  Hard photons from neutron-proton bremsstrahlung in intermediate energy
heavy-ion reactions are examined as a potential probe of the nuclear symmetry
energy within a transport model. Effects of the symmetry energy on the yields
and spectra of hard photons are found to be generally smaller than those due to
the currently existing uncertainties of both the in-medium nucleon-nucleon
cross sections and the photon production probability in the elementary process
$pn\to pn\gamma$. Very interestingly, nevertheless, the ratio of hard photon
spectra $R_{1/2}(\gamma)$ from two reactions using isotopes of the same element
is not only approximately independent of these uncertainties but also quite
sensitive to the symmetry energy. For the head-on reactions of
$^{132}Sn+^{124}Sn$ and $^{112}Sn+^{112}Sn$ at $E_{beam}/A=50$ MeV, for
example, the $R_{1/2}(\gamma)$ displays a rise up to 15% when the symmetry
energy is reduced by about 20% at $\rho=1.3\rho_0$ which is the maximum density
reached in these reactions.
