Polarization curves of GRB afterglows predicted by the universal jet structure

Detections of linear polarization in the optical afterglow of GRBs are accumulating fast since the first measurements in GRB 990510 [1], [15]. It was soon proposed that the observed polarization could arise from observing a collimated fireball with a slightly off axis line of sight ([3], [14]). As a consequence, it was also realized that the degree of polarization could be connected with the achromatic break in the lightcurve expected in jetted fireballs. The polarization starts to grow when the observer perceives the nearest edge of the jet and the lightcurve begins to steepen; it has two maxima with the position angle being orthogonal to each other and then it slowly fades away while the jet front becomes entirely visible and the flux accomplishes the transition between the two asymptotic power–laws (Fig.4 in [3]). Recently a different structure for the emission pattern has been proposed: a jet with a brighter (maybe faster) spine surrounded by dimmer (and slower) wings, with a standard energy reservoir [9], [11], [13], [16],. The main parameters of such a structured jet are the angular size of the core θcore and the wings luminosity distribution. We [11] have shown that if the luminosity goes as θ for θ > θcore the lightcurves from such a jet are virtually indistinguishable from an homogeneous one (see 1) and both models can reproduce the break time–luminosity correlation [2], [8]. Actually while in the standard model the time at which there is a steepening in the power–law decay of the afterglow, is related to the cone angle of the jet, in the structured jet such a break in the afterglow lightcurve occurs at a time that depends on the viewing angle. Instead of implying a range of intrinsically different jets – some very narrow, and others with similar power spread over a wider cone – the data on afterglow breaks are consistent with a standardized jet, viewed from different angles. Since each luminosity is univocally related to a viewing angle, it is possible to calculate the predicted luminosity function [11]; in the simplest version of this model, n(L) ∝ L. This prediction, if robustly confirmed by observations, could support the universal jet structure, but it cannot rule out the standard model.