Gamma-rays from the vicinity of accreting neutron stars inside compact high-mass X-ray binaries

Department of Astrophysics, University of Łódź, 90-236 Łódź, ul. Pomorska 149/153, Poland e-mail: bednar@fizwe4.phys.uni.lodz.pl

Received: 1 October 2008
Accepted: 22 November 2008

Abstract

The dense wind of a massive star can be captured partially by a neutron star (NS) inside a compact binary system. Depending on the parameters of the NS and the wind, the matter can penetrate the inner NS magnetosphere. At some distance from the NS, a turbulent and magnetized transition region is formed due to the balance between the magnetic pressure and the pressure inserted by accreting matter. This region provides good conditions for acceleration of particles to relativistic energies. The matter at the transition region can farther accrete onto the NS surface (the accretor phase) or be expelled from the NS vicinity (the propeller phase). We consider the consequences of the acceleration of electrons at the transition region, concentrating on the situation in which at least part of the matter falls onto the NS surface. This matter creates a hot spot on the NS surface which emits thermal radiation. Relativistic electrons lose energy to the synchrotron process and the inverse Compton (IC) scattering of this thermal radiation. We calculate the synchrotron spectra (from X-rays to soft γ-rays) and IC spectra (above a few tens of MeV) expected in this scenario. We argue that a population of massive binaries discovered by the INTEGRAL observatory, which contain neutron stars hidden inside dense stellar winds of massive stars, is detectable by the Fermi LAT telescope in the GeV energy range. As an example, we predict the expected γ-ray flux from the source IGR J19140+0951.