  The PHENIX experiment at BNL-RHIC recently reported the single transverse
spin asymmetry $A_N$ for forward neutrons measured in polarized proton-nucleus
collisions at $\sqrt{s_{NN}} = 200\,$GeV; $A_N$ in proton-aluminum and
proton-gold collisions are -0.015 and 0.18, respectively, which are clearly
different from $A_N=-0.08$ in proton-proton collisions. In this paper, we
propose large $A_N$ for forward neutrons in ultraperipheral polarized
proton-nucleus collisions as an explanation of the PHENIX measurements. The
proposed model is demonstrated using two Monte Carlo simulations organized as
follows. In the ultraperipheral collision simulation, we use the STARlight
event generator for the simulation of the virtual photon flux and then use the
MAID2007 unitary isobar model for the simulation of the neutron production in
the interactions of a virtual photon with a polarized proton. In the
proton-nucleus hadronic interaction simulation, the differential cross sections
of forward neutron production are predicted by a simple one-pion exchange model
and the Glauber model. The simulated $A_N$ values accumulating the both
contribution of ultraperipheral collisions and hadronic interactions are in
good agreement with the PHENIX results.
