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Slipping Magnetic Reconnection of Flux Rope Structures as a Precursor to an Eruptive X-class Solar Flare

We present the quasi-periodic slipping motion of flux rope structures prior to the onset of an eruptive X-class flare on 2015 March 11, obtained by the \emph{Interface Region Imaging Spectrograph} (\emph{IRIS}) and the \emph{Solar Dynamics Observatory} (\emph{SDO}). The slipping motion occurred at the north part of the flux rope and seemed to successively peel off the flux rope. The speed of the slippage was 30$-$40 km s$^{-1}$, with an average period of 130$\pm$30 s. The Si {\sc iv} 1402.77 Å line showed a redshift of 10$-$30 km s$^{-1}$ and a line width of 50$-$120 km s$^{-1}$ at the west legs of slipping structures, indicative of reconnection downflow. The slipping motion lasted about 40 min and the flux rope started to rise up slowly at the late stage of the slippage. Then an X2.1 flare was initiated and the flux rope was impulsively accelerated. One of the flare ribbons swept across a negative-polarity sunspot and the penumbral segments of the sunspot decayed rapidly after the flare. We studied the magnetic topology at the flaring region and the results showed the existence of a twisted flux rope, together with quasi-separatrix layers (QSLs) structures binding the flux rope. Our observations imply that quasi-periodic slipping magnetic reconnection occurs along the flux-rope-related QSLs in the preflare stage, which drives the later eruption of the flux rope and the associated flare.