Update on the GRB universal scaling EX,iso-Eγ,iso-Epk with 10 years of Swift data

Abstract

From a comprehensive statistical analysis of Swift X-ray light curves of gamma-ray bursts (GRBs) collected from 2004 December to the end of 2010, we found a three-parameter correlation between the isotropic energy emitted in the rest-frame 1-10<SUP>4</SUP> keV energy band during the prompt emission (E<SUB>γ,iso</SUB>), the rest-frame peak of the prompt emission energy spectrum (E<SUB>pk</SUB>), and the X-ray energy emitted in the rest-frame 0.3-30 keV observed energy band (E<SUB>X,iso</SUB>), computed excluding the contribution of the flares. In this paper, we update this correlation with the data collected until 2014 June, expanding the sample size with ∼35 per cent more objects, where the number of short GRBs doubled. With this larger sample, we confirm the existence of a universal correlation that connects the prompt and afterglow properties of long and short GRBs. We show that this correlation does not depend on the X-ray light-curve morphology and that further analysis is necessary to firmly exclude possible biases derived by redshift measurements. In addition, we discuss about the behaviour of the peculiar objects as ultra-long GRBs and we propose the existence of an intermediate group between long and short GRBs. Interestingly, two GRBs with uncertain classification fall into this category. Finally, we discuss the physics underlying this correlation, in the contest of the efficiency of conversion of the prompt gamma-ray emission energy into the kinetic energy of the afterglow, the photospheric model, and the cannonball model.