arXiv subjects

Carmelo Evoli

Publications and source records attributed to Carmelo Evoli.

At least 37 records · Page 2Linked to original sources

arXiv2021

Simulating the Galactic Multi-messenger Emissions with HERMES

The study of non-thermal processes such as synchrotron emission, inverse Compton scattering, bremsstrahlung and pion production is crucial to understand the properties of the Galactic cosmic-ray population, to shed light on their origin and confinement mechanisms, and to assess the significance of exotic signals possibly associated to new physics. We present a public code called HERMES aimed at generating sky maps associated to a variety of multi-messenger and multi-wavelength radiative processes, spanning from the radio domain all the way up to high-energy gamma-ray and neutrino production. We describe the physical processes under consideration, the code concept and structure, and the user interface, with particular focus on the python-based interactive mode. We especially present the modular and flexible design that allows to easily further extend the numerical package according to the user's needs. In order to demonstrate the capabilities of the code, we describe in detail a comprehensive set of sky maps and spectra associated to all physical processes included in the code. We comment in particular on the radio, gamma-ray, and neutrino maps, and mention the possibility to study signals stemming from dark matter annihilation. HERMES can be successfully applied to constrain the properties of the Galactic cosmic-ray population, improve our understanding of the diffuse Galactic radio, gamma--ray, and neutrino emission, and search for signals associated to particle dark matter annihilation or decay.

astro-ph.HE↗

arXiv2021

Turbulent axion-photon conversions in the Milky-Way

The Milky-Way magnetic field can trigger conversions between photons and axion-like particles (ALPs), leading to peculiar features on the observable photon spectra. Previous studies considered only the regular component of the magnetic field. However, observations consistently show the existence of an additional turbulent component, with a similar strength and correlated on a scale of a few 10$\,$pc. We investigate the impact of the turbulent magnetic field on the ALP-photon conversions, characterizing the effects numerically and analytically. We show that the turbulent magnetic field can change the conversion probability by up to a factor of two and may lead to observable irregularities in the observable photon spectra from different astrophysical sources.

hep-ph↗

arXiv2021

Intermediate mass and heavy Galactic cosmic-ray nuclei: the case of new AMS-02 measurements

The recent measurement of the spectra of intermediate mass nuclei and iron nuclei carried out with the AMS-02 experiment provided us with the most complete set of data on cosmic ray fluxes to date, and allowed us to test the standard model for the transport of these particles through the Galaxy to the finest details. We show that the parameters derived from lighter primary and secondary elements in the cosmic radiation also lead to a good description of the data on heavier nuclei, with no need to invoke different injection spectra for such nuclei, provided the whole chain of fragmentation is properly accounted for. The only exception to this finding is represented by iron nuclei, which show a very unusual trend at rigidity $\lesssim 100$ GV. This trend reflects in a Fe/O ratio that is at odds with the results of the standard model of cosmic ray transport, and is in contradiction with data collected by HEAO, ACE-CRIS and Voyager at lower energy. We speculate on possible origins of such findings.

astro-ph.HE↗

arXiv2021

Galactic factories of cosmic-ray electrons and positrons

We present a novel calculation of the spectrum of electrons and positrons from random sources, supernova remnants and pulsars, distributed within the spiral arms of the Galaxy. The pulsar emissivity in terms of electron-positron pairs is considered as time dependent, following the magnetic dipole spin-down luminosity, and the temporal evolution of the potential drop is accounted for. Moreover each pulsar, with the magnetic field and initial spin period selected at random from the observed distribution, is considered as a source of pairs only after it leaves the parent supernova due to its birth kick velocity (also selected at random from the observed distribution). We show that (i) the spectrum of electrons is characterized by a feature at $\gtrsim 50$ GeV that proves that their transport is dominated by radiative losses. The flux reduction at $E\gtrsim 1$ TeV is explained as a result of lepton transport from sources in the spiral arms. (ii) The spectrum of positrons is very well described by the contribution of pulsars and the rising positron fraction originates naturally. The implications of pulsars as positron sources in terms of positron fraction at very high energies are also discussed. (iii) The role of fluctuations in the high-energy regime is thoroughly discussed and used to draw conclusions on the possibility to single out the contribution of local sources to the lepton spectrum with current and upcoming experiments.

