Implications of gamma-ray observations on proton models of UHECR

The origin of ultra high energy cosmic rays (UHECR) is still unknown. However, great progress has been achieved in past years due to the good quality and large statistics in experimental data collected by the current observatories. The data of the Pierre Auger Observatory show that the composition of the UHECRs becomes progressively lighter starting from $10^{17}$ eV up to $\sim 10^{18.3}$ eV and then, beyond that energy, it becomes increasingly heavier. These analyses are subject to important systematic uncertainties due to the use of hadronic interaction models that extrapolate lower energy accelerator data to the highest energies. Although proton models of UHECRs are disfavored by these results, they cannot be completely ruled out. It is well known that the energy spectra of gamma rays and neutrinos, produced during propagation of these very energetic particles through the intergalactic medium, are a useful tool to constrain the spectrum models. In particular, it has recently been shown that the neutrino upper limits obtained by IceCube challenge the proton models at 95% CL. In this work we study the constraints imposed by the extragalactic gamma-ray background, measured by Fermi-LAT, on proton models of UHECRs. In particular, we make use of the extragalactic gamma-ray background flux, integrated from 50 GeV to 2 TeV, that originates in point sources, which has recently been obtained by the Fermi-LAT collaboration, in combination with the neutrino upper limits, to constrain the emission of UHECRs at high redshits ($z>1$), in the context of the proton models

Gamma rays and neutrinos from a cosmic ray source in the Galactic Center region

The center of the our Galaxy is a region where very energetic phenomena take place. In particular powerful cosmic ray sources can be located in that region. The cosmic rays accelerated in these sources may interact with ambient protons and/or low energy photons producing gamma rays and neutrinos. The observation of these two types of secondary particles can be very useful for the identification of the cosmic ray sources and for the understanding of the physical processes occurring during acceleration. Motivated by the excess in the neutrino spectrum recently reported by the IceCube Collaboration, we study in detail the shape of the gamma ray and neutrino spectra originated from the interaction of cosmic ray protons with ambient protons for sources located in the Galactic Center region. We consider different models for proton acceleration and study the impact on the gamma ray and neutrino spectra. We also discuss the possibility to constrain and even identify a particular neutrino source by using the information given by the gamma ray spectrum taking advantage of the modification of the spectral shape, caused by the interaction of the gamma rays with the photons of the radiation field present in the interstellar medium, which strongly depends on the source distance.Comment: Accepted for publication in Physical Review

Implications of gamma-ray and neutrino observations on source models of ultrahigh energy cosmic rays

The origin and nature of the ultrahigh energy cosmic rays (UHECRs) are still unknown. However, great progress has been achieved in past years due to the observations performed by the Pierre Auger Observatory and Telescope Array. Above $10^{18}$ eV the observed energy spectrum presents two features: a hardening of the slope at about $10^{18.7}$ eV, which is known as the ankle and a suppression at approximately $10^{19.6}$ eV. The composition inferred from the experimental data, interpreted by using the current high energy hadronic interaction models, seems to be light below the ankle, showing a trend to heavier nuclei for increasing values of the primary energy. Current high energy hadronic interaction models, updated by using Large Hadron Collider data, are still subject to large systematic uncertainties, which makes difficult the interpretation of the experimental data in terms of composition. On the other hand, it is very well known that gamma rays and neutrinos are produced by UHECRs during propagation from their sources, as a consequence of their interactions with the radiation field present in the universe. The flux at Earth of these secondary particles depends on the source models of UHECRs including the chemical composition at injection. Therefore, both gamma-ray and neutrino observations can be used to constrain source models of UHECRs, including the composition in a way which is independent of the high energy hadronic interaction models. In this article I will review recent results obtained by using the latest gamma-ray and neutrino observations.Comment: Talk presented at International Conference on Black Holes as Cosmic Batteries: UHECRs and Multimessenger Astronomy (BHCB) 2018, Foz do Igua\c{c}u, Brasil. PoS(BHCB2018)00

A new method for reconstructing the muon lateral distribution with an array of segmented counters

Although the origin of ultra high energy cosmic rays is still unknown, significant progress has been achieved in last decades with the construction of large arrays that are currently taking data. One of the most important pieces of information comes from the chemical composition of primary particles. It is well known that the muon content of air showers generated by the interaction of cosmic rays with the atmosphere is rather sensitive to primary mass. Therefore, the measurement of the number of muons at ground level is an essential ingredient to infer the cosmic ray mass composition. In this work we present a new method for reconstructing the muon lateral distribution function with an array of segmented counters. The energy range from .4 to 2.5 EeV is considered. For a triangular array spaced at 750 m we found that 450 m is the optimal distance to evaluate the number of muons. The corresponding statistical and systematic uncertainties of the new and of a previous reconstruction methods are compared. Since the statistical uncertainty of the new reconstruction is less than in the original one, the power to discriminate between heavy and light cosmic ray primaries is enhanced. The detector dynamic range is also extended in the new reconstruction, so events falling closer to a detector can be included in composition studies.Comment: Accepted for publication in Astroparticle Physic

Ensemble fluctuations of the cosmic ray energy spectrum and the intergalactic magnetic field

The origin of the most energetic cosmic ray particles is one of the most important open problems in astrophysics. Despite a big experimental effort done in the past years, the sources of these very energetic particles remain unidentified. Therefore, their distribution on the Universe and even their space density are still unknown. It has been shown that different spatial configurations of the sources lead to different energy spectra and composition profiles (in the case of sources injecting heavy nuclei) at Earth. These ensemble fluctuations are more important at the highest energies because only nearby sources, which are necessarily few, can contribute to the flux observed at Earth. This is due to the interaction of the cosmic rays with the low energy photons of the radiation field, present in the intergalactic medium, during propagation. It is believed that the intergalactic medium is permeated by a turbulent magnetic field. Although at present it is still unknown, there are several constraints for its intensity and coherence length obtained from different observational techniques. Charged cosmic rays are affected by the intergalactic magnetic field because of the bending of their trajectories during propagation through the intergalactic medium. In this work, the influence of the intergalactic magnetic field on the ensemble fluctuations is studied. Sources injecting only protons and only iron nuclei are considered. The ensemble fluctuations are studied for different values of the density of sources compatible with the constraints recently obtained from cosmic ray data. Also, the possible detection of the ensemble fluctuations in the context of the future JEM-EUSO mission is discussed.Comment: Accepted for publication in Physical Review

On the possibility of neutrino flavor identification at the highest energies

High energy astrophysical neutrinos carry relevant information about the origin and propagation of cosmic rays. They can be created as a by-product of the interactions of cosmic rays in the sources and during propagation of these high energy particles through the intergalactic medium. The determination of flavor composition in this high energy flux is important because it presents a unique chance to probe our understanding of neutrino flavor oscillations at gamma factors >10^21. In this work we develop a new statistical technique to study the flavor composition of the incident neutrino flux, which is based on the multipeak structure of the longitudinal profiles of very deep electron and tau neutrino horizontal air showers. Although these longitudinal profiles can be observed by means of fluorescence telescopes placed over the Earth's surface, orbital detectors are more suitable for neutrino observations owing to their much larger aperture. Therefore, we focus on the high energy region of the neutrino spectrum relevant for observations with orbital detectors like the planned JEM-EUSO telescope.Comment: Accepted for publication in Physical Review