Impulsive and gradual nonthermal emissions in an X-class flare

In this paper we present a comprehensive case study of an X-class flare observed on 2001 April 6. The flare consists of two episodes, the first characterized by impulsive spiky bursts and the second by gradual smooth emission at hard X-ray and microwave wavelengths. Emissions in the two episodes are regarded as the impulsive and gradual components, respectively. We compare the temporal, spatial, and spectral evolution of the two components in hard X-rays and microwaves. For this event, the most important finding is that both the impulsive and gradual hard X-rays at ≥50 keV are thick-target emissions at conjugate footpoints. Evolution of hard X-rays and microwaves during the gradual phase exhibits a separation motion between two footpoint sources, which reflects progressive magnetic reconnection. Observations further reveal distinct spectral properties of the gradual component. It is most prominently observed in the high-energy (>20 keV) range, and the gradual hard X-rays have a harder and hardening spectrum compared with the impulsive component. The gradual component is also a microwave-rich event, with the microwaves lagging the hard X-rays by tens of seconds. A correlation analysis of the hard X-ray light curves shows energy-dependent time delays, with the 200 keV hard X-rays lagging the 40 keV emission by 20 s. The observations and analyses suggest that magnetic reconnection occurs during both the impulsive and gradual phases that generate nonthermal electrons, primarily precipitating at the footpoints. However, the temporal and spectral properties of the gradual component must be produced by an acceleration mechanism different from that of the impulsive phase. We propose that the "collapsing-trap" effect, as a product of progressive magnetic reconnection in bipolar magnetic fields, is a viable mechanism that continuously accelerates the gradual-phase electrons in a low-density trap before they precipitate into the footpoints.