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arXiv2020

Novel aspects of cosmic ray diffusion in synthetic magnetic turbulence

The diffusive motion of charged particles in synthetic magnetic turbulence with different properties is investigated by using numerical simulations with unprecedented dynamical range, which allow us to ensure that both the inertial range and the long wavelength part of the turbulent spectrum are properly described. This is of particular importance in evaluating previous suggestions that parallel and perpendicular diffusion coefficients differ in their energy dependence, an assertion at odds with the many claims of universality of the $D_{\perp}$ and $D_{\parallel}$ as functions of particle energy. Cases with and without an ordered magnetic field are discussed. Results of the numerical simulations are compared with available theoretical models, for slab, slab/2D and isotropic turbulence. We find widespread evidence that universality is broken, and that the ratio $D_{\perp}/D_{\parallel}$ is not independent of energy. The implications of this finding for the physics of cosmic ray transport are discussed in depth.

astro-ph.HE↗

arXiv2020

Signature of Energy Losses on the Cosmic Ray Electron Spectrum

We show that the fine structure of the electron spectrum in cosmic rays, especially the excess claimed by AMS-02 at energies $\sim$42 GeV, is fully accounted for in terms of inverse Compton losses in the photon background dominated by ultraviolet, infrared and CMB photons, plus the standard synchrotron losses in the Galactic magnetic field. The transition to the Klein-Nishina regime on the ultraviolet background causes the feature. Hence, contrary to previous statements, observations do not require the overlap of different components. We stress that the feature observed by AMS-02 at energies $\sim$42 GeV is not related to the positron excess, which instead requires the existence of positron sources, such as pulsars. Because energy losses are the physical explanation of this feature, we indirectly confirm that the transport of leptons in the Galaxy is loss-dominated down to energies of the order of tens of GeV. This finding imposes strong constraints on the feasibility of alternative theories of cosmic transport in which the grammage is accumulated in cocoons concentrated around sources, requiring that electrons and positrons become loss dominated only at very high energies.

astro-ph.HE↗

arXiv2020

AMS-02 beryllium data and its implication for cosmic ray transport

The flux of unstable secondary cosmic ray nuclei, produced by spallation processes in the interstellar medium, can be used to constrain the residence time of cosmic rays inside the Galaxy. Among them, $^{10}$Be is especially useful because of its relatively long half-life of 1.39 Myr. In the framework of the diffusive halo model we describe cosmic ray transport taking into account all relevant interaction channels and accounting for the decay of unstable secondary nuclei. We then compare our results with the data collected by the Alpha Magnetic Spectrometer (AMS-02) on board the International Space Station for the flux ratios Be/C, B/C, Be/O, B/O, C/O and Be/B as well as C, N and O absolute fluxes. These measurements, and especially the Be/B ratio, allow us to single out the flux of $^{10}$Be and infer a best fit propagation time of CRs in the Galaxy. Our results show that, if the cross sections for the production of secondary elements through spallation are taken at face value, AMS-02 measurements are compatible with the standard picture based on CR diffusion in a halo of size $H \gtrsim 5$ kpc. Taking into account the uncertainties in the cross sections, this conclusion becomes less reliable, although still compatible with the standard picture. Implications of our findings for alternative models of CR transport are discussed.

