Positive charge prevalence in cosmic rays: Room for dark matter in the positron spectrum

PHYSICAL REVIEW D 94, 063006 (2016) Positive charge prevalence in cosmic rays: Room for dark matter in the positron spectrum M. A. Malkov, 1 P. H. Diamond, 1 and R. Z. Sagdeev 2 CASS and Department of Physics, University of California, San Diego, California 92093-0424, USA University of Maryland, College Park, Maryland 20742-0280, USA (Received 12 July 2016; published 15 September 2016) The unexpected energy spectrum of the positron/electron ratio is interpreted astrophysically, with a possible exception of the 100–300 GeV range. The data indicate that this ratio, after a decline between 0.5 and 8 GeV, rises steadily with a trend towards saturation at 200–400 GeV. These observations (except for the trend) appear to be in conflict with the diffusive shock acceleration (DSA) mechanism, operating in a single supernova remnant (SNR) shock. We argue that e þ =e − ratio can still be explained by the diffusive shock acceleration if positrons are accelerated in a subset of SNR shocks which (i) propagate in clumpy gas media and (ii) are modified by accelerated cosmic ray protons. The protons penetrate into the dense gas clumps upstream to produce positrons and charge the clumps positively. The induced electric field expels positrons into the upstream plasma where they are shock accelerated. Since the shock is modified, these positrons develop a harder spectrum than that of the cosmic ray electrons accelerated in other SNRs. Mixing these populations explains the increase in the e þ =e − ratio at E > 8 GeV. It decreases at E 8 GeV. Below this energy, it declines, thus creating a deficit. The decline, the rise, and the clear minimum between them (at 8 GeV) are all pivotal to the mechanism proposed here. These aspects are intrinsic to a single-source mechanism proposed, revealing unique characteristics of the acceler- ator. By contrast, assuming two or more independent positron contributions to the spectrum (as, e.g., in Refs. [21–23]), one fits the nonmonotonic positron fraction, but with no constraints on the underlying acceleration mechanisms. The position of the minimum in the positron fraction is then coincidental, and the fit does not add © 2016 American Physical Society