The 2021 X-ray outburst of magnetar SGR J1935+2154 -- I. Spectral properties

Over a period of four active episodes between January 2021 and January 2022, the magnetar SGR J1935+2154 emitted a total of 343 bursts observed by the \textit{Fermi}/GBM and 82 bursts observed by GECAM-B. Temporal and spectral analyses reveal that the bursts have an average duration of 145 ms and a fluence ranging from $1.2 \times 10^{-9} \ \mathrm{erg \cdot cm^{-2}}$ to $4.1 \times 10^{-4} \ \mathrm{erg \cdot cm^{-2}}$ (8 - 200 keV). The spectral properties of these bursts are similar to those of earlier active episodes. Specifically, we find that the emission area of the Double Black Body (BB2) model shows a Log-Linear correlation to its temperature, and there is a weak relation between fluence and $E_{\mathrm{peak}}$/$\alpha$ in the CPL model. However, we note that the temperature distributions of BB2/BB models in GECAM-B are different from those in \textit{Fermi}/GBM, due to differences in the energy range used for fitting. To understand this difference, we propose a Multi-Temperature Black Body (MBB) model for analyzing thermal radiation, assuming that the BB temperatures follow a power law distribution. Our analysis shows the minimum temperature $kT_{\mathrm{min}} \sim 5$ keV of the MBB model is consistent between \textit{Fermi}/GBM and GECAM-B, and reveals the spectra of magnetar bursts tending to be soft, which may be composed of multiple BB components. The slope of the temperature distribution is steep which indicates that the majority of the BB temperatures are concentrated around the minimum temperature.