The UV flash accompanying GRBs from neutron-rich internal shocks

In the neutron-rich internal shocks model for Gamma-ray Burts (GRBs), the Lorentz factors (LFs) of ions shells are variable, so are the LFs of accompanying neutron shells. For slow neutron shells with a typical LF $\sim {\rm tens}$, the typical $\beta-$decay radius reads $R_{\rm \beta,s}\sim {\rm several}\times10^{14}{\rm cm}$, which is much larger than the typical internal shocks radius $\sim 10^{13}{\rm cm}$, so their impact on the internal shocks may be unimportant. However, as GRBs last long enough ($T_{90}>20(1+z){\rm s}$), one earlier but slower ejected neutron shell will be swept successively by later ejected ion shells in the range $\sim 10^{13}{\rm cm}-10^{15}{\rm cm}$, where slow neutrons have decayed significantly. We show in this work that ion shells interacting with the $\beta-$decay products of slow neutron shells can power a ultraviolet (UV) flash bright to 12th magnitude during the prompt $\gamma-$ray emission phase or slightly delayed, which can be detected by the upcoming Satellite SWIFT in the near future.