  We report microwave spectral imaging observations of an erupting magnetic
flux rope during the early impulsive phase of the X8.2-class limb flare on 2017
September 10, obtained by the Expanded Owens Valley Solar Array. A few days
prior to the eruption, when viewed against the disk, the flux rope appeared as
a reverse S-shaped dark filament along the magnetic polarity inversion line.
During the eruption, the rope exhibited a "hot channel" structure in extreme
ultraviolet and soft X-ray passbands sensitive to ~10 MK plasma. The central
portion of the flux rope was nearly aligned with the line of sight, which
quickly developed into a teardrop-shaped dark cavity during the early phase of
the eruption. A long and thin plasma sheet formed below the cavity, interpreted
as the reconnection current sheet viewed edge-on. A nonthermal microwave source
was present at the location of the central current sheet, which extended upward
encompassing the dark cavity. A pair of nonthermal microwave sources were
observed for several minutes on both sides of the main flaring region. They
shared a similar temporal behavior and spectral property to the central
microwave source below the cavity, interpreted as the conjugate footpoints of
the erupting flux rope. These observations are broadly consistent with the
magnetic topology and the associated energy release scenario suggested in the
three-dimensional standard model for eruptive solar flares. In particular, our
detection of nonthermal emission at conjugate flux rope footpoints provides
solid evidence of particle transport along an erupting magnetic flux rope.
