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

In the neutron-rich internal shocks model for Gamma-ray Burts (GRBs), theLorentz factors (LFs) of ions shells are variable, so are the LFs ofaccompanying neutron shells. For slow neutron shells with a typical LF tens,the typical beta-decay radius reads R_{\beta,s} several 10^{14} cm, which ismuch larger than the typical internal shocks radius 10^{13} cm, so their impacton the internal shocks may be unimportant. However, as GRBs last long enough(T_{90}>20(1+z) s), one earlier but slower ejected neutron shell will be sweptsuccessively by later ejected ion shells in the range 10^{13}-10^{15} cm, whereslow neutrons have decayed significantly. We show in this work that ion shellsinteracting with the beta-decay products of slow neutron shells can power aultraviolet (UV) flash bright to 12th magnitude during the prompt gamma-rayemission phase or slightly delayed, which can be detected by the upcomingSatellite SWIFT in the near future.Comment: 6 pages (2 eps figures), accepted for publication in ApJ