arXiv subjects

Silvia Mollerach

Publications and source records attributed to Silvia Mollerach.

At least 19 recordsLinked to original sources

arXiv2026

Impact of the EPOS.LHC-R hadronic interaction model on the Centaurus A ultrahigh-energy cosmic-ray scenario

We discuss the impact of the recent EPOS.LHC-R hadronic interaction model on the scenario in which most of the cosmic rays with energies above 5 EeV originate in the nearby Centaurus A radiogalaxy. The heavier composition inferred from this hadronic model has important implications for the interpretation of the spectral features and for the anisotropies. In particular, the amount of H and He present above the ankle is now very suppressed. The elements of the CNO group from the source play a predominant role in the instep region just above the ankle, while the elements of the Si and Fe groups contribute significantly in the suppression region of the energy spectrum above 50 EeV. The lack of He from the source above the ankle also leads to a smaller expected anisotropy around the Cen A direction at energies of 10 to 20 EeV. The expected dipolar anisotropy in different energy bins above 4 EeV is well consistent with the measurements for appropriate values of the extragalactic turbulent magnetic field and source lifetime. The overall fit to the different observables improves considerably with the new hadronic model. A new method is introduced to extract the information on the cosmic ray masses from that of the depth of the maximum development of the air showers, which significantly improves the existing method.

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arXiv2024

Case for Centaurus A as the main source of ultrahigh-energy cosmic rays

We discuss the possibility that a dominant fraction of the cosmic rays above the ankle is due to a single nearby source, considering in particular the radio galaxy Centaurus A. We focus on the properties of the source spectrum and composition required to reproduce the observations, showing that the nuclei are strongly suppressed for E>10Z EeV, either by a rigidity dependent source cutoff or by the photodisintegration interactions with the CMB at the giant dipole resonance. The very mild attenuation effects at lower energies imply that the secondary nuclei from this source only provide a small contribution. Given the moderate anisotropies observed, the deflections in extragalactic and Galactic magnetic fields should play a crucial role in determining the cosmic ray arrival direction distribution. The diffusion in extragalactic fields as well as the finite source lifetime also significantly affect the shape of the observed spectrum. The cosmic ray flux at tens of EeV is dominated by the CNO component, and we show that it is actually better reproduced by a mixture of C and O nuclei rather than by the usual assumption of a N component effectively describing this mass group. The Si and Fe group components become dominant above 70 EeV, in the energy range in which a strong spectral suppression is present. If the localised flux excess appearing above 40 EeV around the Centaurus A direction is attributed to the CNO component, the He nuclei from the source in the energy range from 10 to 20 EeV could lead to a similar anisotropy unless its contribution is suppressed. The cosmic ray flux at a few EeV should mostly result from a more isotropic light component associated to a population of extragalactic sources. The inclusion of the subdominant contribution of heavy nuclei from the Galactic component helps to reproduce the observations around 1 EeV.

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arXiv2022

Large-scale anisotropies of extragalactic cosmic rays below the ankle

We study the anisotropies on large angular scales which can be present in the flux of cosmic rays reaching the Earth from a population of extragalactic sources, focusing on the energy range between the second knee and the ankle. In this energy range the particles are significantly affected by the Galactic magnetic field, which then plays a relevant role in shaping the expected anisotropies. The Galactic magnetic field deflects the cosmic-ray trajectories and thus modifies the anisotropies present outside the halo of the Galaxy, in particular the dipolar one associated with the translational motion of the observer (Compton-Getting effect). Also, due to the Galactic rotation, in the reference frame of an observer at Earth there is an electric component of the Galactic field that produces a small change in the particles' momentum. This acceleration depends on the cosmic-ray arrival direction and it hence induces anisotropies in the flux observed in a given energy range. We analyse the expected amplitude and phase of the resulting dipolar component of the flux and discuss the possibility to explain via these effects the change in the phase of the right-ascension distribution which is observed at energies around 1 EeV.

