arXiv subjects

Luca Maccione

Publications and source records attributed to Luca Maccione.

At least 19 recordsLinked to original sources

arXiv2014

PAMELA and AMS-02 positron and electron spectra are reproduced by three-dimensional cosmic-ray modeling

The PAMELA collaboration recently released the $e^+$ absolute spectrum between 1 and 300 GeV in addition to the positron fraction and the $e^-$ spectrum previously measured in the same period. We use the newly developed three-dimensional upgrade of the DRAGON package to model those data. This code allows us to consider a realistic spiral arm source distribution in the Galaxy, which impacts the high-energy shape of the propagated spectra. At low energy we treat solar modulation with the HelioProp code and compare its results with those obtained using the usual force-field approximation. We show that all PAMELA data sets can be consistently, and accurately, described in terms of a standard background on top of which a charge symmetric $e^+$ + $e^-$ extra component with harder injection spectrum is added; this extra contribution is peaked at about 1 - 10 TeV and may originate from a diffuse population of sources located in the Galactic arms. For the first time, we compute the energy required to sustain such a relevant positron flux in the Galaxy, finding that it is naturally compatible with an astrophysical origin. We considered several reference propagation setups; we find that models with a low (or null) reacceleration - tuned against light nuclei data - nicely describe both PAMELA leptonic and hadronic data with no need to introduce a low-energy break in the proton and helium spectra, as it would be required for high reacceleration models. We also compare our models with the preliminary $e^-$ and $e^+$ absolute spectra recently measured by AMS-02. We find that those data, differently from what is inferred from the positron fraction alone, favor a high energy cutoff (about 10 TeV) of the extra component if this is uniquely generated in the Galactic arms. A lower cutoff may be allowed if a relevant contribution from powerful $e^-$ + $e^+$ nearby accelerators is invoked.

astro-ph.HE↗

arXiv2014

Astrophysical constraints on Planck scale dissipative phenomena

The emergence of a classical spacetime from any quantum gravity model is still a subtle and only partially understood issue. If indeed spacetime is arising as some sort of large scale condensate of more fundamental objects then it is natural to expect that matter, being a collective excitations of the spacetime constituents, will present modified kinematics at sufficiently high energies. We consider here the phenomenology of the dissipative effects necessarily arising in such a picture. Adopting dissipative hydrodynamics as a general framework for the description of the energy exchange between collective excitations and the spacetime fundamental degrees of freedom, we discuss how rates of energy loss for elementary particles can be derived from dispersion relations and used to provide strong constraints on the base of current astrophysical observations of high energy particles.

gr-qc↗

arXiv2013

Model independent interpretation of recent CR lepton data after AMS-02

We model the CR leptonic fluxes above 20 GeV in terms of a superposition of a standard and a charge symmetric extra component, which we generically describe as power-laws in momentum. We investigate under these hypotheses the compatibility between AMS-02, Fermi-LAT and PAMELA datasets on positron fraction, electron+positron spectrum and electron spectrum respectively. We find that it is possible to reconcile AMS and Fermi-LAT data within uncertainties, if energy-dependent effects are present in Fermi-LAT systematics. We also study possible deviations from charge symmetry in the extra component and find no compelling evidence for them.

astro-ph.HE↗

arXiv2013

Three-Dimensional Model of Cosmic-Ray Lepton Propagation Reproduces Data from the Alpha Magnetic Spectrometer on the International Space Station

We study the compatibility of Alpha Magnetic Spectrometer (AMS-02) data on the cosmic-ray (CR) positron fraction with data on the CR electron and positron spectra provided by PAMELA and Fermi LAT. We do that in terms of a novel propagation model in which sources are distributed in spiral arm patterns in agreement with astrophysical observations. While former interpretations assumed an unrealistically steep injection spectrum for astrophysical background electrons, the enhanced energy losses experienced by CR leptons due to the larger average source distance from Earth allow us to reproduce the data with harder injection spectra as expected in a shock acceleration scenario. Moreover, we show that in this approach, and accounting for AMS-02 results, the contribution of nearby accelerators to the fluxes at very high energy can be significantly reduced, thus avoiding any tension with anisotropy upper limits.

