We review open questions and prospects for progress in ultrahigh-energy cosmic ray (UHECR) research, based on a series of discussions that took place during the “The High-Energy Universe: Gamma-Ray, Neutrino, and Cosmic-ray Astronomy” MIAPP workshop in 2018. Specifically, we overview open questions on the origin of the bulk of UHECRs, the UHECR mass composition, the origin of the end of the cosmic-ray spectrum, the transition from Galactic to extragalactic cosmic rays, the effect of magnetic fields on the trajectories of UHECRs, anisotropy expectations for specific astrophysical scenarios, hadronic interactions, and prospects for discovering neutral particles as well as new physics at ultrahigh energies. We also briefly overview upcoming and proposed UHECR experiments and discuss their projected science reach.

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Open questions in cosmic-ray research at ultrahigh energies

The interaction of ultra-high-energy cosmic rays (UHECRs) with pervasive photon fields generates associated cosmogenic fluxes of neutrinos and photons due to photohadronic and photonuclear processes taking place in the intergalactic medium. We perform a fit of the UHECR spectrum and composition measured by the Pierre Auger Observatory for four source emissivity scenarios: power-law redshift dependence with one free parameter, active galactic nuclei, gamma-ray bursts, and star formation history. We show that negative source emissivity evolution is favoured if we treat the source evolution as a free parameter. In all cases, the best fit is obtained for relatively hard spectral indices and low maximal rigidities, for compositions at injection dominated by intermediate nuclei (nitrogen and silicon groups). In light of these results, we calculate the associated fluxes of neutrinos and photons. Finally, we discuss the prospects for the future generation of high-energy neutrino and gamma-ray observatories to constrain the sources of UHECRs.

Cosmogenic photon and neutrino fluxes in the Auger era

In this white paper we introduce the IMAGINE Consortium and its scientific background, goals and structure. The purpose of the consortium is to coordinate and facilitate the efforts of a diverse group of researchers in the broad areas of the interstellar medium, Galactic magnetic fields and cosmic rays, and our overarching goal is to develop more comprehensive insights into the structures and roles of interstellar magnetic fields and their interactions with cosmic rays within the context of Galactic astrophysics. The ongoing rapid development of observational and numerical facilities and techniques has resulted in a widely felt need to advance this subject to a qualitatively higher level of self-consistency, depth and rigour. This can only be achieved by the coordinated efforts of experts in diverse areas of astrophysics involved in observational, theoretical and numerical work. We present our view of the present status of this research area, identify its key unsolved problems and suggest a strategy that will underpin our work. The backbone of the consortium is the Interstellar MAGnetic field INference Engine, a publicly available Bayesian platform that employs robust statistical methods to explore the multi-dimensional likelihood space using any number of modular inputs. This tool will be used by the IMAGINE Consortium to develop an interpretation and modelling framework that provides the method, power and flexibility to interfuse information from a variety of observational, theoretical and numerical lines of evidence into a self-consistent and comprehensive picture of the thermal and non-thermal interstellar media. An important innovation is that a consistent understanding of the phenomena that are directly or indirectly influenced by the Galactic magnetic field, such as the deflection of ultra-high energy cosmic rays or extragalactic backgrounds, is made an integral part of the modelling. The IMAGINE Consortium, which is informal by nature and open to new participants, hereby presents a methodological framework for the modelling and understanding of Galactic magnetic fields that is available to all communities whose research relies on a state of the art solution to this problem.

http://iopscience.iop.org/article/10.1088/1475-7516/2018/08/049/pdf

IMAGINE: A comprehensive view of the interstellar medium, Galactic magnetic fields and cosmic rays

