Gamma-ray spectroscopy of positron annihilation in the Milky Way

Abstract

Context. The annihilation of positrons in the Galaxy's interstellar medium produces characteristic gamma-rays with a line at 511 keV. This gamma-ray emission has been observed with the spectrometer SPI on ESA's INTEGRAL observatory, confirming a puzzling morphology with bright emission from an extended bulge-like region, while emission from the disk is faint. Most known or plausible sources of positrons are, however, believed to be distributed throughout the disk of the Milky Way. Aims: We aim to constrain characteristic spectral shapes for different spatial components in the disk and bulge using data with an exposure that has doubled since earlier reports. Methods: We exploit high-resolution gamma-ray spectroscopy with SPI on INTEGRAL based on a new instrumental background method and detailed multi-component sky model fitting. Results: We confirm the detection of the main extended components of characteristic annihilation gamma-ray signatures, altogether at 58σ significance in the 511 keV line. The total Galactic 511 keV line intensity amounts to (2.74 ± 0.25) × 10-3 ph cm-2 s-1 for our assumed model of the spatial distribution. We derive spectra for the bulge and disk, and a central source modelled as point-like and at the position of Sgr A*, and discuss spectral differences. The bulge (56σ) shows a 511 keV line intensity of (0.96 ± 0.07) × 10-3 ph cm-2 s-1 together with ortho-positronium continuum equivalent to a positronium fraction of (1.080 ± 0.029). The two-dimensional Gaussian that represents the disk emission (12σ) has an extent of 60+10-5 degrees in longitude and a rather large latitudinal extent of 10.5+2.5-1.5 degrees; the line intensity is (1.66 ± 0.35) × 10-3 ph cm-2 s-1 with a marginal detection of the annihilation continuum and an overall diffuse Galactic continuum of (5.85 ± 1.05) × 10-5 ph cm-2 s-1 keV-1 at 511 keV. The disk shows no flux asymmetry between positive and negative longitudes, although spectral details differ. The flux ratio between bulge and disk is (0.58 ± 0.13). The central source (5σ) has an intensity of (0.80 ± 0.19) × 10-4 ph cm-2 s-1.