Positron annihilation in the Milky Way and beyond

Abstract

The electron-positron annihilation gamma-ray signal at 511 keV in the Milky Way is investigated towards a possible dark matter interpretation. If all bulge positrons were created by dark matter particle annihilation, the satellite galaxies of the Milky Way, apparently being dominated by dark matter, should also show measurable 511 keV signals. Using INTEGRAL/SPI, we test for emission in 39 neighbouring dwarf satellite galaxies, and found a consistent trend against a dark matter scenario. One galaxy, Reticulum II, shows up as a strong source of annihilation emission, which we interpret as the presence of a microquasar, ejecting pair-plasma into the galaxy's interstellar medium.

The Positron Puzzle

“…I want to stress, that this is not an absolute measure of how good the fit is, nor is this intended. Figure 3, shows the χ 2 -values as a function of tracer map photon energy for the ortho-Ps band, the 511 keV line band, and the continuum band above the e + -annihilation line (Siegert 2017). It can be seen that the line and ortho-Ps behave similarly as a function of tracer map photon energy, Fig.…”

“…more granular images from reconstruction algorithms show a similar trend of where the flux is enhanced, and could reveal details if were it not for the strong instrumental background in MeV telescopes. While the basic structure is also found with Richardson-Lucy (Knoedlseder et al 2005), Maximum Likelihood (Bouchet et al 2010), or Maximum Entropy (Siegert 2017) deconvolutions, image artefacts naturally emerge from the finite number of photons detected and to be distributed over a large number of pixels. High-resolution spectroscopy of the 511 keV line for the bulge and disk components suggests that the annihilation of e + s occurs dominantly in the interstellar medium (ISM), which would partly explain the 'diffuse' nature of the image.…”

“…Stars may be both, the dominant sources and the natural sinks, of e + s in the Galaxy. In fact, contemporaneous to Siegert (2017), Bisnovatyi-Kogan and Pozanenko (2017) suggested flaring stars to be a major contributor to the Galactic 511 keV signal.…”

“…It can be seen that the line and ortho-Ps behave similarly as a function of tracer map photon energy, Fig. 3 Likelihood comparisons of different tracer maps as a function of photon energy in three bands, including the ortho-Ps continuum, the 511 keV line, and the diffuse Galactic continuum emission (Siegert 2017) except for the 10-200 keV range. This is understandable because the continuum emission above the line is nicely traced by X-ray sources as seen by the dip (improvement) in χ 2 in the lower panel, and also impacts the emission below the line.…”

“…Shallow gradients in the morphology are also difficult to reliably detect with coded aperture masks (Siegert et al 2022d) where A i is the nucleon number of isotope i. For example, given the estimate of 2.8 M of 26 Al in the Milky Way (Siegert 2017;Pleintinger 2020), and a probability for β + -decay of 26 Al of p 26 = 82%, we obtain a production rate 5 of 3.2 × 10 42 e + s −1 . These are at most 10% of the total annihilate rate in the Milky Way, which means that 26 Al alone cannot explain the measured 511 keV line, unless there was a star burst from which the e + s are still annihilating now (e.g., Prantzos et al 2011;Alexis et al 2014).…”