On the origin of a highly dispersed coherent radio burst

We discuss the possible source of a highly dispersed radio transient discovered in the Parkes Multibeam Pulsar Survey (PMPS). The pulse has a dispersion measure of 746 cm−3 pc, a peak flux density of 400 mJy for the observed pulse width of 7.8 ms and a flat spectrum across a 288-MHz band centred on 1374 MHz. The flat spectrum suggests that the pulse did not originate from a pulsar, but is consistent with radio-emitting magnetar spectra. The non-detection of subsequent bursts constrains any possible pulsar period to ≳1 s, and the pulse energy distribution to being much flatter than typical giant pulse emitting pulsars. The burst is also consistent with the radio signal theorized from an annihilating mini black hole. Extrapolating the PMPS detection rate provides a limit of ΩBH ≲ 5 × 10−14 on the density of these objects. We investigate the consistency of these two scenarios, plus several other possible solutions, as potential explanations to the origin of the pulse, as well as for another transient with similar properties: the Lorimer burst.