Probing Nearby CR Accelerators and ISM Turbulence with Milagro Hot Spots

TL;DR

This paper proposes a mechanism involving anisotropic Alfven wave scattering to explain the narrow, energy-independent cosmic ray anisotropy observed by MILAGRO, linking it to local turbulence and potential nearby sources.

Contribution

It introduces a systematic transport theory explaining narrow CR beams via anisotropic Alfven wave scattering, matching observed anisotropy features and estimating source distances.

Findings

The CR beam width and excess are consistent with a 1 pc scale turbulence.

The model predicts a distance to the source within a few hundred parsecs.

The large-scale anisotropy is energy independent, matching observations.

Abstract

Both the acceleration of cosmic rays (CR) in supernova remnant shocks and their subsequent propagation through the random magnetic field of the Galaxy deem to result in an almost isotropic CR spectrum. Yet the MILAGRO TeV observatory discovered a sharp ($\sim10^{\circ})$ arrival anisotropy of CR nuclei. We suggest a mechanism for producing a weak and narrow CR beam which operates en route to the observer. The key assumption is that CRs are scattered by a strongly anisotropic Alfven wave spectrum formed by the turbulent cascade across the local field direction. The strongest pitch-angle scattering occurs for particles moving almost precisely along the field line. Partly because this direction is also the direction of minimum of the large scale CR angular distribution, the enhanced scattering results in a weak but narrow particle excess. The width, the fractional excess and the maximum momentum of the beam are calculated from a systematic transport theory depending on a single scale $l$ which can be associated with the longest Alfven wave, efficiently scattering the beam. The best match to all the three characteristics of the beam is achieved at $l\sim1$pc. The distance to a possible source of the beam is estimated to be within a few 100pc. Possible approaches to determination of the scale $l$ from the characteristics of the source are discussed. Alternative scenarios of drawing the beam from the galactic CR background are considered. The beam related large scale anisotropic CR component is found to be energy independent which is also consistent with the observations.