Is the large-scale sidereal anisotropy of the galactic cosmic-ray intensity really instable at TeV energies?

Abstract

The sidereal cosmic-ray anisotropy at TeV energies has a large-scale deficit region distributed between 150 and 240 degrees in right ascension, which is called the “loss-cone”. The Milagro experiment reported the detection of a steady increase in the loss-cone amplitude at 6TeV from July 2000 to July 2007. In this paper, we examine Milagro’s claim using the data collected by the Tibet air-shower experiment from November 1999 through December 2008. No time dependence was found in the loss-cone amplitude at 4.4, 6.2, and 11TeV. If the increase in the loss-cone amplitude were, as Milagro argued, due to variations in the heliosphere in relation to solar activities, the same tendency would be seen at sub-TeV energies where the anisotropy is far more sensitive to solar activities. At 0.6TeV, however, Matsushiro underground muon observatory reported no significant increase in the loss-cone amplitude during the corresponding period.