The sources of apparently non-repeating FRB

Repeating and apparently non-repeating fast radio bursts (FRBs) differ qualitatively and are produced by distinct classes of sources. Apparently non-repeating FRBs must actually repeat many times in their lifetimes because there are insufficient catastrophic events to explain their rate. Yet no such infrequent repetitions, in contrast to the frequent activity of FRBs known to repeat, have been observed, constraining their repetition rates. The large gap in energy between FRB 200428 and its much weaker repetitions resembles the large gap in energy between rare giant outbursts of soft gamma repeaters (SGRs) and their lesser outbursts. This phenomenological similarity suggests that the sources of apparently non-repeating FRBs are related to SGRs. Apparently non-repeating FRBs are associated with SGR outbursts more energetic than the 2020 April 28 outburst of SGR 1935+2154 associated with FRB 200428 but less energetic than the 2004 outburst of SGR 1806-20 that had no associated FRB. At 20-1000 times its actual distance, FRB 200428 would have been an apparent non-repeater.