2 research outputs found
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
micro-physics is considered -- shows that the decay indices of the X-ray and
R-band light curves, and , obey a certain
relation, and that in particular, 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 level, so that a fraction of
events are outliers of the classical external shock model at the ``normal''
decay phase.Comment: Accepted for publication in ApJL. 12 page, 2 figures, 2 table