The peaks of 30 optical afterglows and 14 X-ray light-curves display a good anticorrelation of the peak flux with the peak epoch : F _ { p } \propto t _ { p } ^ { -2.0 } in the optical , F _ { p } \propto t _ { p } ^ { -1.6 } in the X-ray , the distributions of the peak epochs being consistent with each other .
We investigate the ability of two forward-shock models for afterglow light-curve peaks – an observer location outside the initial jet aperture and the onset of the forward-shock deceleration – to account for those peak correlations .
For both models , the slope of the F _ { p } - t _ { p } relation depends only on the slope of the afterglow spectrum .
We find that only a conical jet seen off-aperture and interacting with a wind-like medium can account for both the X-ray peak relation , given the average X-ray spectral slope \beta _ { x } = 1.0 , and for the larger slope of the optical peak relation .
However , any conclusion about the origin of the peak flux – peak epoch correlation is , at best , tentative , because the current sample of X-ray peaks is too small to allow a reliable measurement of the F _ { p } - t _ { p } relation slope and because more than one mechanism and/or one afterglow parameter may be driving that correlation .