Rapid Spectral Evolution of SGR 1935+2154 during Its 2022 Outburst

Abstract

During the 2022 outburst of SGR 1935+2154, a fast radio burst (FRB)-like event (FRB 20221014A) and X-ray activities occurred between two spin-up glitches, suggesting these glitches may connect to multiwavelength phenomenology. However, the mechanisms altering the magnetar’s magnetosphere to enable radio emission remain unclear. This study presents high-cadence Neutron Star Interior Composition Explorer and Nuclear Spectroscopic Telescope Array observations revealing spectral changes in burst and persistent emission. Hardness ratio and spectral analysis reveal significant changes during an “intermediate flare” 2.5 hr before FRB 20221014A. This 40 s flare, releasing >(6.3 ± 0.2) × 10⁴⁰ erg, coincides with a rapid spectral softening in both burst and persistent emission, and a notable decrease in the burst occurrence rate. The intermediate flare is bright enough to be detected if placed at a few megaparsecs, and would appear as a fast X-ray transient. This implies that the connection between magnetar X-ray activity and FRBs can be observed in the local Universe. Postflare burst spectra peak near 5 keV, resembling the characteristics of the FRB-associated X-ray burst of 2020. Such change persisted for a few hours, implying magnetospheric evolution on similar timescales. However, no radio emission was detected from postflare bursts, suggesting that FRB emission requires conditions beyond peculiar short bursts. The burst waiting times exhibit a broken power-law distribution, likely resulting from contamination by enhanced persistent emission. Although the bursts appear randomly distributed in the spin phase, the hardness ratio profile as a function of spin phase follows that of the persistent emission, indicating that X-ray bursts originate at low altitudes.