Intrinsic parameters of GRB 990123 from its prompt optical flash and afterglow

Abstract

We have constrained the intrinsic parameters, such as the magnetic energy density fraction (∈B), the electron energy density fraction (∈e), the initial Lorentz factor (Γ0) and the Lorentz factor of the reverse external shock (Γrs), of GRB 990123, in terms of the afterglow information (forward shock model) and the optical flash information (reverse shock model). Our result shows: (1) the inferred values of ∈e and ∈B are consistent with the suggestion that they may be universal parameters, comparing to those inferred for GRB 970508; (2) the reverse external shock may have become relativistic before it passed through the ejecta shell. Other intrinsic parameters of GRB 990123, such as the energy contained in the forward shock E and the ambient density n, are also determined and discussed in this paper.