Measuring the smearing of the Galactic 511-keV signal: positron propagation or supernova kicks?

Abstract

Abstract We use 15 years of γ-ray data from INTEGRAL/SPI in a refined investigation of the morphology of the Galactic bulge positron annihilation signal. Our spatial analysis confirms that the signal traces the old stellar population, revealing for the first time that it traces the boxy bulge and nuclear stellar bulge, while disfavouring the presence of additional dark matter components. Using a 3D smoothing kernel, we find that the signal is smeared out over a characteristic length scale of 150 ± 50 pc, suggesting either annihilation in situ at astrophysical sources kicked at formation or positron propagation away from sources. The former is disfavoured by its requiring kick velocities different between the Galactic nucleus (≳ 50 km s−1) and wider bulge (≲ 15 km s−1) source. Positron propagation prior to annihilation can explain the overall phenomenology of the 511 keV signal for positrons injection energies ≲ 1.4 MeV, suggesting a nucleosynthesis origin.