A Universal Energy Distribution for FRB 121102

Fast radio bursts (FRBs) are millisecond radio transients with large dispersion measures. An outstanding question is the relation between repeating FRBs and those with a single burst. In this paper, we study the energy distribution of the repeating FRB 121102. For a power-law distribution of energy , we show that the value of E is in a narrow range 1.6-1.8 for bursts observed by different telescopes at different frequencies, which indicates a universal energy distribution for FRB 121102. Interestingly, a similar power-law index of energy distribution for nonrepeating FRBs observed by Parkes and ASKAP is also found. However, if low-energy bursts below the completeness threshold of Arecibo are discarded for FRB 121102, the slope could be up to 2.2. Implications of such a universal energy distribution are discussed.