Peaks of optical and X-ray afterglow light curves

Abstract

The peaks of 30 optical afterglows and 14 X-ray light curves display a good anticorrelation of the peak flux with the peak epoch: Fp α t-2p-2.0 in the optical and Fp α tp-1.6 in the X-ray, the distributions of the peak epochs being consistent with each other. We investigate the abilityof 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 Fp-tp relation depends only on the slopeof the afterglow spectrum. We find that only a conical jet seen off-aperture and interactingwith a wind-like medium can account both for the X-ray peak relation, given the averageX-ray spectral slope β 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 Fp-tp relation slope and because more than one mechanism and/or one afterglow parameter may be driving that correlation. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.