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arXiv2019

The origin of Galactic cosmic rays: challenges to the standard paradigm

A critical review of the standard paradigm for the origin of Galactic cosmic rays is presented. Recent measurements of local and far-away cosmic rays reveal unexpected behaviours, which challenge the commonly accepted scenario. These recent findings are discussed, together with long-standing open issues. Despite the progress made thanks to ever-improving observational techniques and theoretical investigations, at present our understanding of the origin and of the behaviour of cosmic rays remains incomplete. We believe it is still unclear whether a modification of the standard paradigm, or rather a radical change of the paradigm itself is needed in order to interpret all the available data on cosmic rays within a self-consistent scenario.

astro-ph.HE↗

arXiv2019

Atmospheric neutrinos and the knee of the Cosmic Ray spectrum

The nature of the knee in the all-particle spectrum of cosmic rays remains subject of much investigation, especially in the aftermath of recent measurements claiming the detection of a knee-like feature in the spectrum of the light component of cosmic rays at energy $\sim$700 TeV, at odds with the standard picture in which the knee in the all-particle spectrum is dominated by light cosmic rays. Here we investigate the implications of this and other scenarios in terms of the measured flux of atmospheric neutrinos. In particular we discuss the possibility that the spectrum of atmospheric neutrinos in the region $\gtrsim$50 TeV may provide information about the different models for the mass composition in the knee region. We investigate the dependence of the predicted atmospheric neutrino flux on the shape of the light cosmic ray spectra and on the interaction models describing the development of showers in the atmosphere. The implications of all these factors for the identification of the onset of an astrophysical neutrino component are discussed.

astro-ph.HE↗

arXiv2019

Galactic cosmic rays after the AMS-02 observations

The unprecedented quality of the data collected by the AMS-02 experiment onboard the International Space Station allowed us to address subtle questions concerning the origin and propagation of cosmic rays. Here we discuss the implications of these data for the injection spectrum of elements with different masses and for the diffusion coefficient probed by cosmic rays through their journey from the sources to the Earth. We find that the best fit to the spectra of primary and secondary nuclei requires (1) a break in the energy dependence of the diffusion coefficient at energies $\sim 300$ GV; (2) an injection spectrum that is the same for all nuclei heavier than helium, and different injections for both protons and helium. Moreover, if to force the injection spectrum of helium to be the same as for heavier nuclei, the fit to oxygen substantially worsens. Accounting for a small, $X_{s}\sim 0.4~\rm g~cm^{-2}$, grammage accumulated inside the sources leads to a somewhat better fit to the B/C ratio but makes the difference between He and other elements even more evident. The statistic and systematic error bars claimed by the AMS collaboration exceed the error that is expected from calculations once the uncertainties in the cross sections of production of secondary nuclei are taken into account. In order to make this point more quantitative, we present a novel parametrization of a large set of cross sections, relevant for cosmic ray physics, and we introduce the uncertainty in the branching ratios in a way that its effect can be easily grasped.

astro-ph.HE↗

arXiv2019

Blazar VHE spectral alterations induced by photon-ALP oscillations

Prompted by the increasing interest of axion-like particles (ALPs) for very-high-energy (VHE) astrophysics, we have considered a full scenario for the propagation of a VHE photon/ALP beam emitted by a BL Lac and reaching us in the light of the most up-to-date astrophysical information and for energies up to above $100 \, \rm TeV$. During its trip, the beam -- generated in a small region of a BL Lac jet -- crosses a variety of magnetic structures in very different astronomical environments: the BL Lac jet, the host elliptical galaxy, the extragalactic space and the Milky Way. We have taken an effort to model all these magnetic fields in the most realistic fashion and using a new model developed by us concerning the extragalactic magnetic field. Assuming an intrinsic spectrum with a power law exponentially truncated at a fixed cut-off energy, we have evaluated the resulting observed spectra of Markarian 501, the extreme BL Lac 1ES 0229+200 and a similar source located at $z = 0.6$ up to above $100 \, \rm TeV$. We obtain interesting results: the model with photon-ALP oscillations possesses features (spectral energy oscillatory behaviour and photon excess above $20 \, \rm TeV$) which can be tested by $γ$-ray observatories like CTA, HAWC, GAMMA 400, LHAASO, TAIGA-HiSCORE and HERD. In addition, our ALP can be detected in dedicated laboratory experiments like the upgrade of ALPS II at DESY, the planned IAXO and STAX experiments, as well as with other techniques developed by Avignone and collaborators.

