2018ApJ...855...88D

The fraction of long duration gamma-ray bursts (GRBs) without an associated bright supernova (SN-less GRBs) at low redshifts is comparable to that of GRBs associated with bright supernovae (SN-GRBs). The prompt emission and its fast decline phase in both types of GRBs are well described by the cannonball model of GRBs, where inverse Compton scattering of ambient light is the dominant γ-ray production mechanism. However, in SN-less GRBs, the fast decay of the prompt emission appears to be overtaken by an afterglow powered by a millisecond pulsar, while, in SN-GRBs, the late-time X-ray afterglow is well described by synchrotron radiation from the decelerating jet in the interstellar medium. We use their different X-ray light curves to determine the ratio of SN-less GRBs/SN-GRBs at very high redshifts. We find that at z > 4, this ratio is the same as that at small redshifts. Such a z-independent ratio suggests that the origin of SN-less GRBs is a phase transition of neutron stars to quark stars in high-mass X-ray binaries, rather than the merger of neutron stars.