Spectral Evolution of Persistent and X-ray Burst Emission of SGR 1935+2154 during its 2022 Outburst

Chin-Ping Hu^1*

¹Departmemt of Physics, National Changhua University of Education, Taiwan

* Presenter:Chin-Ping Hu, email:cphu0821@gm.ncue.edu.tw

SGR 1935+2154 stands out as a key example linking fast radio bursts (FRBs) with magnetar activity. During its 2022 outburst, an FRB occurred between two spin-up glitches, suggesting that these glitches may have triggered the multiwavelength energetic phenomena observed. However, the specific mechanisms that change the magnetar environment to allow for radio emission remain unclear. In this study, we present high-cadence NICER and NuSTAR observations that reveal spectral changes in both burst and persistent emissions. Analysis of the hardness ratio evolution and detailed spectral behavior indicates significant spectral change during an intermediate flare approximately 2.5 hours prior to the FRB on October 14, 2022. The hardness-intensity diagram shows a distinct counterclockwise pattern, with the intermediate flare, lasting 80 seconds and releasing $(6.3\pm0.2)\times10^{40}$ erg, marking a pivotal transition characterized by rapid spectral softening and a notable decrease in burst occurrence rate. The burst spectra observed before the flare are hard, with a curved shape peaking around 30 keV. In contrast, the spectra detected after the flare peak near 5 keV and exhibit a soft tail, resembling the characteristics of the FRB-associated X-ray burst. We suggest that the flare resulted in an untwisting of magnetic field lines, facilitating the production of collimated radio emission. However, the narrow opening angle of this emission may explain the relative rarity of FRB detections from magnetars, even though many post-flare bursts exhibit spectral characteristics similar to the FRB-associated X-ray burst.

Keywords: magnetars, neutron stars, fast radio bursts