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Hinode XRT tracked AR12488 for several days back in January 2016. Just after 15 UT on 27 January, a small area, circled in the above image, begins to flicker with activity.
The increased activity starts as sporadic nanoflares in the upper left portion of the active region. Eventually, a small bright dome envelopes all of these little flares and then with a big flash, the tiny region separates from the big active region.
The emergence of magnetic flux probably caused the separation as the local magnetic field reorganizes due to the new fields presence. The active region continued to produce several small and mid-sized flares in the days following the separation.