Two-zone Diffusion of Electrons and Positrons from Geminga Explains the Positron Anomaly

Abstract

The recent HAWC observations of very-high-energy γ-ray halo around Geminga and Monogem indicate a very slow diffusion of cosmic rays which results in tiny contribution of positrons from these two pulsars to the local flux. This makes the cosmic positron excess anomaly observed by PAMELA and AMS-02 even more puzzling. However, from the Boron-to-Carbon ratio data one can infer that the average diffusion coefficient in the Galaxy should be much larger. In this work we propose a two-zone diffusion model that the diffusion is slow only in a small region around the source, outside of which the propagation is as fast as usual. We find that such a scenario can naturally explain the positron excess data with parameters even more reasonable than that in the conventional one-zone diffusion model. The reason is that during the life time of Geminga (∼ 300 kyr) the electrons/positrons have propagated too far away with a fast diffusion and lead to a low local flux. The slow diffusion region in the two-zone model helps to confine the electrons/positrons for a long time and lead to an enhancement of the local flux. So under the constraint of the HAWC observations, pulsars are still the probable origin of the cosmic-ray positron excess.

Some new hints on cosmic-ray propagation from AMS-02 nuclei spectra

“…It infers that there exists slower diffusion zone (SDZ) around PWN, which could be extended to CR sources [49]. Some previous works [50,51] show that positron excess in CRs could be explained by this twozone model. If we locate in such a SDZ [52], a smaller D 0 can lead to produce more secondary nuclei species' flux than the uniform diffusion in the whole galaxy.…”

Cosmic ray models

“…Refs. [237][238][239]. Using a standard diffusion coefficient outside the sources, the positron flux is strongly increased relative to the analysis of Ref.…”

Cosmic-Ray Propagation in Light of the Recent Observation of Geminga

“…Profumo et al (2018) highlighted that such nearby sources have not been identified and proposed a two-zone diffusion model with the slow diffusion confined to a small region around the PWN. Other authors have considered similar scenarios with varying details (Tang & Piran 2018;Fang et al 2018;Evoli et al 2018). Interestingly, a similar suppression of the CR diffusion was observed in the Large Magellanic Cloud around the 30 Doradus starforming region, where an analysis of combined γ-ray and radio observations yielded a diffusion coefficient, averaged over a region with radius 200-300 pc, an order of magnitude smaller than the typical value in the Milky Way (Murphy et al 2012).…”