The Ultraviolet Flash Accompanying Gamma-Ray Bursts from Neutron-rich Internal Shocks

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Abstract
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 $sim { m tens}$, the typical $eta-$decay radius reads $R_{ m eta,s}sim { m several} imes10^{14}{ m cm}$, which is much larger than the typical internal shocks radius $sim 10^{13}{ m cm}$, so their impact on the internal shocks may be unimportant. However, as GRBs last long enough ($T_{90}>20(1+z){ m s}$), one earlier but slower ejected neutron shell will be swept successively by later ejected ion shells in the range $sim 10^{13}{ m cm}-10^{15}{ m cm}$, where slow neutrons have decayed significantly. We show in this work that ion shells interacting with the $eta-$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