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The Question
(Submitted July 19, 1997)
I've been reading up on the recent positron findings at our
galaxy center and wondered if we know that electron-positron
annihilation always yields energy in the gamma-ray band?
My background in this area is limited to college physics, and
book (net) research in plasma and astrophysics.
The Answer
Thank you for your question. The answer is yes, we know the physics of
electron-positron interactions quite well, because it has been measured
in particle physics labs. As it turns out the mass of an electron
(9.1E-28 grams) times the speed of light squared (E = m c2) is
8.12E-07 ergs of energy. In more common units this is 511 keV (kilo
electron volts).
When an electron and positron annihilate they produce 2 photons, each with
511 keV of energy (so no net energy is gained or lost). When we
observe a spectral emission line at 511 keV, we can be pretty sure it
is caused by this positron/electron interactions.
Jonathan Keohane
for Ask a High-Energy Astronomer
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