The 2021 Activity of the Fast-radio-burst-emitting Galactic Magnetar SGR 1935+2154 as Observed by NASA's Fermi Gamma-Ray Burst Monitor

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 T 100 ∼77.34 ms, a cutoff power-law peak energy E peak ∼26.62 keV, and a photon index "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 S-0.93 . The anticorrelation between the blackbody temperature and the emission area is weaker in the hard component at RBBh2 kTBBh-4.56 but is improved in the soft component at RBBs2 kTBBs-1.63, compared to previous activities, with an overall RBB2 kTBB-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.