Peaks of optical and X-ray afterglow light-curves

TL;DR

This study examines the correlation between peak flux and peak epoch in optical and X-ray afterglows, testing models of jet geometry and medium interaction, and finds that off-axis jets in wind-like media best explain the observed relations.

Contribution

It evaluates two forward-shock models against observed peak correlations, highlighting the potential role of off-axis jets in wind media as the explanation.

Findings

Optical and X-ray peak fluxes are anticorrelated with peak epochs.

Only off-axis conical jets in wind media can explain the observed relations.

Current data is insufficient for definitive conclusions.

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: F_p ~ t_p^{-2.0} in the optical, F_p ~ 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.