Precision measurement of cosmic-ray spectra

Broadly viewed, the energy spectra of the dozen or so nuclear species that dominate the flux of cosmic rays (CRs) bombarding the top of the atmosphere all look much the same, exhibiting a featureless E ^−2.7 power-law falloff with increasing energy E up to about 10⁶ GeV. That seeming universality has bolstered a standard paradigm, which presumes that CRs originating in the Milky Way come almost entirely from supernova remnants whose expanding shock fronts can accelerate ions to as high as 10⁶ GeV. But now PAMELA , the first satellite-borne magnetic spectrometer dedicated to CR observation, has taken a closer look at the spectra of the two principal CR species: hydrogen and helium. Having analyzed two years of data, the PAMELA collaboration reports results that conflict with the standard paradigm. The power-law falloff of the H spectrum is distinctly steeper than that of the He spectrum, and each spectrum exhibits an abrupt upward kink at a few hundred GeV. Both results had been hinted at by earlier, less precise balloon data, so theoretical speculation has had something of a head start. The data suggest that in addition to supernova remnants, other classes of astrophysical accelerators within the galaxy contribute significantly to the CR flux. The list of candidates includes nova explosions on white dwarf stars, superbubbles of diffuse hot gas hundreds of light-years across, and even larger-scale plasma winds associated with the galaxy’s spiral arms. (O. Adriani et al., Science, in press, doi:10.1126/science.1199172 .)—Bertram Schwarzschild