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 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 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 10 21 e and total energy 10 23 erg. Bursts from Gpc distances require incoherent superposition of 10 12 d 2 Gpc shot pulses or a total energy 10 35 d 2 Gpc erg. The energy reservoir near the light cylinder limits the detection distance to few 100 Mpc for a fluence 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.