[Paper Review] Imprint of a 2 Myr old source on the cosmic ray anisotropy

V. Savchenko, M. Kachelrieß+1 more|arXiv (Cornell University)|May 11, 2015

Dark Matter and Cosmic PhenomenaPhysics and Astronomy20 references47 citations

TL;DR

This paper proposes that the plateau in cosmic ray dipole anisotropy between 2–20 TeV arises from a single nearby source aged 2 Myr, with its flux anisotropy determined solely by its distance (~200 pc) and age, independent of magnetic fields or CR energy. Numerical simulations of CR trajectories confirm that the dipole anisotropy δ ≈ 3R/(2cT) holds even with regular and turbulent Galactic magnetic fields, providing a natural explanation for the energy-independent plateau and linking it to local sources responsible for positron and antiproton enhancements.

ABSTRACT

We study numerically the anisotropy of the cosmic ray (CR) flux emitted by a single source calculating the trajectories of individual CRs. We show that the contribution of a single source to the observed anisotropy is instead determined solely by the fraction the source contributes to the total CR intensity, its age and its distance,and does not depend on the CR energy at late times. Therefore the observation of a constant dipole anisotropy indicates that a single source dominates the CR flux in the corresponding energy range. A natural explanation for the plateau between 2--20 TeV observed in the CR anisotropy is thus the presence of a single, nearby source. For the source age of 2 Myr, as suggested by the explanation of the antiproton and positron data from PAMELA and AMS-02 through a local source [arXiv:astro-ph/1504.06472], we determine the source distance as $\sim 200$ pc. Combined with the contribution of the global CR sea calculated in the escape model, we can explain qualitatively the data for the dipole anisotropy. Our results suggest that the assumption of a smooth CR source distribution should be abandoned between 200 GeV and 1 PeV.

Motivation & Objective

• To explain the observed plateau in cosmic ray dipole anisotropy between 2–20 TeV, which contradicts standard diffusive propagation models.
• To investigate whether a single nearby source can dominate the anisotropy in this energy range, given its age and distance.
• To test whether the dipole anisotropy from a single source is independent of CR energy and magnetic field structure in the quasi-Gaussian diffusion regime.
• To reconcile the observed anisotropy with local sources that also explain anomalies in antiproton and positron spectra from PAMELA and AMS-02.
• To challenge the assumption of a smooth, global cosmic ray source distribution in the 200 GeV–1 PeV range.

Proposed method

• Numerical simulation of individual cosmic ray trajectories using the Lorentz equation in turbulent and regular Galactic magnetic fields.
• Use of nested grids to resolve small-scale magnetic fluctuations relative to Larmor radii.
• Modeling the source as an instantaneous injection at a point, with momentum distribution measured on concentric spheres from 1 pc to 1 kpc.
• Computation of flux anisotropy A = (Fmax − Fmin)/(Fmax + Fmin) and conversion to intensity anisotropy δ = 3A/2.
• Systematic averaging over symmetric regions (spherical or axial) to reduce statistical uncertainty.
• Validation of the analytical formula A = R/(cT) in the quasi-Gaussian regime under varying magnetic field conditions, including both turbulent and regular components.

Experimental results

Research questions

• RQ1 Can a single, nearby cosmic ray source explain the energy-independent plateau in dipole anisotropy between 2–20 TeV?
• RQ2 Is the dipole anisotropy of a single source independent of CR energy and magnetic field structure in the quasi-Gaussian diffusion regime?
• RQ3 What distance and age are required for a single source to produce the observed anisotropy amplitude and plateau shape?
• RQ4 How does the presence of a dominant local source affect the validity of the standard assumption of a smooth, global cosmic ray source distribution?
• RQ5 Can the same source explain both the anisotropy plateau and the enhancements in positron and antiproton fluxes observed by PAMELA and AMS-02?

Key findings

• The dipole anisotropy δ of a single cosmic ray source is independent of CR energy and magnetic field configuration in the quasi-Gaussian regime, with δ ≈ 3R/(2cT).
• For a source age of 2 Myr and distance of ~200 pc, the predicted anisotropy matches the observed plateau in the 2–20 TeV range.
• The presence of a regular magnetic field does not alter the anisotropy, confirming that δ ≈ 3R/(2cT) holds even with aligned field lines.
• The observed anisotropy plateau is best explained by a single local source dominating the flux in the 2–20 TeV range, with the lower energy decline due to decreasing source contribution.
• The model implies that the assumption of a smooth, global cosmic ray source distribution breaks down between ~200 GeV and 1 PeV, requiring a local source treatment.
• The results are consistent with a 2 Myr-old supernova remnant at ~200 pc, potentially responsible for the 60Fe overabundance in deep-sea crust, and with the local source explanation for positron and antiproton anomalies.