  We investigate how the splitting of the leading sunspot and associated flux
emergence and cancellation in active region NOAA 11515 caused an eruptive M5.6
flare on 2012 July 2. Our study employs multi-wavelength observations from HMI,
AIA and ChroTel. Emerging flux formed a neutral line ahead of the leading
sunspot and new satellite spots. The sunspot splitting caused a long-lasting
flow toward this neutral line, where a filament formed. Further flux emergence,
partly of mixed-polarity, as well as episodes of flux cancellation occurred
repeatedly at the neutral line. Following a nearby C-class precursor flare with
signs of interaction with the filament, the filament erupted nearly
simultaneously with the onset of the M5.6 flare and evolved into a coronal mass
ejection. The sunspot stretched without forming a light bridge, splitting
unusually fast (within about a day, complete approximately 6 hours after the
eruption) in two nearly equal parts. The front part separated strongly from the
active region to approach the neighbouring active region where all its coronal
magnetic connections were rooted. It also rotated rapidly (by 4.9 degree/hr)
and caused significant shear flows at its edge. The eruption resulted from a
complex sequence of processes in the (sub-)photosphere and corona. The
persistent flows toward the neutral line likely caused the formation of a flux
rope which held the filament. These flows and their associated flux
cancellation, the emerging flux, and the precursor flare all contributed to the
destabilization of the flux rope. We interpret the sunspot splitting as the
separation of two flux bundles differently rooted in the convection zone and
only temporarily joined in the spot. This explains the rotation as continued
rise of the separating flux and implies that at least this part of the sunspot
was still connected to its roots deep in the convection zone.
