Propagation of ultra-high-energy cosmic ray nuclei in cosmic magnetic fields and implications for anisotropy measurements

Abstract

Recent results from the Pierre Auger Observatory (PAO) indicate that the composition of ultra-high-energy cosmic rays (UHECRs) with energies above 10 19 eV may be dominated by heavy nuclei. An important question is whether the distribution of arrival directions for such UHECR nuclei can exhibit observable anisotropy or positional correlations with their astrophysical source objects despite the expected strong deflections by intervening magnetic fields. For this purpose, we have simulated the propagation of UHECR nuclei including models for both the extragalactic magnetic field (EGMF) and the Galactic magnetic field (GMF). We find that the GMF is particularly crucial for suppressing the anisotropy as well as source correlations. Assuming that only iron nuclei are injected steadily from sources with equal luminosity and spatially distributed according to the observed large scale structure in the local Universe, at the number of events published by the PAO so far (69 events above 5.5 × 10 19 eV), the arrival distribution of UHECRs would be consistent with no auto-correlation at 95% confidence if the mean number density of UHECR sources n s 10 −6 Mpc −3 , and consistent with no cross-correlation with sources within 95% errors for n s 10 −5 Mpc −3 . On the other hand, with 1000 events above 5.5 × 10 19 eV in the whole sky, next generation experiments can reveal auto-correlation with more than 99% probability even for n s 10 −3 Mpc −3 , and cross-correlation with sources with more than 99% probability for n s 10 −4 Mpc −3 . In addition, we find that the contribution of Centaurus A is required to reproduce the currently observed UHECR excess in the Centaurus region. Secondary protons generated by photodisintegration of primary heavy nuclei during propagation play a crucial role in all cases, and the resulting anisotropy at small angular scales should provide a strong hint of the source location if the maximum energies of the heavy nuclei are sufficiently high.

High-energy neutrino fluxes from AGN populations inferred from X-ray surveys

“…These are summarized in Table 3, and agree with local AGN estimates from UHECR observations (see e.g. Takami et al 2012, and references therein). We find that the higher luminosity AGN are preferentially found at higher redshifts.…”

Ensemble fluctuations of the flux and nuclear composition of ultrahigh energy cosmic ray nuclei

“…A local source density of H 0 = 10 −5 Mpc −3 is consistent with the absence of "repeaters" in CR data [38,39]. Values as low as H 0 = 10 −6 Mpc −3 are still marginally consistent with auto-correlation studies of UHE CR nuclei [40]. We will consider these two cases as fiducial values in the following.…”

“…Typically, neutron-rich isotopes β-decay to a stable or long-lived nucleus with the same mass number. In most cases there is only one stable nucleus per mass number below 56 Fe with the exception of the pairs 54 Cr/ 54 Fe, 46 Ca/ 46 Ti, 40 Ar/ 40 Ca and 36 S/ 36 Ar. We follow here the approach of Puget, Stecker and Bredekamp (PSB) [25] and consider only a single nucleus per mass number A in the decay chain of primary iron 56 Fe.…”

Ultrahigh-energy cosmic ray production by turbulence in gamma-ray burst jets and cosmogenic neutrinos

“…Although jets in active galactic nuclei (AGN) are the most widely considered candidates for the UHECR sources [1], the observed degree of anisotropy in the arrival distribution indicates a source density larger than 10 −4 Mpc −3 for a pure proton compositon (10 −6 Mpc −3 for a pure iron composition) [2,3], which disfavors Fanaroff-Reily II (FR II) galaxies and BL Lac objects. Other types of relatively low luminosity AGNs like Seyfert galaxies may not satisfy the luminosity requirement ( 10 46 erg s −1 for protons) needed for UHECR acceleration [4].…”