2023ApJ...956L..27R

Magnetars are young neutron stars powered by the strongest magnetic fields in the Universe (10^13–15 G). Their transient X-ray emission usually manifests as short (a few hundred milliseconds), bright, energetic (∼10^40–41 erg) X-ray bursts. Since its discovery in 2014, SGR J1935+2154 has become one of the most prolific magnetars, exhibiting very active bursting episodes and other fascinating events, such as pulse timing antiglitches and fast radio bursts. Here we present evidence for possible 42 Hz (24 ms) quasiperiodic oscillations in the νF_ν spectrum peak energy (E_p) identified in a unique burst detected with the Fermi Gamma-ray Burst Monitor in 2022 January. While quasiperiodic oscillations have been previously reported in the intensity of magnetar burst light curves, quasiperiodic oscillations in E_p have not. We also find an additional event from the same outburst that appears to exhibit a similar character in E_p, albeit of lower statistical quality. For these two exceptional transients, such E_p oscillations can be explained by magnetospheric density and pressure perturbations. For burst-emitting plasma consisting purely of e⁺e^– pairs, these acoustic modes propagate along a highly magnetized flux tube of length up to around L ∼ 130 neutron star radii, with L being lower if ions are present in the emission zone. Detailed time-resolved analyses of other magnetar bursts are encouraged to evaluate the rarity of these events and their underlying mechanisms.