A Universal Break in Energy Functions of Three Hyperactive Repeating Fast Radio Bursts

Abstract

Fast radio bursts (FRBs) are millisecond-duration pulses occurring at cosmological distances with a mysterious origin. Observations show that at least some FRBs are produced by magnetars. All magnetar-powered FRB models require some triggering mechanisms, among which the most popular is the cracking of the crust of a neutron star, which is called a starquake. However, so far there has been no decisive evidence for this speculation. Here we report the energy functions of the three most active repeating FRBs, which show a universal break around 10³⁸ erg. Such a break is similar to that of the frequency-magnitude relationship of earthquakes. The break, and the change in the power-law indices below and above it, can be well understood within the framework of FRBs triggered by starquakes in the magnetar models. The seed of weak FRBs can grow both on the magnetar surface and in the deeper crust. In contrast, the triggering of strong FRBs is confined by the crustal thickness, and the seed of strong FRBs can only grow on the surface. This difference in dimensionality causes a break in the scaling properties from weak to strong FRBs, occurring at a point where the penetration depth of starquakes equals the crustal thickness. Our result, together with the earthquake-like temporal properties of these FRBs, strongly supports the idea that FRBs are triggered by starquakes, providing a new opportunity to study the physical properties of the crust of a neutron star.