cosmic-ray anisotropy, regularization and convergence

I had the privilege of serving on the PhD thesis committee for the defense of Brian Kolterman's (NYU) PhD thesis today. He performed a set of very careful statistical tests of the angle and time distributions of about 10¹¹ few-TeV cosmic rays incident on the Milagro experiment. He finds an anisotropy to the distribution in celestial coordinates, he finds a time dependence to that anisotropy, and he finds the (expected, known) effect of the orbit of the Earth around the Sun. The most surprising thing is the time dependence of the (very small but very high significance) anisotropy. After the very nice defense, Gruzinov and I spent some time arguing about whether the anisotropy and its time derivative were reasonable in the context of any simple model in which the cosmic ray population is fed by supernovae events throughout the disk of the Galaxy. I think I concluded that his results must put a strong constraint on the coherence or large-scale structure of the local magnetic field.

Bovy and I discussed the convergence and regularization of the mixture-of-gaussians model that he is fitting to the error-deconvolved velocity distribution in the disk in the Solar Neighborhood. We read some of the literature on EM and it was very instructive. Now Bovy has some serious coding to do. If he succeeds with all these enhancements, he will be hitting this problem with a very large hammer.