NICER View of the 2020 Burst Storm and Persistent Emission of SGR 1935+2154

We report on NICER observations of the magnetar SGR 1935+2154, covering its 2020 burst storm and long-term persistent emission evolution up to ~90 days postoutburst. During the first 1120 s taken on April 28 00:40:58 UTC, we detect over 217 bursts, corresponding to a burst rate of >0.2 bursts s⁻¹. Three hours later, the rate was 0.008 bursts s⁻¹, remaining at a comparatively low level thereafter. The T 90 burst duration distribution peaks at 840 ms; the distribution of waiting times to the next burst is fit with a lognormal with an average of 2.1 s. The 1–10 keV burst spectra are well fit by a blackbody, with an average temperature and area of kT = 1.7 keV and R² = 53 km². The differential burst fluence distribution over ~3 orders of magnitude is well modeled with a power-law form dN/dF ∝ F_(−1.5)^(±0.1). The source persistent emission pulse profile is double-peaked hours after the burst storm. We find that the burst peak arrival times follow a uniform distribution in pulse phase, though the fast radio burst associated with the source aligns in phase with the brighter peak. We measure the source spin-down from heavy-cadence observations covering days 21–39 postoutburst, ν˙=−3.72(3)×10⁻¹² Hz s ⁻¹, a factor of 2.7 larger than the value measured after the 2014 outburst. Finally, the persistent emission flux and blackbody temperature decrease rapidly in the early stages of the outburst, reaching quiescence 40 days later, while the size of the emitting area remains unchanged.