Stringent Constraint on Galactic Positron Production

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 it with the observed diffuse Galactic gamma-ray data. Even with a simplified but conservative treatment, we find that the positron injection energies must be $\lesssim 3\mathrm{MeV}$, which strongly constrains models for Galactic positron production.

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 lines).

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°-diameter region at the Galactic Center. The 0.511 MeV line flux is not shown.