  The first hours following a neutron star merger are considered to provide
several UV/optical/NIR signals: $ \beta $-decay emission from free neutrons,
radioactive decay of shocked heavy elements in the cocoon and cocoon's cooling
emission. Here we consider two additional emission sources: $ \beta $-decay of
free neutrons in the cocoon and synchrotron by the $ \beta $-decay electrons.
We present 3D RHD simulations of jets that propagate in a multi-layer ejecta
from the merger and calculate semi-analytically the resulting light curves. We
find that the free neutrons emission at high latitudes is enhanced by the
cocoon by a factor of a few to power a wide ($ \lesssim 60^\circ $) and brief
($ \sim 1 $ hour) UV signal that can reach an absolute magnitude of $\gtrsim$
-15, comparable with the cooling emission. If the ejected neutron matter mass
is $ M_n \gtrsim 10^{-4}{\rm M_\odot} $, the synchrotron emission may yield a
long ($ \sim 8 $ hours) quasi-isotropic UV/optical signal with an absolute
magnitude between -12 and -15, depending on the magnetic field. Such a high
mass of a mildly-relativistic component may partly obscure the cocoon's shocked
r-process elements, thereby attenuating its radioactive decay emission. Future
observations on these timescales, including null detections, may place
constraints on the ejected neutron matter mass and shed light on the ejecta and
jet-cocoon characteristics.
