Mapping galactic positrons

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 emit at energies above 20 keV. In hard LMXBs, a low-mass star donates material via an accretion disk to a compact companion star—either a black hole or a neutron star. Theorists think that the inner disk gets so hot that electron–positron pairs are spontaneously produced by ambient gamma rays. If, as the two maps suggest, pair production in LMXBs accounts for the 511-keV emission, then other exotic explanations, such as the decay of dark-matter particles, are no longer needed. (G. Weidenspointner et al., Nature 451 , 159, 2008 http://dx.doi.org/10.1038/nature06490 .)