Compton backscattered 511 keV annihilation line emission and the 170 keV line from the Galactic center direction

Abstract

We show that Compton scattering of 511 keV electron-positron annihilation radiation produces a linelike reflection feature at ∼170 keV from backscattered photons. Assuming simple models of clouds and accretion disks around a compact source, we explore the spectrum of Compton-scattered annihilation line emission for a range of geometries, opacities, and observing angles, and we find that the linelike feature is produced under a wide variety of conditions. We further show that such Compton backscattering of slightly redshifted annihilation line emission from the inner edge of an accretion disk could account for the 170 keV line emission and higher energy continuum observed together with the 511 keV annihilation radiation from the direction of the Galactic center. Identification of the observed 170 keV line as a slightly redshifted annihilation line reflection feature provides strong new evidence that the source of this emission is a compact object surrounded by a disk of presumably accreting matter.