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arXiv2022

Anisotropies of ultrahigh-energy cosmic rays in a scenario with nearby sources

The images of ultrahigh-energy cosmic ray sources get distorted, in an energy dependent way, by the effects of Galactic and extragalactic magnetic fields. These deflections can also affect the observed cosmic ray spectrum, specially when the sources are transient. We study scenarios in which one or a few nearby extragalactic sources, such as CenA or M81/M82, provide the dominant contribution to the cosmic ray flux above the ankle of the spectrum. We discuss the effects of the angular dispersion induced by the turbulent extragalactic magnetic fields, and the coherent deflections caused by the regular Galactic magnetic field, with the associated multiple imaging of the sources. We consider the possible contribution from those sources to the dipolar distribution discovered by the Pierre Auger Observatory above 8 EeV, as well as to the hot spots hinted in the observations by the Pierre Auger and Telescope Array observatories at higher energies, taking into account the mixed nature of the cosmic ray composition.

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arXiv2021

Magnetic diffusion and interaction effects on Ultrahigh Energy Cosmic Rays: protons and nuclei

The flux of ultrahigh energy cosmic rays reaching the Earth is affected by the interactions with the cosmic radiation backgrounds as well as with the magnetic fields that are present along their trajectories. We combine the SimProp cosmic ray propagation code with a routine that allows to account for the average effects of a turbulent magnetic field on the direction of propagation of the particles. We compute in this way the modification of the spectrum which is due to the magnetic horizon effect, both for primary nuclei as well as for the secondary nuclei resulting from the photo-disintegration of the primary ones. We also provide analytic parameterizations of the attenuation effects, as a function of the magnetic field parameters and of the density of cosmic ray sources, which make it possible to obtain the expected spectra in the presence of the magnetic fields from the spectra that would be obtained in the absence of magnetic fields. The discrete nature of the distribution of sources with finite density also affects the spectrum of cosmic rays at the highest energies where the flux is suppressed due to the interactions with the radiation backgrounds, and parameterizations of these effects are obtained.

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arXiv2021

Cosmic ray anisotropies from transient extragalactic sources

We study the spectrum and anisotropies of ultrahigh energy cosmic ray transient sources, accounting for the effects of their propagation through the turbulent extragalactic magnetic fields. We consider either bursting sources or sources emitting since a given initial time. We analyse in detail the transition between the diffusive and the quasi-rectilinear regimes, describing some new features that could be present

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arXiv2020

High-energy cosmic particles

A review of the status of the knowledge in the field of High-energy cosmic particles is presented. The spectrum, arrival direction distribution and composition measurements are summarized, together with some implications for the understanding of the cosmic ray origin and their propagation. Special emphasis is put in the ultra-high energy range, corresponding to particles of extragalactic origin.

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arXiv2020

Extragalactic cosmic rays diffusing from two populations of sources

We consider the possibility of explaining the observed spectrum and composition of the cosmic rays with energies above $10^{17}$ eV in terms of two different extragalactic populations of sources in the presence of a turbulent intergalactic magnetic field (including also a fading Galactic cosmic-ray component). The populations are considered to be the superposition of different nuclear species having rigidity dependent spectra. The first extragalactic population is dominant in the energy range $10^{17}$ to $10^{18}$ eV and consists of sources having a relatively large density ($> 10^{-3}$ Mpc$^{-3}$) and a steep spectrum. The second extragalactic population dominates the cosmic ray flux above few EeV, it has a harder spectral slope and has a high-energy cutoff at few $Z$ EeV (where $eZ$ is the associated cosmic ray charge). This population has a lower density of sources ($<10^{-4}$ Mpc$^{-3}$), so that the typical intersource separation is larger than few tens of Mpc, being significantly affected by a magnetic horizon effect that strongly suppresses its flux for energies below $\sim Z$ EeV. We discuss how this scenario could be reconciled with the values of the cosmic-ray source spectral indices that are expected to result from the diffusive shock acceleration mechanism.

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arXiv2019

Ultrahigh energy cosmic rays from a nearby extragalactic source in the diffusive regime

We study the effects that the diffusion of the cosmic rays in the magnetic field of the Local Supercluster can have on the spectrum of a nearby extragalactic source at ultrahigh energies. We find that the strong enhancement of the flux below the energy at which the transition between the diffusive and quasirectilinear regimes takes place, as well as the suppression at lower energies associated with a finite source age, can help to explain the observed features of the cosmic-ray spectrum and the composition. Scenarios are discussed in which a nearby extragalactic source with mixed composition and rigidity-dependent spectrum accounts for most of the observed cosmic rays at energies above a few EeV while the rest of the extragalactic sources lead to a diffuse flux that dominates at lower energies and down to $\sim 0.1$~EeV. The nearby source can also naturally account for the dipolar anisotropy measurements above 4~EeV, and these measurements can also help to constrain its evolution with redshift.

