Recurrence times of γ-ray bursts from neutron star/comet encounters

RECENT observations¹ from Phobos 2 added about one hundred γ-ray bursts to the several hundred detected over the past 20 years. One of the most promising explanations of these bursts² is that they originate from the activation of the magnetospheres of old neutron stars³, for example by encounters with small bodies. Here we use estimates of the number density of small bodies, either interstellar or in circumstellar Oort-like clouds, to predict the recurrence times of γ-ray bursts from individual sources, on the assumption that they are produced from neutron star/comet encounters (direct impacts^4,5, captures^6,7 or passages⁸). We find that 'classical' (non-repeating) bursts may be accounted for by passages of interstellar small bodies through the magnetosphere, whereas the small number of repeating bursts is consistent with the interaction of neutron stars with small bodies within Oort-like clouds. In the latter case, the best agreement with observational statistics is found by adopting the recent model of the Oort cloud⁹ derived from the study of comet Halley by the Vega spacecraft¹⁰.