Conclusions

There are a number of aspects of the calculations relevant to the interpretation of observational data concerning charged particles from a potential Galactic Centre source. The spread of the arrival directions is of interest, as is the mean value of the arrival direction north of the galactic plane. Both of these are of the order of a few degrees at 10¹⁹eV and reduce with increasing energy. We would also like to have an idea of how likely it is that a particle might reach the observational region. This is very unlikely below 10¹⁸eV but reaches a value expected on the basis of solid angle geometry at about 10¹⁹eV where the propagation is becoming rather rectilinear. The way in which the propagation changes between these energies depends on detail of the magnetic field which is presently unknown. In this range of energies, it is clear that the interplay between the galactic regular field and its random field is crucial in the propagation of the particles and thus their likelihood of reaching us and being observed in a particular direction. With the inclusion of a regular field, there is a systematic deviation of the observation direction to the north, based on our best knowledge of the true field.

Thus, for quite different reasons, it appears that a source in the vicinity of the Galactic Centre would be unobservable below 10¹⁸eV with either charged particles or neutrons. At energies above this, the neutrons would produce a point source and the charged particles (protons) would produce a halo of energy-dependent size and location, north of the galactic plane. In previous work it has been noted that charged particles can become attached to the regular spiral arm field lines and we would additionally expect some of the charged particles to be deflected along the spiral arm, producing an excess in the general direction of its magnetic field.

© Copyright Astronomical Society of Australia 1997