On the True Fractions of Repeating and Nonrepeating Fast Radio Burst Sources

Observationally, fast radio bursts (FRBs) can be divided into repeating and apparently nonrepeating (one-off) ones. It is unclear whether all FRBs repeat and whether there are genuine nonrepeating FRBs. We attempt to address these questions using Monte Carlo simulations. We define a parameter Tc at which the accumulated number of nonrepeating sources becomes comparable to the total number of repeating sources, which is a good proxy to denote the intrinsic repeater fraction among FRBs. Assuming that both types of sources exist and their burst energies follow power-law distributions, we investigate how the observed repeater fraction evolves with time for different parameters. If the lifetime of repeaters is sufficiently long that the evolutionary effect can be neglected within the observational time span, unless Tc → ∞ (i.e., there is no genuine nonrepeating FRB source), the observed repeater fraction should increase with time first, reach a peak, and then decline. The peak time Tp and peak fraction Fr,obs,p depend on Tc and other repeating rate parameters. With the current data, we pose a lower limit Tc > 0.1 day for reasonable parameter values. We predict that future continuous monitoring of FRBs with CHIME or similar wide-field radio telescopes would obtain an Fr,obs less than 0.04. The detection of a smaller peak value Fr,obs,p < 0.04 in the near future would disfavor the ansatz that "all FRB sources repeat."