14 June 2010
Hyesung Kang (Pusan National University)
"Intergalactic Magnetic Field and Arrival Direction of Ultra-High-Energy Cosmic Rays"
We studied how the intergalactic magnetic field (IGMF) affects the propagation of
super-Greisen-Zatsepin-Kuz'min (GZK) protons that originate from extragalactic
sources within the local GZK sphere. To this end, we set up hypothetical sources of
ultra-high-energy cosmic rays (UHECRs), virtual observers, and the magnetized cosmic
web in a model universe constructed from cosmological structure formation
simulations. We then arranged a set of reference objects mimicking active galactic
nuclei (AGNs) in the local universe, with which correlations of simulated UHECR
events are analyzed. With our model IGMF, the deflection angle between the arrival
direction of super-GZK protons and the sky position of their actual sources is quite
large with a mean value of ~15 deg and a median value of 7--10 deg. On
the other hand, the separation angle between the arrival direction and the sky
position of nearest reference objects is substantially smaller with 3.5-4 deg, which
is similar to the mean angular distance in the sky to nearest neighbors among the reference
objects. This is a direct consequence of our model that the sources, observers,
reference objects, and the IGMF all trace the matter distribution of the universe.
The result implies that extragalactic objects lying closest to the arrival
direction of UHECRs are not necessarily their actual sources. With our model for
the distribution of reference objects, the fraction of super-GZK proton events,
whose closest AGNs are true sources, is less than 1/3. We discussed implications of
our findings for correlation studies of real UHECR events.