Does galactic magnetic field disturb the correlation of the highest energy cosmic rays with their sources?

Abstract

The propagation trajectories of the highest energy cosmic rays (HECRs) are deflected by not only intergalactic magnetic field but also Galactic magnetic field (GMF). These magnetic fields can weaken the positive correlation between the arrival directions of HECRs and the positions of their sources. In order to explore the effect of GMF on the expected correlation, we simulate the arrival distribution of protons with energy above 6 × 1019 eV taking several GMF models into account, and then test the correlation between the protons and their sources assumed in the simulation. The dependence of the correlation signals on GMF models is also investigated. The correlation can be observed by accumulating ∼200 protons in a half-hemisphere. The typical angular scale at which the positive signal of the correlation is maximized depends on the spiral component of the GMF model. That angular scale is ∼5° for bisymmetric spiral (BS) GMF models and ∼7° for axisymmetric spiral (AS) GMF models if the number density of HECR sources, ns, is ∼10-4 Mpc-3. An additional vertical (dipole) component of GMF affects these angular scales by 0°.5-1°. The difference between the correlation signal for the BS models and that for the AS models is prominent in the northern sky. The significance of the positive correlation depends on source distribution. The probability that the number of simulated HECR events correlating with sources is smaller than the number of random events correlating with the same sources by chance is much less than 10-3 (∼3σ) in almost all the source distributions with ns = 10-4 Mpc-3 for detection of under 200 protons, but ∼10% of source distributions predict a chance probability more than 10-3 in the AS GMF model. In addition, we also briefly discuss the effect of GMF for heavy-nuclei-dominated composition. © 2010. The American Astronomical Society.