Introduction

Recent results from the AGASA cosmic ray air shower array in Japan (Hayashida et al. 1999a, 1999b) have suggested that there is a source of ultra high energy cosmic rays in the general direction of the Galactic Centre. Those results were highly suggestive of a Galactic Centre source with an excess of cosmic ray intensity of about 25% from a region in the galactic plane about from the Centre. This compact source contributed to the broad-scale 4% first harmonic of the cosmic ray anisotropy reported by Hayashida et al.. A region of lesser intensity extends for a few tens of degrees northwards into the AGASA field of view and there is a less convincing enhancement in the Cygnus region of the galactic plane, close to the direction of the inward spiral arm. The high latitude of the array and the southern declination of the Galactic Centre meant that the Centre itself was just outside the view of the array and it was not clear whether the main excess region extended to the Galactic Centre direction.

The major strength of the source signal was at energies a little over 10¹⁸eV and was restricted to a range of energies of about a factor of three. This meant that the only other array capable of observing the source, SUGAR (the Sydney University Giant Air Shower Recorder), had not detected the source due to its statistically poorer dataset. Knowing that a signal might exist, Clay et al. (2000) were able to confirm the observation with the SUGAR dataset. Further detailed examination of the SUGAR data (Bellido et al. 2000) found that the source appeared to be pointlike (and coincident with the strongest AGASA direction) and did not extend to the direction of the Galactic Centre.

A simple explanation of a pointlike source of cosmic rays is that they are neutral particles. The fact that the source is observed only close to 10¹⁸eV is strongly suggestive that they are neutrons, which have a lifetime just sufficient to reach us from the distance of the Galactic Centre at those energies. At lower energies, the effect of time dilation is less and lower energy neutrons would not survive to produce a point source. One expects galactic cosmic ray acceleration to have an energetic upper limit not much greater than about 10¹⁸eV, so the limited range of observed energies is naturally explained (Clay et al. 2000).

One presumes that the neutrons are produced by the conversion of accelerated particles in a target close to the source and so the question of the propagation of the non-converted particles becomes of interest. Jones (1990) has discussed the production of neutrons at energies such as this. Both in the case of neutron production by inelastic charge exchange of protons and by the dissociatiation of primary nuclei, there are more neutrons produced than gamma-rays. If the source emits nuclei together with protons, their propagation paths will be scattered more than for the protons in the discussion below and they would be unlikely to be identifiable over the more general cosmic ray background.

We have modelled the propagation of cosmic ray protons from the general vicinity of the Galactic Centre (as explained above, that would seem to be the appropriate distance and to be roughly the right direction) to the distance (8.5kpc) of the Earth through a simple model of the galactic magnetic field.

© Copyright Astronomical Society of Australia 1997