Deflections Of Ultra-high Energy Cosmic Rays Research Articles

The Coherent Magnetic Field of the Milky Way

We present a suite of models of the coherent magnetic field of the Galaxy based on new divergence-free parametric functions describing the global structure of the field. The model parameters are fit to the latest full-sky Faraday rotation measures (RMs) of extragalactic sources and polarized synchrotron intensity (PI) maps from the Wilkinson Microwave Anisotropy Probe and Planck. We employ multiple models for the density of thermal and cosmic-ray electrons in the Galaxy, needed to predict the sky maps of RMs and PI for a given Galactic magnetic field (GMF) model. The robustness of the inferred properties of the GMF is gauged by studying many combinations of parametric field models and electron density models. We determine the pitch angle of the local magnetic field (11° ± 1°), explore the evidence for a grand-design spiral coherent magnetic field (inconclusive), determine the strength of the toroidal and poloidal magnetic halo fields below and above the disk (magnitudes the same for both hemispheres within ≈10%), set constraints on the half-height of the cosmic-ray diffusion volume (≥2.9 kpc), investigate the compatibility of RM- and PI-derived magnetic field strengths (compatible under certain assumptions), and check if the toroidal halo field could be created by the shear of the poloidal halo field due to the differential rotation of the Galaxy (possibly). A set of eight models is identified to help quantify the present uncertainties in the coherent GMF spanning different functional forms, data products, and auxiliary input. We present the corresponding sky maps of rates for axion–photon conversion in the Galaxy and deflections of ultrahigh-energy cosmic rays.

Using the “least-deflected” subsamples of ultra-high energy cosmic rays to constrain source population(s)

Context. The source population(s) of ultra-high energy cosmic rays (UHECRs) with energies of ≥57 × 1018 eV (57 EeV) are still unknown and continue to be a subject of debate. The published arrival directions and energies of 303 UHECRs from the Pierre Auger Observatory and Telescope Array enable correlations with astronomical sources. However, deflections of UHECRs in the Galactic Magnetic Field (GMF) can be considerable, especially for heavy UHECR compositions. Aims. We aim to constrain the source(s) of UHECRs by focusing on the subset of UHECRs that experience minimal deflections by the Galactic magnetic field (GMF). Methods. We used Monte Carlo simulations of UHECR trajectories in a total of eight posited GMF models (and six UHECR compositions from H to Fe) to identify two subsamples of about 40 UHECRs. The H compositions of these subsamples suffer deflections of less than a few degrees in one (or most) of the GMFs. Both the total UHECR sample and the “least-deflected” (vs. the remaining “more-deflected”) subsamples of UHECRs were cross-correlated with catalogs of astronomical sources (at D ≲ 100 Mpc) and neutrino arrival directions. Results. For H compositions, the full sample of UHECRs is most closely correlated with Cen A, nearby (< 75 Mpc) radiogalaxies, and optically selected active galactic nuclei (AGNs); the least-deflected sub sample shows a most consistent correlation with nearby (D ≲ 25 Mpc) galaxies, although some GMF models show a preference for the IceCube muon-neutrino arrival detections. For oxygen compositions, the out-of-galaxy arrival directions of least-deflected UHECRs fall on the extragalactic plane (and thus close to nearby galaxies) in many GMFs. For Fe compositions, the out-of-galaxy UHECR arrival directions typically originate in a region ∼30 deg offset from Cen A, except in the case of the JF12 model; this offset can be decreased by re-scaling (by ∼50%) the magnitudes of the dipole and disk fields of the relevant GMFs.

Extragalactic magnetic field constraints from ultrahigh-energy cosmic rays from local galaxies

ABSTRACT We interpret the correlation between local star-forming galaxy positions and ultrahigh-energy cosmic-ray (UHECR) directions, recently detected by the Pierre Auger Observatory (PAO), in terms of physical parameters: the local density of sources and the magnetic fields governing the UHECR propagation. We include a Galactic magnetic field model on top of a random extragalactic magnetic field description to determine the level of UHECR deflections expected from an ensemble of source positions. Besides deflections in magnetic fields, we also take into account energy losses with background photon fields as well as spectrum and composition measurements by the PAO. We find consistency between the PAO anisotropy measurement and the local star-forming galaxy density for large extragalactic magnetic field strengths with $B \gt 0.2 \ \rm nG$ (for a coherence length of $1 \ \rm Mpc$) at the 5σ confidence level. Larger source densities lead to more isotropic background and consequently allow for weaker extragalactic magnetic fields. However, the acceleration of UHECR by such abundant sources is more challenging to motivate. Too large source densities and extragalactic magnetic field strengths, on the other hand, are also disfavoured as that decreases the expected level of anisotropy. This leads to upper limits of $B \lt 22 \ \rm nG$ and $\rho _0 \lt 8.4 \times 10^{-2} \ \rm Mpc^{-3}$ at the 90 per cent confidence level.

