  Magnetic flux ropes are the centerpiece of solar eruptions. Direct
measurements for the magnetic field of flux ropes are crucial for understanding
the triggering and energy release processes, yet they remain heretofore
elusive. Here we report microwave imaging spectroscopy observations of an
M1.4-class solar flare that occurred on 2017 September 6, using data obtained
by the Expanded Owens Valley Solar Array. This flare event is associated with a
partial eruption of a twisted filament observed in H{\alpha} by the Goode Solar
Telescope at the Big Bear Solar Observatory. The extreme ultraviolet (EUV) and
X-ray signatures of the event are generally consistent with the standard
scenario of eruptive flares, with the presence of double flare ribbons
connected by a bright flare arcade. Intriguingly, this partial eruption event
features a microwave counterpart, whose spatial and temporal evolution closely
follow the filament seen in H{\alpha} and EUV. The spectral properties of the
microwave source are consistent with nonthermal gyrosynchrotron radiation.
Using spatially resolved microwave spectral analysis, we derive the magnetic
field strength along the filament spine, which ranges from 600-1400 Gauss from
its apex to the legs. The results agree well with the non-linear force-free
magnetic model extrapolated from the pre-flare photospheric magnetogram. We
conclude that the microwave counterpart of the erupting filament is likely due
to flare-accelerated electrons injected into the filament-hosting magnetic flux
rope cavity following the newly reconnected magnetic field lines.
