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

¹ Institute of Applied Physics, Russian Academy of Science, 46 Ulyanov st., 603950 Nizhny Novgorod, Russia
² Dept. of Physics, Texas A& M University, College Station, TX 77843-4242, USA e-mail: belyanin@atlantic.tamu.edu
³ MPI für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany

Corresponding author: E. V. Derishev, derishev@mpi-hd.mpg.de

Received: 21 August 2001
Accepted: 1 October 2001

Abstract

It has been recognized long ago that the presence of hot plasma in the inner accretion disks around black holes could lead to the neutron production via dissociation of helium nuclei. We show that, for a broad range of accretion parameters, neutrons effectively decouple from protons and pile up in the inner disk leading to the formation of self-sustained halo. This means that new neutrons in the halo are supplied mainly by the splitting of helium nuclei in their collisions with existing neutrons. Once formed, such a halo can exist even if the proton temperature is much lower than the energy threshold of helium dissociation. We show that neutron haloes can be the natural source of relativistic electrons and positrons, providing characteristic comptonization spectra and hard spectral tails observed in many black hole candidates, and also giving rise to relativistic outflows. Deuterium gamma-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 rich variety of transient dynamical regimes.