Stellar collisions near a large black hole - a model for compact extragalactic sources

Abstract

A model is proposed to explain the outbursts observed in compact extragalactic sources. It involves collisions between stars deep in the potential well of large (≳ 108M⊙) black holes, which may exist in the nuclei of some active galaxies. The largest outbursts observed have total energy > 1054 erg, which is orders of magnitude greater than the observed output of a supernova. High-energy collisions of stars can produce the required energy to explain these large outbursts as well as the more ‘normal’ variations observed. This model also agrees well with the recent results of Long Baseline Interferometry, which indicate that the outbursts expand relativistically out from a central source rather than the random outburst or ‘Christmas tree’ model of Dent. A discussion is also given on the possible application of the Penrose mechanism. It is concluded that, while the presence of an extreme Kerr black hole enhances the directional transfer of relativistic particles from the collision to infinity, little extra energy can be obtained via the Penrose process.