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arXiv2019

A scenario for the Galactic cosmic rays between the knee and the second-knee

We perform a fit to measurements of the cosmic ray spectrum and of the depth of shower maximum in the energy range between $10^{15}$~eV and $10^{18}$~eV. We consider a Galactic component that is a mixture of five representative nuclear species (H, He, N, Si and Fe), for which we adopt rigidity dependent broken power-law spectra, and we allow for an extragalactic component which becomes strongly suppressed for decreasing energies. The relative abundances of the Galactic components at $10^{15}$~eV are taken to be comparable to those determined by direct measurements at $10^{13}$~eV. The main features of the spectrum and of the composition are reproduced in these scenarios. The spectral knee results from the break of the H spectrum at $E_{\rm k}\simeq 3\times 10^{15}$~eV, although it is broaden by the comparable contribution from He which has a break at about $6\times 10^{15}$~eV. The low-energy ankle at $E_{\rm la}\simeq 2\times 10^{16}$~eV is associated to the strong suppression of the H and He Galactic components and the increasing relative contribution of the heavier ones, but the observed hardening of the spectrum at this energy turns out to result from the growing contribution of the extragalactic component. The second-knee at $E_{\rm sk}\simeq 26 E_{\rm k}\simeq 8\times 10^{16}$~eV is associated with the steepening of the Galactic Fe component. The transition to the regime in which the total cosmic ray flux is dominated by the extragalactic component takes place at an energy of about $10^{17}$~eV. The parameters of the fit depend on the hadronic model that is used to interpret the $X_{\rm max}$ measurements as well as on the specific $X_{\rm max}$ dataset that is considered in the fit. The impact of the possible existence of a maximum rigidity cutoff in the Galactic components is also discussed.

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arXiv2017

Progress in high-energy cosmic ray physics

We review some of the recent progress in our knowledge about high-energy cosmic rays, with an emphasis on the interpretation of the different observational results. We discuss the effects that are relevant to shape the cosmic ray spectrum and the explanations proposed to account for its features and for the observed changes in composition. The physics of air-showers is summarized and we also present the results obtained on the proton-air cross section and on the muon content of the showers. We discuss the cosmic ray propagation through magnetic fields, the effects of diffusion and of magnetic lensing, the cosmic ray interactions with background radiation fields and the production of secondary neutrinos and photons. We also consider the cosmic ray anisotropies, both at large and small angular scales, presenting the results obtained from the TeV up to the highest energies and discuss the models proposed to explain their origin.

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arXiv2016

The shape of the extragalactic cosmic ray spectrum from Galaxy Clusters

We study the diffusive escape of cosmic rays from a central source inside a galaxy cluster to obtain the suppression in the outgoing flux appearing when the confinement times get comparable or larger than the age of the sources. We also discuss the attenuation of the flux due to the interactions of the cosmic rays with the cluster medium, which can be sizeable for heavy nuclei. The overall suppression in the total cosmic ray flux expected on Earth is important to understand the shape of the extragalactic contribution to the cosmic ray spectrum for $E/Z<1$ EeV. This suppression can also be relevant to interpret the results of fits to composition-sensitive observables measured at ultra-high energies.

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arXiv2016

Angular distribution of cosmic rays from an individual source in a turbulent magnetic field

We obtain the angular distribution of the cosmic rays reaching an observer from an individual source and after propagation through a turbulent magnetic field, for different ratios between the source distance and the diffusion length. We study both the high-energy quasi-rectilinear regime as well as the transition towards the diffusive regime at lower energies where the deflections become large. We consider the impact of energy losses, showing that they tend to enhance the anisotropy of the source at a given energy. We also discuss lensing effects, in particular those that could result from the regular galactic magnetic field component, and show that the effect of the turbulent extragalactic magnetic fields can smooth out the divergent magnification peaks that would result for point-like sources in the limit of no turbulent deflections.

