2023ApJ...950..121R

The galactic, fast-radio-burst-emitting magnetar SGR 1935+2154 entered an active X-ray burst episode in 2021 September, nearly a year after emitting a fast radio burst with an X-ray counterpart. We report on the temporal and spectral properties of the activity, which included bright intermediate flares, and present a burst catalog of the Fermi Gamma-Ray Burst Monitor observations. We compare this activity to the previous 6 yr history of the source since its discovery in 2014. The bursts show a mean log-Gaussian duration T₁₀₀ ∼ 77.34 ms, a cutoff power-law peak energy E_peak ∼ 26.62 keV, and a photon index Γ_CPL ∼ 0.49, and blackbody soft and hard temperatures kT_BBs ∼ 5.23 keV and kT_BBh ∼ 9.24 keV, respectively. The burst fluence spans 3 orders of magnitude (10^–8-10^–5 erg cm^–2) and follows ${dN}/{dS}\propto {S}^{-0.93}$. The anticorrelation between the blackbody temperature and the emission area is weaker in the hard component at ${R}_{\mathrm{BBh}}^{2}\propto {{kT}}_{\mathrm{BBh}}^{-4.56}$ but is improved in the soft component at ${R}_{\mathrm{BBs}}^{2}\propto {{kT}}_{\mathrm{BBs}}^{-1.63}$, compared to previous activities, with an overall ${R}_{\mathrm{BB}}^{2}\propto {{kT}}_{\mathrm{BB}}^{-3.82}$ correlation. The 2021 bursts have higher average and integrated fluence than the 2014-2020 episodes and are relatively shorter than the 2019-2020 activities yet comparable to the earlier episodes. During 2014-2021, the burst mean peak energy (E_peak) shows a slight softening and is generally softer than other magnetars. We discuss our results in terms of magnetar physics and emission mechanisms.