Pulsating X-ray Precursors of a Powerful Solar Flare: Evidence of Quasi-Periodic Reconnection Leading to a Flux-Rope Eruption

Solar flares and eruptions have precursors. A special subclass of precursors are quasi-periodic pulsations (QPPs), observed before a significant fraction of flares in the microwave and soft X-ray ranges. Their mechanisms have not yet been established. The study of pre-flare QPPs is important for diagnostics of magnetoplasma structures in active regions (ARs) and understanding the processes of their transition from a stable to an unstable state, triggers of powerful flares and eruptions, which is necessary for the development of forecasting methods for such events. We present the analysis of multiwavelength observations with spatial resolution of X-ray QPP sources with a period of $${{P}_{{{\text{QPP}}}}} \approx 6.7 \pm 1.6$$ min and increasing amplitude before the powerful eruptive X2.8 flare SOL2023-12-14T16:47 in the quasi-dipole NOAA AR 13514. It is shown that pre-flare QPPs were a manifestation of successive episodes of energy release with plasma heating to high temperatures of about 30 MK (and/or electron acceleration to at least tens of keV) in a system of crossed and twisted magnetic flux tubes with a large shear inside the larger arcade of loops remaining from the previous flare. These successive episodes of energy release, accompanied by a steady magnetic flux cancellation on the photosphere under the QPP sources, could lead to the disappearance of the filament due to plasma heating, the formation of an unstable magnetic flux-rope and its eruption associated with the onset of the impulsive phase of the main X2.8 flare. Arguments are presented in favor of the hypothesis that the observed pre-flare QPPs were a manifestation of a repetitive quasi-periodic magnetic reconnection as a cyclic loading-unloading process of supply and transformation of free magnetic energy in the pre-flare current sheet(s).