Mapping galactic positrons Available to Purchase

Thirty years ago, Marvin Leventhal, Crawford MacCallum, and Paul Stang detected 511-keV photons emanating from the center of our galaxy. Evidently, positrons are meeting electrons there and annihilating each other. But what produces the positrons? In the near vacuum of interstellar space, a positron typically travels for 100 000 years before annihilation. The corresponding distance depends on local magnetic fields, but it’s small enough that a map of the galaxy’s 511-keV emission should reveal the locations of positron production and, indirectly, the production mechanism itself. Now, after four years of photon gathering, the Integral spacecraft has produced the best-ever map of the central region of the galaxy. The map (below left) confirms earlier findings that the emission is strongly peaked at the galactic center itself. It also shows for the first time a distinct and significant asymmetry that coincides with the distribution (below right) of “hard” low-mass x-ray binaries that...