Hard X-Ray Emission from an Activated Flux Rope and Subsequent Evolution of an Eruptive Long-duration Solar Flare

In this paper, we present a comprehensive study of the evolutionary phases of a major M6.6 long duration event with special emphasize on its pre-flare phase. The event occurred in NOAA 12371 on 2015 June 22. A remarkable aspect of the event was an active pre-flare phase lasting for about an hour during which a hot EUV coronal channel was in the build-up stage and displayed cospatial hard X-ray (HXR) emission up to energies of 25 keV. This is the first evidence of the HXR coronal channel. The coronal magnetic field configuration based on nonlinear-force-free-field modeling clearly exhibited a magnetic flux rope (MFR) oriented along the polarity inversion line (PIL) and cospatial with the coronal channel. We observed significant changes in the AR's photospheric magnetic field during an extended period of ≈42 hr in the form of rotation of sunspots, moving magnetic features, and flux cancellation along the PIL. Prior to the flare onset, the MFR underwent a slow rise phase (≈14 km s⁻¹) for ≈12 minutes, which we attribute to the faster build-up and activation of the MFR by tether-cutting reconnection occurring at multiple locations along the MFR itself. The sudden transition in the kinematic evolution of the MFR from the phase of slow to fast rise (≈109 km s⁻¹ with acceleration ≈110 m s⁻²) precisely divides the pre-flare and impulsive phase of the flare, which points toward the feedback process between the early dynamics of the eruption and the strength of the flare magnetic reconnection.