Supergiant pulses from extragalactic neutron stars

We consider radio bursts that originate from extragalactic neutron stars (NSs) by addressing three questions about source distances. What are the physical limitations on coherent radiation at GHz frequencies? Do they permit detection at cosmological distances? How many bursts per NS are needed to produce the inferred burst rate similar to 10(3)-10(4)sky(-1) d(-1)? The burst rate is comparable to the NS formation rate in a Hubble volume, requiring only one per NS if they are bright enough. Radiation physics suggests a closer population, requiring more bursts per NS and increasing the chances for repeats. Bursts comprise sub-ns, coherent shot pulses superposed incoherently to produce ms-duration similar to 1 Jy amplitudes; each shot pulse can be much weaker than 1 Jy, placing less restrictive requirements on the emission process. None the less, single shot pulses are similar to the extreme, unresolved (<0.4 ns) MJy shot pulse seen from the Crab pulsar, consistent with coherent curvature radiation emitted near the light cylinder by an almost neutral clump with net charge similar to +/- 10(21)e and total energy greater than or similar to 10(23) erg. Bursts from Gpc distances require incoherent superposition of similar to 10(12)d(Gpc)(2) shot pulses or a total energy greater than or similar to 10(35)d(Gpc)(2) erg. The energy reservoir near the light cylinder limits the detection distance to less than or similar to few x 100 Mpc for a fluence similar to 1 Jy ms unless conditions are more extreme than for the Crab pulsar, such as in magnetars. We discuss contributions to dispersion measures from galaxy clusters and we propose tests for the overall picture presented.