Positron Annihilation Radiation from the Galactic Center: ``Cheshire Cat'' Compton Scattering and the Origin of Excess Continuum
Abstract
Two observations of the gamma-ray spectrum from the direction of the Galactic center were made by HEAO 3 in the fall of 1979 and the spring of 1980. The 2-gamma 511 keV annihilation line flux decreased by a factor of about three during the 6 months between these observations, while the excess gamma-ray continuum below the annihilation line, often interpreted as 3-gamma decay of orthopositronium, barely changed. This discrepancy in temporal behavior makes the identification of the bulk of excess continuum as 3-gamma decay of positronium difficult. It is shown that Compton scattering of the line and high-energy radiation provides a natural explanation for the surprisingly small changes seen in the excess continuum. Scattered photons are delayed by a time corresponding to the size of the scattering region. For the annihilation source in the Galactic center, this distance is probably a fraction of a parsec. Thus, even after the high-energy continuum and annihilation line are gone, low-energy Compton-scattered photons can still be detected with an almost unchanged flux.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- June 1988
- DOI:
- 10.1086/166371
- Bibcode:
- 1988ApJ...329..212B
- Keywords:
-
- Compton Effect;
- Elementary Particles;
- Emission Spectra;
- Galactic Nuclei;
- Gamma Ray Astronomy;
- Positron Annihilation;
- Gas Mixtures;
- Heao 3;
- Ionized Gases;
- Astrophysics;
- GALAXIES: NUCLEI;
- GALAXIES: THE GALAXY;
- GAMMA RAYS: GENERAL