  Direct hard photon emissions from incoherent proton-neutron bremsstrahlung in
collisions of the neutron-rich projectile $^{50}$Ca with $^{12}$C and $^{40}$Ca
targets are simulated in the framework of the isospin-dependent quantum
molecular dynamics (IQMD) model. By adjusting the diffuseness parameter of
neutron density in the droplet model to obtain different neutron skin
thicknesses for $^{50}$Ca, the effects of neutron skin thickness on direct hard
photon emission are investigated via several probes. The results show that more
direct hard photons are produced with increasing neutron skin thickness in
peripheral collisions. Meanwhile, we find that the multiplicity yield ratio
$R_{cp}(\sigma_{\gamma})$ between central collisions and peripheral collisions
as well as the rapidity dependence of multiplicity for direct hard photons are
sensitive to neutron skin thickness. The results indicate that direct hard
photon emission can be taken as an experimental observable to extract
information on neutron skin thickness.