astro-ph.HE↗

arXiv2018

Self-Generated Cosmic-Ray Confinement in TeV Halos: Implications for TeV Gamma-Ray Emission and the Positron Excess

Recent observations have detected extended TeV gamma-ray emission surrounding young and middle-aged pulsars. The morphology of these "TeV halos" requires cosmic-ray diffusion to be locally suppressed by a factor of ~100-1000 compared to the typical interstellar medium. No model currently explains this suppression. We show that cosmic-ray self-confinement can significantly inhibit diffusion near pulsars. The steep cosmic-ray gradient generates Alfven waves that resonantly scatter the same cosmic-ray population, suppressing diffusion within ~20 pc of pulsars younger than ~100 kyr. In this model, TeV halos evolve through two phases, a growth phase where Alfven waves are resonantly generated and cosmic-ray diffusion becomes increasingly suppressed, and a subsequent relaxation phase where the diffusion coefficient returns to the standard interstellar value. Intriguingly, cosmic-rays are not strongly confined early in the TeV halo evolution, allowing a significant fraction of injected e+e- to escape. If these e+e- also escape from the surrounding supernova remnant, they would provide a natural explanation for the positron excess observed by PAMELA and AMS-02. Recently created TeV cosmic-rays are confined in the TeV halo, matching observations by HAWC and H.E.S.S. While our default model relaxes too rapidly to explain the confinement of TeV cosmic rays around mature pulsars, such as Geminga, models utilizing a Kraichnan turbulence spectrum experience much slower relaxation. Thus, observations of TeV halos around mature pulsars may provide a probe into our understanding of interstellar turbulence.

astro-ph.HE↗

arXiv2018

Cosmic-ray propagation with DRAGON2: II. Nuclear interactions with the interstellar gas

Understanding the isotopic composition of cosmic rays (CRs) observed near Earth represents a milestone towards the identification of their origin. Local fluxes contain all the known stable and long-lived isotopes, reflecting the complex history of primaries and secondaries as they traverse the interstellar medium. For that reason, a numerical code which aims at describing the CR transport in the Galaxy must unavoidably rely on accurate modelling of the production of secondary particles. In this work we provide a detailed description of the nuclear cross sections and decay network as implemented in the forthcoming release of the galactic propagation code DRAGON2. We present the secondary production models implemented in the code and we apply the different prescriptions to compute quantities of interest to interpret local CR fluxes (e.g., nuclear fragmentation timescales, secondary and tertiary source terms). In particular, we develop a nuclear secondary production model aimed at accurately computing the light secondary fluxes (namely: Li, Be, B) above 1 GeV/n. This result is achieved by fitting existing empirical or semi-empirical formalisms to a large sample of measurements in the energy range 100 MeV/n to 100 GeV/n and by considering the contribution of the most relevant decaying isotopes up to iron. Concerning secondary antiparticles (positrons and antiprotons), we describe a collection of models taken from the literature, and provide a detailed quantitative comparison.

astro-ph.HE↗

arXiv2018

Origin of the Cosmic Ray Galactic Halo Driven by Advected Turbulence and Self-Generated Waves

The diffusive paradigm for the transport of Galactic cosmic rays is central to our understanding of the origin of these high energy particles. However, it is worth recalling that the normalization, energy dependence, and spatial extent of the diffusion coefficient in the interstellar medium are fitted to the data and typically are not derived from more basic principles. Here, we discuss a scenario in which the diffusion properties of cosmic rays are derived from a combination of wave self-generation and advection from the Galactic disc, where the sources of cosmic rays are assumed to be located. We show for the first time that a halo naturally arises from these phenomena, with a size of a few kiloparsecs, compatible with the value that typically best fits observations in simple parametric approaches to cosmic ray diffusion. We also show that transport in such a halo results in a hardening in the spectra of primary cosmic rays at ~300 GV.

