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Magnetars are young neutron stars powered by the strongest magnetic fields inthe Universe (10$^{13-15}$ G). Their transient X-ray emission usually manifestsas short (a few hundred milliseconds), bright, energetic ($\sim$ 10$^{40-41}$erg) X-ray bursts. Since its discovery in 2014, magnetar J1935+2154 has becomeone of the most prolific magnetars, exhibiting very active bursting episodes,and other fascinating events such as pulse timing anti-glitches and Fast RadioBursts. Here, we present evidence for possible 42 Hz (24 ms) quasi-periodicoscillations in the $\nu F_{\nu}$ spectrum peak energy (Ep) identified in aunique burst detected with the Fermi Gamma-ray Burst Monitor in January 2022.While quasi-periodic oscillations have been previously reported in theintensity of magnetar burst lightcurves, quasi-periodic oscillations in the Ephave not. We also find an additional event from the same outburst that appearsto exhibit similar character in Ep, albeit of lower statistical quality. Forthese two exceptional transients, such Ep oscillations can be explained bymagnetospheric density and pressure perturbations. For burst-emitting plasmaconsisting purely of $e^+e^-$ pairs, these acoustic modes propagate along ahighly magnetized flux tube of length up to around $L\sim 130$ neutron starradii, with $L$ being lower if ions are present in the emission zone. Detailedtime-resolved analyses of other magnetar bursts are encouraged to evaluate therarity of these events and their underlying mechanisms.

Quasi-Periodic Peak Energy Oscillations in X-ray Bursts from SGR J1935+2154