Galactic annihilation emission from nucleosynthesis positrons

Abstract

The Galaxy hosts a widespread population of low-energy positrons revealed by successive generations of gamma-ray telescopes through a bright annihilation emission from the bulge region, with a fainter contribution from the inner disk. The exact origin of these particles remains currently unknown. We estimate the contribution to the annihilation signal of positrons generated in the decay of radioactive 26Al, 56Ni and 44Ti. We adapted the GALPROP propagation code to simulate the transport and annihilation of radioactivity positrons in a model of our Galaxy. Using plausible source spatial distributions, we explored several possible propagation scenarios to account for the large uncertainties on the transport of ~1MeV positrons in the interstellar medium. We then compared the predicted intensity distributions to the INTEGRAL/SPI observations. We obtain similar intensity distributions with small bulge-to-disk ratios, even for extreme large-scale transport prescriptions. At least half of the positrons annihilate close to their sources, even when they are allowed to travel far away. In the high-diffusion, ballistic case, up to 40% of them escape the Galaxy. In proportion, this affects bulge positrons more than disk positrons because they are injected further off the plane in a tenuous medium, while disk positrons are mostly injected in the dense molecular ring. The predicted intensity distributions are fully consistent with the observed longitudinally-extended disk-like emission, but the transport scenario cannot be strongly constrained by the current data. Nucleosynthesis positrons alone cannot account for the observed annihilation emission in the frame of our model. An additional component is needed to explain the strong bulge contribution, and the latter is very likely concentrated in the central regions if positrons have initial energies in the 100keV-1MeV range.

16 pages, 7 figures, accepted for publication in A&A

References in corpus (10)

• Rates and properties of type Ia supernovae as a function of mass and star-formation in their host galaxies
• The Vertical Structure of Warm Ionised Gas in the Milky Way
• Global cosmic-ray related luminosity and energy budget of the Milky Way
• Inverse Compton Origin of the Hard X-Ray and Soft Gamma-Ray Emission from the Galactic Ridge
• Spatial distribution of interstellar gas in the innermost 3 kpc of our Galaxy
• The signature of 44Ti in Cassiopeia A revealed by IBIS/ISGRI on INTEGRAL
• The Galactic Positron Annihilation Radiation & The Propagation of Positrons in the Interstellar Medium
• Continuum gamma-ray emission from light dark matter positrons and electrons
• Spectral and intensity variations of Galactic 26^Al emission
• Late Light Curves of Normal Type Ia Supernovae

Cited by in corpus (15)

• Gamma-ray spectroscopy of Positron Annihilation in the Milky Way
• Nuclear-Astrophysics Lessons from INTEGRAL
• The Positron Puzzle
• Monte Carlo modelling of the propagation and annihilation of nucleosynthesis positrons in the Galaxy
• Supernova bounds on axion-like particles coupled with nucleons and electrons
• 511 keV line constraints on feebly interacting particles from supernovae
• Positron Annihilation in the Nuclear Outflows of the Milky Way
• Starbursts and High-Redshift Galaxies are Radioactive: High Abundances of ${}^{26}$Al and Other Short Lived Radionuclides
• Extinct radio pulsars as a source of subrelativistic positrons
• Galactic Positrons from Thermonuclear Supernovae
• The effect of positron-alkali metal atom interactions in the diffuse ISM
• Gamma-ray line emission from the Local Bubble
• Updated Constraints on the Injection Energy of Positrons Generating the Galactic 511 keV $\gamma$-ray line
• Galactic Electron-Positron Annihilation Emission from X-Ray Binaries with Transient and Persistent Jets
• Measuring the smearing of the Galactic 511 keV signal: positron propagation or supernova kicks?