Stringent Constraint on Galactic Positron Production

TL;DR

The gamma-ray spectrum due to inflight annihilation is calculated and it is found that the positron injection energies must be less than or similar to 3 MeV, which strongly constrains models for Galactic positron production.

Abstract

The intense 0.511 MeV gamma-ray line emission from the Galactic Center observed by INTEGRAL requires a large annihilation rate of nonrelativistic positrons. If these positrons are injected at even mildly relativistic energies, higher-energy gamma rays will also be produced. We calculate the gamma-ray spectrum due to inflight annihilation and compare to the observed diffuse Galactic gamma-ray data. Even in a simplified but conservative treatment, we find that the positron injection energies must be $\lesssim 3$ MeV, which strongly constrains models for Galactic positron production.

Figures (2)

• Figure 1: Gamma-ray spectra (cusped lines, inflight annihilation; smooth lines, internal bremsstrahlung) from relativistic positrons, normalized to the 0.511 MeV flux; the injection energies are 1, 3, and 10 MeV (dotted, dot-dashed and dashed).
• Figure 2: The INTEGRAL and COMPTEL diffuse gamma ray flux measurements are shown with a black solid line, and their $\pm$30% uncertainties by the shaded band. For positron injection energies of 1, 3, and 10 MeV (dotted, dot-dashed and dashed lines), the thick lines show how this would be increased by the inflight annihilation gamma ray flux (thin lines also include the internal bremsstrahlung flux). All results are for a $5^{\circ}$-diameter region at the Galactic Center. The 0.511 MeV line flux is not shown.