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

Abstract

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 micro-physics is considered -- shows that the decay indices of the X-ray and R-band light curves, ${\alpha }_{\mathrm{X}}$ and ${\alpha }_{\mathrm{O}}$, obey a certain relation, and that in particular, ${\alpha }_{\mathrm{O}}-{\alpha }_{\mathrm{X}}$ 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 4 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.

Accepted for publication in ApJL. 12 page, 2 figures, 2 tables

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