There are many attempts to detect early optical emission and there is a good chance that this emission will be observed in the near future. A strong 5th magnitude optical flash would have been produced if the fluence of a moderately strong GRBs, 10-5 erg/s/cm2 would have been released on a time scale of 10 s in the optical band. Even a small fraction of this will be easily observed. It is important, therefore, to explore the expected optical emission from the GRB and the early afterglow.
During the GRB and the initial emission from the forward shock the
emission peaks in -rays, and only extremely small fraction is
emitted in the optical band. For example, the prompt optical flash from the
GRB would be of 21st magnitude if the flux drops according to the
synchrotron low energy tail of
.
A considerably stronger flux is obtained from the reverse shock. The
reverse shock contains, at the time it crosses the shell, a comparable
amount of energy to the forward shock. However, its effective
temperature is significantly lower (typically by a factor of ) than that of the forward shock. The resulting peak
frequency is therefore lower by
. A more detailed
calculation shows that the reverse shock frequency is
![]() |
(6) |
Copyright The European Southern Observatory (ESO)