  Within an isospin- and momentum-dependent Boltzmann-Uehling-Uhlenbeck (IBUU)
transport model, we investigate the hard photons emission from neutron-proton
bremsstrahlung in reaction system of $^{208}Pb+^{208}Pb$ around Fermi energy.
Effects of neutron skins in coordinate ($r$) space and proton skins in momentum
($k$) space on the time evolution, the angular distribution, and the transverse
momentum spectra of hard photons with different energies are studied. It is
shown that the emission of direct hard photons is sensitive to the neutron
skin, which has larger effects for more energetic hard photons. Meanwhile, we
find that the proton skins have an important influence on the emission of
direct hard photons, and its effect is actually even larger than that of
neutron skins. It needs to take the effect of proton skins into account when we
determine the size of neutron skins by comparing transport mode predictions of
hard photons with the corresponding experiment measurements.
