Introduction

Data from the AGASA cosmic ray detector in Japan have recently been presented which show an excess of events from the general direction of the Galactic Centre at energies a little above 10¹⁸eV (Hayashida et al. 1999). The Galactic Centre is just below the southern limit of the acceptance of the AGASA array and data from the southern SUGAR array were used to confirm the result (Bellido et al. 2000). Those data indicated that the source direction was probably several degrees from the actual direction of the Galactic Centre and that there was a point source component (within the array angular resolution). The AGASA array data suggested the existence of a broader scale component around the Galactic Centre direction and it may be possible to describe the phenomenon in terms of a two component model with a neutral point source beam (possible neutrons) together with a charged particle beam which scatters in the galactic magnetic field (Clay 2000). The limited energy range over which the anisotropy and the point source are strongly detectable supports the idea of a neutron component from a source roughly at the distance of the galactic centre since a time dilation threshold for such neutrons would be close to 10¹⁸eV. Clay et al. (2000) have shown that conditions in the region of the Galactic Centre might be suitable for cosmic ray acceleration to these energies, interactions in material close to the source could easily then produce a neutron beam.

There are two further features in the AGASA directional data. There is the suggestion of an excess towards our spiral arm inward direction, which might have a natural interpretation in terms of some propagation from inner galactic regions guided by the large scale spiral arm magnetic field. There is also a strongly significant deficit close to the galactic anticentre. The magnitude of this deficit is not so large as that of the galactic centre excess, but is significant due to its declination, which is much better observed by AGASA.

The existence of an excess and a deficit of cosmic ray flux at opposite directions on the celestial sphere is strongly suggestive of a unidirectional anisotropy (e.g. Jacklyn 1986). In this case, either the observer moves through a frame of reference in which the cosmic ray flux is isotropic or the flux is diffusing past the observer. The solar velocity through the galaxy is not sufficiently great to produce an anisotropy of the appropriate magnitude. Our interpretation of the AGASA data is that there is the diffusion of a cosmic ray component at 10¹⁸eV past us from the inner galactic regions.

© Copyright Astronomical Society of Australia 1997