Full sky harmonic analysis hints at large ultra-high energy cosmic ray deflections

The full-sky multipole coefficients of the ultra-high energy cosmic ray (UHECR) flux have been measured for the first time by the Pierre Auger and Telescope Array collaborations using a joint data set with E > 10 EeV. We calculate these harmonic coefficients in the model where UHECR are protons and sources trace the local matter distribution, and compare our results with observations. We find that the expected power for low multipoles (dipole and quadrupole, in particular) is sytematically higher than in the data: the observed flux is too isotropic. We then investigate to which degree our predictions are influenced by UHECR deflections in the regular Galactic magnetic field. It turns out that the UHECR power spectrum coefficients C l are quite insensitive to the effects of the Galactic magnetic field, so it is unlikely that the discordance can be reconciled by tuning the Galactic magnetic field model. On the contrary, a sizeable fraction of uniformly distributed flux (representing for instance an admixture of heavy nuclei with considerably larger deflections) can bring simulations and observations to an accord.

The Galactic magnetic field and ultrahigh-energy cosmic ray deflections

Our understanding of the Galactic magnetic field (GMF) has increased considerably in recent years, while at the same time remaining far from adequate. By way of illustration, the Jansson and Farrar (2012) (JF12) GMF model is described, emphasizing how it is constrained and which features are robust or likely to change, as modelling and constraining data improve. The most urgent requirements for the next phase of modelling are a more realistic model for the relativistic electron distribution (in order to reduce the systematic error associated with interpreting synchrotron data) and a better theoretical understanding of the origin of the large-scale coherent field (in order to develop a better phenomenological parameterization of the field). Even in its current stage of development, the JF12 model allows some important conclusions about UHECR deflections in the GMF to be formulated.

SEARCH FOR ANISOTROPY OF ULTRAHIGH ENERGY COSMIC RAYS WITH THE TELESCOPE ARRAY EXPERIMENT

We study the anisotropy of Ultra-High Energy Cosmic Ray (UHECR) events collected by the Telescope Array (TA) detector in the first 40 months of operation. Following earlier studies, we examine event sets with energy thresholds of 10 EeV, 40 EeV, and 57 EeV. We find that the distributions of the events in right ascension and declination are compatible with an isotropic distribution in all three sets. We then compare with previously reported clustering of the UHECR events at small angular scales. No significant clustering is found in the TA data. We then check the events with E>57 EeV for correlations with nearby active galactic nuclei. No significant correlation is found. Finally, we examine all three sets for correlations with the large-scale structure of the Universe. We find that the two higher-energy sets are compatible with both an isotropic distribution and the hypothesis that UHECR sources follow the matter distribution of the Universe (the LSS hypothesis), while the event set with E>10 EeV is compatible with isotropy and is not compatible with the LSS hypothesis at 95% CL unless large deflection angles are also assumed. We show that accounting for UHECR deflections in a realistic model of the Galactic magnetic field can make this set compatible with the LSS hypothesis.

Formation of the spectrum of ultra-high-energy cosmic rays: Effect of extragalactic magnetic fields and expected contribution of individual sources

The effect of the extragalactic magnetic field on the propagation of ultra-high-energy cosmic rays (UHECRs) is investigated. We use the infrared galaxy catalog IRAS PSCz to reconstruct the magnetic field distribution in the Local Universe. The magnetic field induction is considered as a power function of the galactic infrared luminosity density: B = Kρβ. In contrast to some earlier studies in which the exponent β = 2/3 corresponded to the freezing-in condition, the parameters K and β are estimated from the field inductions normalized by the expected maximum inductions (strong field) and minimum inductions (weak field) in galaxy clusters and voids, respectively. Maps of angular deflections of UHECRs are presented for these magnetic field models. We found that the protons with energies E > 4 × 1019 eV are not significantly deflected from their sources in a sphere with a radius of 100 Mpc only in the case of the weak magnetic field model (the deflections are comparable to the errors of modern detectors). The effect of the extragalactic magnetic field on the UHECR spectrum is investigated, with Virgo A and Arp 299 taken as potential sources.

Mapping Deflections of Extragalactic Ultra High Energy Cosmic Rays in Magnetohydrodynamic Simulations of the Local Universe

We construct a map of deflections of ultrahigh-energy cosmic rays by extragalactic magnetic fields using a magnetohydrodynamical simulation of cosmic structure formation that realistically reproduces the positions of known galaxy clusters in the Local Universe. Large deflection angles occur in cluster regions, which, however, cover only an insignificant fraction of the sky. More typical deflections of order ≲1° are caused by crossings of filaments. For protons with energies E≥4×1019eV, deflections do not exceed a few degrees over most of the sky up to a propagation distance of 500 Mpc. Given that the field strength of our simulated intergalactic magnetic field forms a plausible upper limit, we conclude that charged particle astronomy is in principle possible.

Mapping deflections of extragalactic ultrahigh-energy cosmic rays in magnetohydrodynamic simulations of the local universe

We construct a map of deflections of ultrahigh-energy cosmic rays by extragalactic magnetic fields using a magnetohydrodynamical simulation of cosmic structure formation that realistically reproduces the positions of known galaxy clusters in the Local Universe. Large deflection angles occur in cluster regions, which, however, cover only an insignificant fraction of the sky. More typical deflections of order ≲1° are caused by crossings of filaments. For protons with energies E≥4×1019eV, deflections do not exceed a few degrees over most of the sky up to a propagation distance of 500 Mpc. Given that the field strength of our simulated intergalactic magnetic field forms a plausible upper limit, we conclude that charged particle astronomy is in principle possible.