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Constraining primordial black hole abundance with the Galactic 511 keV line

William DeRocco, Peter W. Graham

TL;DR

The paper addresses whether primordial black holes (PBHs) with masses around $M\sim 10^{16}-10^{17}$ g can make up all dark matter by linking Hawking-evaporated positrons to the Galactic 511 keV line observed by INTEGRAL. It models positron production from PBHs, propagates these positrons through the ISM under conservative bounds (notably $E\lesssim 1$ MeV), and uses the measured 511 keV flux to bound the PBH injection rate, deriving limits on both monochromatic and log-normal PBH mass distributions. The results show that such PBHs cannot constitute all DM in this mass window and improve existing bounds by about an order of magnitude, with stronger constraints for narrower distributions and potential further tightening with better propagation knowledge. The work provides a robust, morphology-insensitive constraint on PBH DM that can be refined with future measurements of Galactic magnetic fields and ISM structure.

Abstract

Models in which dark matter consists entirely of primordial black holes (PBHs) with masses around $10^{17}$ g are currently unconstrained. However, if PBHs are a component of the Galactic dark matter density, they will inject a large flux of energetic particles into the Galaxy as they radiate. Positrons produced by these black holes will subsequently propagate throughout the Galaxy and annihilate, contributing to the Galactic 511 keV line. Using measurements of this line by the INTEGRAL satellite as a constraint on PBH positron injection, we place new limits on PBH abundance in the mass range $10^{16} - 10^{17}$ g, ruling out models in which these PBHs constitute the entirety of dark matter.

Constraining primordial black hole abundance with the Galactic 511 keV line

TL;DR

The paper addresses whether primordial black holes (PBHs) with masses around g can make up all dark matter by linking Hawking-evaporated positrons to the Galactic 511 keV line observed by INTEGRAL. It models positron production from PBHs, propagates these positrons through the ISM under conservative bounds (notably MeV), and uses the measured 511 keV flux to bound the PBH injection rate, deriving limits on both monochromatic and log-normal PBH mass distributions. The results show that such PBHs cannot constitute all DM in this mass window and improve existing bounds by about an order of magnitude, with stronger constraints for narrower distributions and potential further tightening with better propagation knowledge. The work provides a robust, morphology-insensitive constraint on PBH DM that can be refined with future measurements of Galactic magnetic fields and ISM structure.

Abstract

Models in which dark matter consists entirely of primordial black holes (PBHs) with masses around g are currently unconstrained. However, if PBHs are a component of the Galactic dark matter density, they will inject a large flux of energetic particles into the Galaxy as they radiate. Positrons produced by these black holes will subsequently propagate throughout the Galaxy and annihilate, contributing to the Galactic 511 keV line. Using measurements of this line by the INTEGRAL satellite as a constraint on PBH positron injection, we place new limits on PBH abundance in the mass range g, ruling out models in which these PBHs constitute the entirety of dark matter.

Paper Structure

This paper contains 6 sections, 7 equations, 2 figures.

Table of Contents

  1. Introduction
  2. Positron propagation in the galaxy
  3. Positron production by PBHs
  4. INTEGRAL bound
  5. Results
  6. Conclusion

Figures (2)

  • Figure 1: New bounds on PBH DM with a monochromatic mass distribution are shown in green. The blue dotted line is computed in the same manner as the green region but without an upper limit on positron injection energy (see Section \ref{['sec: prop']} for a discussion of the 1 MeV limit used to set our final bounds). We display the existing bounds from gamma-ray observations Carr:2009jmCarr:2016hva and positron measurements by the Voyager spacecraft Boudaud:2018hqb in gray.
  • Figure 2: Bounds on PBHs with a log-normal distribution centered at a mass $\mu$ for various widths $\sigma$. The positron injection bounds are shown as solid lines, while the existing Voyager bounds are shown as dotted lines Boudaud:2018hqb.