astro-ph.HE↗

arXiv2017

A signature of anisotropic cosmic-ray transport in the gamma-ray sky

A crucial process in Galactic cosmic-ray (CR) transport is the spatial diffusion due to the interaction with the interstellar turbulent magnetic field. Usually, CR diffusion is assumed to be uniform and isotropic all across the Galaxy. However, this picture is clearly inaccurate: Several data-driven and theoretical arguments, as well as dedicated numerical simulations, show that diffusion exhibits highly anisotropic properties with respect to the direction of a background (ordered) magnetic field (i.e., parallel or perpendicular to it). In this paper we focus on a recently discovered anomaly in the hadronic CR spectrum inferred by the Fermi-LAT gamma-ray data at different positions in the Galaxy, i.e. the progressive hardening of the proton slope at low Galactocentric radii. We propose the idea that this feature can be interpreted as a signature of anisotropic diffusion in the complex Galactic magnetic field: In particular, the harder slope in the inner Galaxy is due, in our scenario, to the parallel diffusive escape along the poloidal component of the large-scale, regular, magnetic field. We implement this idea in a numerical framework, based on the DRAGON code, and perform detailed numerical tests on the accuracy of our setup. We discuss how the effect proposed depends on the relevant free parameters involved. Based on low-energy extrapolation of the few focused numerical simulations aimed at determining the scalings of the anisotropic diffusion coefficients, we finally present a set of plausible models that reproduce the behavior of the CR proton slopes inferred by gamma-ray data.

astro-ph.HE↗

arXiv2017

Cosmic-ray transport in the heliosphere with HelioProp

Before being detected at Earth, charged cosmic rays propagate across the Solar System and undergo interactions with the turbulent solar wind and with the heliospheric magnetic field. As a result, they are subject to a series of processes that include diffusion, convection, energy losses and drifts, which significantly affect the shape and the intensity of the cosmic-ray fluxes at low energies. Here we illustrate how all these mechanisms can be realistically modelled with HelioProp, our public tool designed to treat cosmic-ray transport through the heliosphere in a charge-dependent way. We present a detailed description of the features of the code and we illustrate in a quantitative way the effects that the propagation in the heliosphere can have on the different cosmic-ray species with a particular emphasis on the antiparticle channels relevant for dark matter indirect detection.

astro-ph.HE↗

arXiv2017

Cosmogenic neutrinos and gamma-rays and the redshift evolution of UHECR sources

If ultra-high-energy cosmic rays (UHECRs) have extragalactic origins, as is widely assumed to be the case at least for the majority of cosmic rays with energies above a few EeV, secondary neutrinos and photons can be expected to be produced during the propagation of UHECRs through intergalactic space via interactions with cosmic background photons. The fluxes of such secondary particles are strongly dependent on the redshift evolution of the emissivity (number density times luminosity) of UHECR sources. We show how cosmic rays, neutrinos, and gamma rays can potentially provide complementary information about UHECR source evolution.

astro-ph.HE↗

arXiv2017

Cosmic-ray propagation with DRAGON2: I. numerical solver and astrophysical ingredients

We present version 2 of the DRAGON code designed for computing realistic predictions of the CR densities in the Galaxy. The code numerically solves the interstellar CR transport equation (including inhomogeneous and anisotropic diffusion, either in space and momentum, advective transport and energy losses), under realistic conditions. The new version includes an updated numerical solver and several models for the astrophysical ingredients involved in the transport equation. Improvements in the accuracy of the numerical solution are proved against analytical solutions and in reference diffusion scenarios. The novel features implemented in the code allow to simulate the diverse scenarios proposed to reproduce the most recent measurements of local and diffuse CR fluxes, going beyond the limitations of the homogeneous galactic transport paradigm. To this end, several applications using DRAGON2 are presented as well. The new version facilitates the users to include their own physical models by means of a modular C++ structure.

astro-ph.HE↗