Low Energy Galactic Center Gamma Rays from Low Mass X-Ray Binaries

Abstract

The hard X-ray and low energy γ-ray emission from the galactic center region (GCR) has four components1,2: a power-law continuum between 20/50 keV and 200/300 keV with a power-law photon index β in the range ~ 2.5 to ~ 3.1; a harder spectrum with β ~ 1.–1.5 between 200/300 keV and ~ 511 keV; a narrow electron-positron annihilation line at 511 keV, reported to disappear3,4 in less than < 1/2 yr, although the temporal variation is controversial5; and an equally variable continuum emission between 511 keV and several MeV (“MeV bump”). All four have luminosities 1037-1038 erg s-1, if they are located 10 kpc away. We propose non-thermal processes in low mass X-ray binaries (LMXB’s) concentrated in the galactic bulge as the direct source of the three continuum components of the emission, as well as of an escaping electron-positron e ± wind whose positron annihilation relatively far from the star could be the source of the 511 keV line. We consider a model for energetic emission from LMXB’s that reproduces the softer power-law component of the GCR continuum through synchrotron emission of relativistic electrons in the strongly non-uniform (dipolar) magnetic field of the neutron star. We also explain, with less confidence, the variable MeV bump as the result of interaction of harder γ-rays with the power-law photons. The harder power law might be due to Compton scattering of relativistic electrons or photons.