  Repeating and apparently non-repeating fast radio bursts (FRB) differ by
orders of magnitude in duty factors, energy and rotation measure. Extensive
monitoring of apparently non-repeating FRB has failed to find any repetitions.
This suggests the two types differ qualitatively, rather than in repetition
rate, and are produced by distinct kinds of sources. The absence of periodicity
in repeating FRB argues that they are not produced by neutron stars. They may
originate in dilute relativistic jets produced by low luminosity black hole
accretion. Non-repeating FRB may be produced by catastrophic events such as the
collapse of an accreting magnetic neutron star to a black hole or of an
accreting magnetic white dwarf to a neutron star, during which a disappearing
magnetic moment radiates dipole radiation that accelerates electrons in nearby
matter. If they are emitted by collimated beams of relativistic particles or
charge "bunches" with Lorentz factor $\gamma$, their radiation is collimated
into a solid angle $\sim \gamma^{-2}$ sterad, reducing the energy requirement.
If powered by magnetic reconnection, a pulse of length $\Delta t$ may draw on
the magnetic energy in a volume $\sim \gamma^4 (c \Delta t)^3$, although only a
fraction $\sim 1/\gamma^2$ of this may be dissipated without decollimation.
