Inhomogeneity in the Supernova Remnants as a Natural Explanation of the PAMELA Observations
by
Prof.Tsvi Piran(The Hebrew University)
→
Europe/Stockholm
Nordita seminar room (132:028)
Nordita seminar room (132:028)
Description
Recent measurements of the positron/electron ratio in the cosmic ray (CR)
ux exhibits an apparent anomaly, whereby this ratio increases between 10 and 100 GeV. In contrast, this
ratio should decrease according to the standard scenario, in which CR positrons are secon-
daries formed by hadronic interactions between the primary CR protons and the interstellar
medium (ISM). The positron excess is therefore interpreted as evidence for either an anni-
hilation/decay of weakly interacting massive particles, or for a direct astrophysical source of
pairs. The common feature of all proposed models is that they invoke new physics or new
astrophysical sources. However, this line of argumentation relies implicitly on the assumption
of a relatively homogeneous CR source distribution. 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 low energy electrons (1 GeV) 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 supernovae to the galac-
tic spiral arms. Assuming supernova remnants (SNRs) as the sole primary source of CRs, and
taking into account their concentration near the galactic spiral arms, we consistently predict
the observed positron fraction between 1 and 100 GeV, while abiding to dierent constraints
such as the observed electron spectrum and the CRs cosmogenic age. Electron spec-
excess at around 600 GeV can be explained, in this picture, as the contribution of a few
known nearby SNRs.
