Formation and dynamics of self-sustained neutron haloes in disk accreting sources

It has been recognized long ago that the presence of hot plasma in the inne r accretion disks around black holes could lead to the neutron production v ia dissociation of helium nuclei. We show that, for a broad range of accret ion parameters, neutrons effectively decouple from protons and pile up in t he inner disk leading to the formation of self-sustained halo. This means t hat new neutrons in the halo are supplied mainly by the splitting of helium nuclei in their collisions with existing neutrons. Once formed, such a hal o can exist even if the proton temperature is much lower than the energy th reshold of helium dissociation. We show that neutron haloes can be the natu ral source of relativistic electrons and positrons, providing characteristi c comptonization spectra and hard spectral tails observed in many black hol e candidates, and also giving rise to relativistic outflows. Deuterium gamm a-ray line at 2.2 MeV resulting from neutron capture is also expected at a level detectable by future INTEGRAL mission. Furthermore, the presence of a neutron halo strongly affects the dynamics of accretion and leads to the r ich variety of transient dynamical regimes.