[Paper Review] Annihilation Radiation in the Galaxy

C. D. Dermer, R. J. Murphy|arXiv (Cornell University)|Jul 12, 2001

Dark Matter and Cosmic Phenomena1 references3 citations

TL;DR

This paper reviews astrophysical mechanisms producing positrons in the Galaxy, focusing on annihilation radiation from positron-electron pairs via 2γ and 3γ decay channels. It presents detailed calculations of spectral lines and positronium fractions in various interstellar medium (ISM) conditions, showing that quenching of positronium in hot plasmas or dust can explain discrepancies between 0.511 MeV line and 3γ continuum maps, with implications for INTEGRAL's ability to map positron sources.

ABSTRACT

Observations of annihilation radiation in the Galaxy are briefly reviewed. We summarize astrophysical mechanisms leading to positron production, and recent estimates for production rates from nova and supernova nucleosynthesis in the Galaxy. The physical processes involved in the production of annihilation radiation in the interstellar medium are described. These include positron thermalization, charge exchange, radiative recombination, and direct annihilation. Calculations of 2γand 3γspectra and the positronium (Ps) fraction due to the annihilation of positrons in media containing H and He at different temperatures and ionization states are presented. Quenching of Ps by high temperature plasmas or dust could account for differences between 0.511 MeV and 3γPs continuum maps. These results are presented in the context of the potential of INTEGRAL to map sites of annihilation radiation in the Galaxy. Positron production by compact objects is also considered.

Motivation & Objective

• To summarize the state of annihilation radiation astrophysics in the Galaxy following the Compton Gamma Ray Observatory and before INTEGRAL's launch.
• To evaluate the contributions of novae, supernovae, cosmic rays, and compact objects to positron production rates.
• To model the physical processes governing 2γ and 3γ annihilation spectra in diverse ISM conditions, including thermalization, charge exchange, and radiative recombination.
• To explain discrepancies between 0.511 MeV line and 3γ positronium continuum maps via positronium quenching in hot plasmas or dust.
• To assess INTEGRAL's potential to identify the origin of Galactic positrons through high-resolution imaging and spectral analysis.

Proposed method

• Modeling positron injection into ISM phases with varying temperatures, ionization states, and compositions (H, He) to compute 2γ and 3γ spectra.
• Calculating the 3γ/2γ flux ratio and FWHM line widths as functions of temperature and ionization, using theoretical frameworks for positronium formation and decay.
• Applying the thermal broadening limit ΔE_FWHM ≈ 11.0 T₆^{1/2} keV (T₆ in 10⁶ K) to predict line width evolution in high-temperature media.
• Assessing positronium quenching by high-temperature plasmas and dust as a mechanism suppressing the 3γ continuum relative to the 2γ line.
• Using OSSE and SMM data to constrain omnidirectional 0.511 MeV flux and infer total Galactic annihilation rate (~2×10⁴³ ph s⁻¹).
• Evaluating INTEGRAL’s sensitivity (SPI instrument) for detecting 511 keV line transients and spatially resolved annihilation features with sub-degree resolution.

Experimental results

Research questions

• RQ1 What are the dominant astrophysical sources of positrons in the Galaxy, and what are their estimated production rates?
• RQ2 How do temperature, ionization state, and composition of the ISM affect the 2γ and 3γ annihilation spectra?
• RQ3 Why is the 3γ positronium continuum less detected than the 0.511 MeV line in high-latitude regions, despite theoretical expectations?
• RQ4 Can positronium quenching in hot plasmas or dust explain the observed suppression of the 3γ continuum?
• RQ5 Can INTEGRAL’s imaging and spectral capabilities resolve the origin of the 0.511 MeV line, particularly in relation to compact objects or galactic halo structures?

Key findings

• The omnidirectional 0.511 MeV annihilation line flux is φ₂γ ≈ 2.6×10⁻³ ph cm⁻² s⁻¹, with a total Galactic annihilation rate of ~2×10⁴³ ph s⁻¹ assuming emission from the Galactic Center.
• The 3γ positronium continuum flux is φ₃γ ≈ 10.5×10⁻⁴ ph cm⁻² s⁻¹, with a 3γ/2γ flux ratio of ~5.8 in the Galactic Center region.
• In fully neutral media, the 3γ/2γ ratio approaches 9/2 due to ortho-positronium dominance, but decreases at higher temperatures due to reduced charge exchange and Ps formation.
• Thermal broadening increases the 2γ line FWHM with temperature, approaching ΔE_FWHM ≈ 11.0 T₆^{1/2} keV in hot plasmas.
• Quenching of positronium in high-temperature plasmas or dust can explain the absence of a high-latitude 3γ enhancement despite a strong 0.511 MeV line at high latitudes.
• INTEGRAL’s SPI instrument has a sensitivity of 2–3×10⁻⁵ ph cm⁻² s⁻¹ for on-axis point sources over 10⁶ s, enabling detection of transient or localized annihilation sources.