Natural explanation for the anomalous positron to electron ratio with supernova remnants as the sole cosmic ray source

Recent measurements of the positron/electron ratio in the cosmic ray (CR) flux exhibits an apparent anomaly1, whereby this ratio increases with energy between 10 and 100 GeV. In contrast, this ratio should decrease in the standard scenario, in which CR positrons are secondaries formed by hadronic interactions between the primary CR protons and the interstellar medium (ISM)2. The positron excess is therefore explained as evidence for either an annihilation/decay product of weakly interacting massive particles (e.g., refs. 3, 4) or for a direct astrophysical source of pairs, such as Pulsars5–10. This line of argumentation, however, implicitly relies on the assumption of a relatively homogeneous CR source distribution. Here we show that allowing for inhomogeneity of CR sources on a scale of order a kpc, can naturally explain this anomaly. If the nearest major CR source is about a kpc away, then at low energies (∼ 1 GeV) electrons can easily reach us. At higher energies (& 10 GeV), the source electrons cool via synchrotron and inverse-Compton before reaching the solar vicinity. Pairs formed in the local vicinity through the proton/ISM interactions can reach the solar system also at high energies, thus increasing the positron/electron ratio. A natural origin of source inhomogeneity is the strong concentration of star formation in the galactic spiral arms. In fact, we show that by assuming supernova remnants as the sole primary source of CRs, and taking into account that most supernovae are expected to take place near the galactic spiral arms, we consistently predict the observed positron to electron ratio between 1 and 100 GeV, while abiding to different constraints such as the observed electron spectrum and the CRs cosmogenic age. ATIC’s11 electron spectrum excess at ∼ 600 GeV can be explained, in this picture, simply as the contribution of a few nearby supernova remnants.