The 80 Ms follow-up of the X-ray afterglow of GRB 130427A challenges the standard forward shock model

Abstract

GRB 130427A was the brightest gamma-ray burst detected in the last 30 yr. With an equivalentisotropic energy output of 8.5 × 1053 erg and redshift z = 0.34, it uniquely combined veryhigh energetics with a relative proximity to Earth. As a consequence, its X-ray afterglowhas been detected by sensitive X-ray observatories such as XMM-Newton and Chandra fora record-breaking baseline longer than 80 million seconds. We present the X-ray light curveof this event over such an interval. The light curve shows a simple power-law decay with aslope α = 1.309 ± 0.007 over more than three decades in time (47 ks-83 Ms). We discuss theconsequences of this result for a few models proposed so far to interpret GRB 130427A, andmore in general the significance of this outcome in the context of the standard forward shockmodel. We find that this model has difficulty in explaining our data, in both cases of constantdensity and stellar-wind circumburst media, and requires far-fetched values for the physicalparameters involved.