  White-light observations by the Solar Dynamics Observatory's Helioseismic and
Magnetic Imager of a loop-prominence system occurring in the aftermath of an
X-class flare on 2013 May 13 near the eastern solar limb show a linearly
polarized component, reaching up to $\sim$20% at an altitude of $\sim$33 Mm,
about the maximal amount expected if the emission were due solely to Thomson
scattering of photospheric light by the coronal material. The mass associated
with the polarized component was 8.2$\times$10$^{14}$ g. At 15 Mm altitude, the
brightest part of the loop was 3(+/-0.5)% linearly polarized, only about 20% of
that expected from pure Thomson scattering, indicating the presence of an
additional unpolarized component at wavelengths near Fe I (617.33 nm), probably
thermal emission. We estimated the free electron density of the white-light
loop system to possibly be as high as 1.8$\times$10$^{12}$ cm$^{-3}$.
