Testing the External-Shock Model of Gamma-Ray Bursts Using the Late-Time Simultaneous Optical and X-Ray Afterglows




We study the "normal" decay phase of the X- ray afterglows of gamma- ray bursts ( GRBs), which follows the shallow decay phase, using the events simultaneously observed in the R band. The classical external- shock model - in which neither the delayed energy injection nor time dependency of shock microphysics is considered shows that the decay indices of the X- ray and R- band light curves, alpha(x) and alpha(o), obey a certain relation, and that in particular, alpha(o) - alpha(x) should be larger than -1/4 unless the ambient density increases with the distance from the central engine. For our selected 14 samples, we have found that four events violate the limit at more than the 3 sigma level, so that a fraction of events are outliers of the classical external-shock model at the "normal" decay phase.


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