The Milagro anticenter hot spots: cosmic rays from the Geminga supernova ?

Abstract

The Milagro experiment has announced the discovery of an excess flux of TeV cosmic rays from the general direction of the heliotail, also close to the Galactic anticenter. We investigate the hypothesis that the excess cosmic rays were produced in the SN explosion that gave birth to the Geminga pulsar. The assumptions underlying our proposed scenario are that the Geminga supernova occurred about 3.4 10^5 years ago (as indicated by the spin down timescale), that a burst of cosmic rays was injected with total energy 10^49 erg (i.e., about 1% of a typical SN output), and that the Geminga pulsar was born with a positive radial velocity of 100--200 km s^-1. We find that our hypothesis is consistent with the available information. In a first variant (likely oversimplified), the cosmic rays have diffused according to the Bohm prescription (i.e., with a diffusion coefficient on the order of c times r_L, with c the speed of light and r_L the Larmor radius). An alternative scheme assumes that diffusion only occurred initially, and the final propagation to the Sun was a free streaming in a diverging magnetic field. If the observed cosmic ray excess does indeed arise from the Geminga SN explosion, the long--sought "smoking gun" connecting cosmic rays with supernovae would finally be at hand. It could be said that, while looking for the "smoking gun", we were hit by the bullets themselves.