We report observations of an eruptive X2.8 flare on 2013 May 13 , which shows two distinct episodes of energy release in the impulsive phase .
The first episode is characterized by the eruption of a magnetic flux rope , similar to the energy-release process in most standard eruptive flares .
While the second episode , which is stronger than the first normal one and shows enhanced high-energy X-ray and even \gamma -ray emissions , is closely associated with magnetic reconnection of a large-scale loop in the aftermath of the eruption .
The reconnection inflow of the loop leg is observed in the Solar Dynamics Observatory ( SDO ) /Atmospheric Imaging Assembly ( AIA ) 304 Å passband and accelerates towards the reconnection region to a speed as high as \sim 130 km/s .
Simultaneously the corresponding outflow jets are observed in the AIA hot passbands with a speed of \sim 740 km/s and mean temperature of \sim 14 MK . RHESSI observations show a strong burst of hard X-ray ( HXR ) and \gamma -ray emissions with hard electron spectra of \delta \approx 3 , exhibiting a soft-hard-harder behavior .
A distinct altitude decrease of the HXR loop-top source coincides with the inward swing of the loop leg observed in the AIA 304 Å passband , which is suggested to be related to the coronal implosion .
This fast inflow of magnetic flux contained in the loop leg greatly enhances the reconnection rate and results in very efficient particle acceleration in the second-step reconnection , which also helps to achieve a second higher temperature peak up to T \approx 30 MK .