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arXiv2015

Anisotropies of ultra-high energy cosmic ray nuclei diffusing from extragalactic sources

We obtain the dipolar anisotropies in the arrival directions of ultra-high energy cosmic ray nuclei diffusing from nearby extragalactic sources. We consider mixed-composition scenarios in which different cosmic ray nuclei are accelerated up to the same maximum rigidity, so that $E<ZE_\text{max}^p$, with $Z$ the atomic number and $E_\text{max}^p$ the maximum proton energy. We adopt $E_\text{max}^p\simeq 6$ EeV so as to account for an increasingly heavier composition above the ankle. We obtain the anisotropies through Monte Carlo simulations that implement the cosmic ray diffusion in extragalactic turbulent fields as well as the effects of photo-disintegrations and other energy losses. Dipolar anisotropies at the level of 5 to 10\% at energies $\sim 10$~EeV are predicted for plausible values of the source density and magnetic fields.

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arXiv2013

Anisotropies of ultra-high energy cosmic rays diffusing from extragalactic sources

We obtain the dipolar anisotropies in the arrival directions of ultra-high energy cosmic rays diffusing from nearby extragalactic sources. We discuss both the energy regime of spatial diffusion and the quasi-rectilinear one leading to just angular diffusion at higher energies. We obtain analytic results for the anisotropies from a single source which are validated using two different numerical simulations. For a scenario with a few sources in the local supercluster (with the closest source at a typical distance of few to tens of Mpc), we discuss the possible transition between the case in which the anisotropies are dominated by a few sources at energies below few EeV towards the regime in which many sources contribute at higher energies. The effect of a non-isotropic source distribution is also discussed, showing that it can significantly affect the observed dipole.

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arXiv2013

Magnetic diffusion effects on the Ultra-High Energy Cosmic Ray spectrum and composition

We discuss the effects of diffusion of high energy cosmic rays in turbulent extra-galactic magnetic fields. We find an approximate expression for the low energy suppression of the spectrum of the different mass components (with charge $Z$) in the case in which this suppression happens at energies below $\sim Z$ EeV, so that energy losses are dominated by the adiabatic ones. The low energy suppression appears when cosmic rays from the closest sources take a time comparable to the age of the Universe to reach the Earth. This occurs for energies $E< Z\, {\rm EeV}\,(B/{\rm G})\sqrt{l_c/{\rm Mpc}}(d_s/70\ {\rm Mpc})$ in terms of the magnetic field RMS strength $B$, its coherence length $l_c$ and the typical separation between sources $d_s$. We apply this to scenarios in which the sources produce a mixed composition and have a relatively low maximum rigidity ($E_{max}\sim (2$--$10) Z $ EeV), finding that diffusion has a significant effect on the resulting spectrum, the average mass and on its spread, in particular reducing this last one. For reasonable values of $B$ and $l_c$ these effects can help to reproduce the composition trends observed by the Auger Collaboration for source spectra compatible with Fermi acceleration.

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arXiv2013

PeV neutrinos from the propagation of ultra-high energy cosmic rays

We discuss the possibility that the PeV neutrinos recently observed by IceCube are produced by the interactions of extragalactic cosmic rays during their propagation through the radiation backgrounds. We show that the fluxes resulting from the decays of neutrons produced in the interactions of cosmic ray protons with the CMB background are suppressed ($E_ν^2$d$Φ_ν/$d$E< 10^{-10}$ GeV/cm$^2$ s sr), with those resulting from the decays of pions produced in the interactions with the UV/optical/IR backgrounds being the dominant ones at PeV energies. The anti-neutrino fluxes produced by the decay of neutrons resulting from the photodisintegration of heavy nuclei with CMB photons are also shown to be quite suppressed ($E_ν^2$d$Φ_ν/$d$E< 10^{-11}$ GeV/cm$^2$ s sr), while those produced by photo-pion processes with UV/optical/IR backgrounds may be larger, although they are not expected to be above those achievable in the pure proton case. Scenarios with mixed composition and low cutoff rigidities can lead to PeV neutrino fluxes enhanced with respect to those in the pure Fe scenarios. We also discuss the possible impact of the Glashow resonance for the detection of these scenarios, showing that it plays a moderate role.

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arXiv2011

Searching for signals of magnetic lensing in ultra-high energy cosmic rays

Ultra-high energy cosmic rays are mostly charged particles and they are therefore deflected by magnetic fields on their path from their sources to Earth. An interesting phenomenon arising from these deflections is the appearance of multiple images of a source, i.e. cosmic rays with the same energy coming from the same source that can arrive to the Earth from different directions. In this work we present a technique to identify secondary images, produced by the regular component of the galactic magnetic field, benefiting from the fact that near caustics the flux is significantly magnified.

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