Evidence for an anticorrelation between the duration of the shallow decay phase of GRB X-ray afterglows and redshift

Context One of the most intriguing features discovered by Swift is a plateau phase in the X-ray flux decay of about 70% of the afterglows of gamma-ray bursts (GRBs). The physical origin of this feature is still being debated. <BR />Aims We constrain the proposed interpretations, based on the intrinsic temporal properties of the plateau phase. <BR />Methods We selected and analyzed all the Swift/XRT GRB afterglows at known redshift observed between March 2005 and June 2008 featuring a shallow decay phase in their X-ray lightcurves. <BR />Results For our sample of 21 GRBs we find an anticorrelation of the logarithm of the duration of the shallow phase with redshift, with a Spearman rank-order correlation coefficient of r = -0.4 and a null hypothesis probability of 5%. When we correct the durations for cosmological dilation, the anticorrelation strenghtens, with r = -0.6 and a null hypothesis probability of 0.4%. Considering only those GRBs in our sample that have a well-measured burst peak energy (8 out of 21), we find an anticorrelation between the energy of the burst and the shallow phase duration, with r = -0.80 and a null hypothesis probability of 1.8%. <BR />Conclusions If the burst energy anticorrelation with the shallow phase duration is real, then the dependence of the shallow phase on redshift could be the result of a selection effect, since on average high-redshift bursts with lower energies and longer plateaus would be missed. A burst energy anticorrelation with the shallow phase duration would be expected if the end of the plateau arises from a collimated outflow. Alternative scenarios are briefly discussed involving a possible cosmological evolution of the mechanism responsible for the X-ray shallow decay.