Universal scaling law in long gamma-ray bursts

Abstract

Overwhelming diversity of long gamma-ray bursts (LGRBs), discovered after the launch of {\it Swift} satellite, is a major obstacle to LGRB studies. Recently, it is shown that the prompt emission of LGRBs is classified into three subclasses: Type I, Type II LGRBs populating separate fundamental planes in a 3D space defined by the peak luminosity, the duration, and the spectral peak energy, and outliers not belonging to either of the planes. Here we show that Type I LGRBs (LGRBs I) exhibit different shapes of light curves from Type II LGRBs (LGRBs II). Furthermore, we demonstrate that this classification has uncovered a new scaling law in the light curves of LGRBs II spanning 8 orders of magnitude from the prompt to late X-ray afterglow emission. The scaled light curve has four distinct phases. The first phase has a characteristic time scale while the subsequent three phases exhibit power law behaviors with different exponents. We discuss its possible interpretation in terms of the emission from an optically thick fireball propagating in the cricumstellar matter at relativistic speeds and argue that the observed four phases correspond to its hydrodynamical phases. Our classification scheme can pin down the intrinsic luminosities of LGRBs II through the scaling law from a sample of polymorphic GRBs. Further refinement of this scheme and scaling law will make a subclass of LGRBs a standard candle as reliable and accurate as Type Ia supernovae in the more distant universe than supernovae can reach.