We study the implications of Ultra-High Energy Cosmic Ray (UHECR) data from the Pierre Auger Observatory for potential accelerator candidates and cosmogenic neutrino fluxes for different combinations of nuclear disintegration and air-shower models. We exploit the most recent spectral and mass composition data (2017) with a new, computationally very efficient simulation code PriNCe. We extend the systematic framework originally developed by the Pierre Auger Collaboration with the cosmological source evolution as an additional free parameter. In this framework, an ensemble of generalized UHECR accelerators is characterized by a universal spectral index (equal for all injection species), a maximal rigidity, and the normalizations for five nuclear element groups. We find that the 2017 data favor a small but constrained contribution of heavy elements (iron) at the source. We demonstrate that the results moderately depend on the nuclear disintegration (PSB, Peanut, or Talys) model, and more strongly on the air-shower (EPOS-LHC, Sibyll-2.3, or QGSjet-II-04) model. Variations of these models result in different source evolutions and spectral indices, limiting the interpretation in terms of a particular class of cosmic accelerators. Better constrained parameters include the maximal rigidity and the mass composition at the source. Hence, the cosmogenic neutrino flux can be robustly predicted, since it originates from interactions with the cosmic infrared background and peaks at $10^8 \, \mathrm{GeV}$. Depending on the source evolution at high redshifts the flux is likely out of reach of future neutrino observatories in most cases, and a minimal cosmogenic neutrino flux cannot be claimed from data without assuming a cosmological distribution of the sources.

A new view on Auger data and cosmogenic neutrinos in light of different nuclear disintegration and air-shower models

Cosmogenic neutrinos are produced when ultrahigh-energy cosmic rays (UHECRs) interact with cosmological photon fields. Limits on the diffuse flux of these neutrinos can be used to constrain the fraction of protons arriving at Earth with energies ${E}_{p}\ensuremath{\gtrsim}30\text{ }\text{ }\mathrm{EeV}$, thereby providing bounds on the composition of UHECRs without fully relying on hadronic interaction models. We show to which extent current neutrino telescopes already constrain this fraction of protons and discuss the prospects for next-generation detectors to further constrain it. Additionally, we discuss the implications of these limits for several popular candidates for UHECR source classes.

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Determining the fraction of cosmic-ray protons at ultrahigh energies with cosmogenic neutrinos

Ultra-high-energy cosmic rays from radio galaxies

On the non-thermal electron-to-proton ratio at cosmic ray acceleration sites

For the first time, we characterize the rigidity regimes of the diffusion coefficient $\kappa$ for arbitrary rigidities and guide fields, which we derive as a function of physical and numerical parameters. We show that at turbulence levels above 5\% of the total magnetic field, the approximation of an energy dependence $\kappa\propto E^{1/3}$ as predicted for a Kolmogorov spectrum within Quasi-Linear Theory does not hold. Consequently, a proper description of cosmic-ray propagation can only be achieved by using a turbulence-level dependent diffusion coefficient and can contribute to solving the Galactic cosmic-ray gradient problem.

Deepening the understanding of cosmic-ray diffusion

Cosmic-ray transport in astrophysical environments is often dominated by the diffusion of particles in a magnetic field composed of both a turbulent and a mean component. This process needs to be understood in order to properly model cosmic-ray signatures. One of the most important aspects in the modeling of cosmic-ray diffusion is that fully resonant scattering, the most effective such process, is only possible if the wave spectrum covers the entire range of propagation angles. By taking the wave spectrum boundaries into account, we quantify cosmic-ray diffusion parallel and perpendicular to the guide field direction at turbulence levels above 5% of the total magnetic field. We apply our results of the parallel and perpendicular diffusion coefficient to the Milky Way. We show that simple purely diffusive transport is in conflict with observations of the inner Galaxy, but that just by taking a Galactic wind into account, data can be matched in the central 5 kpc zone. Further comparison shows that the outer Galaxy at $>5$ kpc, on the other hand, should be dominated by perpendicular diffusion, likely changing to parallel diffusion at the outermost radii of the Milky Way.

L. Merten

Papers

Ultra-high-energy cosmic rays from radio galaxies

On the non-thermal electron-to-proton ratio at cosmic ray acceleration sites

Deepening the understanding of cosmic-ray diffusion

Regimes of cosmic-ray diffusion in Galactic turbulence