Low-energy Galactic-Centre γ-rays from low-mass X-ray binaries

Abstract

The hard X-ray and low-energy γ-ray emission from the Galactic Centre region (GCR) has four components^1,2: a power-law continuum between 20/50 keV and 200/300 keV with inverse power-law photon index a in the range ∼2.5 to ∼3.1; a harder spectrum between 200/300keV and ∼511 keV with α ≈ 1.0–1.5; a narrow electron–positron annihilation line at 511 keV, reported to disappear^3,4 in < l/2yr, although the temporal variation is controversial⁵; and an equally variable continuum emission between 511 keV and several MeV ('MeV bump'). All four have luminosities of 10³⁷–10³⁸ erg s⁻¹, if the source is located 10 kpc away. We propose non-thermal processes in low-mass X-ray binaries concentrated in the galactic bulge as the direct source of the three continuum components of the emission and also, possibly, as the indirect source of the 511-keV line. We suggest that the softer power-law component of the GCR continuum arises from synchrotron emission of relativistic electrons in the strongly non-uniform magnetic field of the neutron star, and, more tentatively, that the MeV bump is the result of interaction of harder γ-rays with the power-law photons. The hardest power law may be due to Compton scattering of relativistic electrons or photons.