altitudes.