astro-ph.HE↗

arXiv2013

A unified solution to the anisotropy and gradient problems

The Fermi-LAT collaboration recently confirmed a discrepancy between the observed longitudinal profile of gamma-ray diffuse emission from the Galaxy and that computed with numerical codes assuming that Cosmic Rays (CRs) are produced by Galactic supernova remnants; the accurate Fermi-LAT measurements make this anomaly hardly explainable in terms of conventional diffusion schemes. Moreover, experimental data from both Muon detector and Extensive Air Shower experiments about the large scale dipole anisotropy of CRs can hardly be compatible with model predictions within the framework of conventional isotropic and homogeneous propagation. We argue that, accounting for a well physically motivated correlation between the CR escape time and the spatially dependent magnetic turbulence power, it is possible to solve both problems at the same time in a very natural way. Indeed, by exploiting this correlation, we find propagation models that fit a wide set of CR primary and secondary spectra, and consistently reproduce the CR anisotropy in the energy range 100 GeV - 10 TeV and the gamma-ray longitude distribution recently measured by Fermi-LAT.

astro-ph.HE↗

arXiv2013

Three dimensional modeling of CR propagation

We present here a major upgrade of DRAGON, a numerical package that computes the propagation of a wide set of CR species from both astrophysical and exotic origin in the Galaxy in a wide energy range from tens of MeV to tens of TeV. DRAGON takes into account all relevant processes in particular diffusion, convection, reacceleration, fragmentation and energy losses. For the first time, we present a full 3D version of DRAGON with anisotropic position-dependent diffusion. In this version, the propagation is calculated within a 3D cartesian grid and the user is able to implement realistic and structured three dimensional source, gas and regular magnetic field distributions. Moreover, it is possible to specify an arbitrary function of position and rigidity for the diffusion coefficients in the parallel and perpendicular direction to the regular magnetic field of the Galaxy. The code opens many new possibilities in the study of CR physics. In particular, we can study for the first time the impact of the spiral arm structure on the leptonic spectra: taking into account the fact that we live in an interarm region, far from most sources, we obtain - due to increased energy losses - a steeper electron spectrum compared to the assumption of a smooth source term. We discuss the implications of these results on our understanding on leptonic spectra and we briefly mention future studies that can be performed with our new 3D code.

astro-ph.HE↗

arXiv2013

Inferring the local interstellar spectrum of cosmic ray protons from PAMELA data

We take advantage of the cosmic-ray (CR) proton data collected by the PAMELA detector in several campaigns covering the period 2006-2009 to derive the local interstellar CR proton spectrum (LIS), outside the solar system. We describe the propagation of CR protons in the solar system by means of a detailed model of the charge-sign dependent drifts occurring while CRs diffuse in the irregular solar magnetic field. We fit PAMELA data for each year of data taking and find that they are well described by a unique time-independent LIS. We then discuss the consequences of this LIS on galactic propagation models. We find that diffusive reacceleration is strongly constrained, thus confirming previous results derived from different channels.

astro-ph.HE↗

arXiv2013

Cosmic Ray Electrons, Positrons and the Synchrotron emission of the Galaxy: consistent analysis and implications

A multichannel analysis of cosmic ray electron and positron spectra and of the diffuse synchrotron emission of the Galaxy is performed by using the DRAGON code. This study is aimed at probing the interstellar electron source spectrum down to E ~ 1 GeV and at constraining several propagation parameters. We find that above 4 GeV the electron source spectrum is compatible with a power-law of index -2.5. Below 4 GeV instead it must be significantly suppressed and the total lepton spectrum is dominated by secondary particles. The positron spectrum and fraction measured below a few GeV are consistently reproduced only within low reacceleration models. We also constrain the scale-height zt of the cosmic-ray distribution using three independent (and, in two cases, original) arguments, showing that values of z_t < 2 kpc are excluded. This result may have strong implications for particle dark matter searches.

astro-ph.HE↗

arXiv2013

Violations of Lorentz invariance in the neutrino sector: an improved analysis of anomalous threshold constraints

Recently there has been a renewed activity in the physics of violations of Lorentz invariance in the neutrino sector. Flavor dependent Lorentz violation, which generically changes the pattern of neutrino oscillations, is extremely tightly constrained by oscillation experiments. Flavor independent Lorentz violation, which does not introduce new oscillation phenomena, is much more weakly constrained with constraints coming from time of flight and anomalous threshold analyses. We use a simplified rotationally invariant model to investigate the effects of finite baselines and energy dependent dispersion on anomalous reaction rates in long baseline experiments and show numerically that anomalous reactions do not necessarily cut off the spectrum quite as sharply as currently assumed. We also present a revised analysis of how anomalous reactions can be used to cast constraints from the observed atmospheric high energy neutrinos and the expected cosmogenic ones.

hep-ph↗

arXiv2013

Low energy cosmic ray positron fraction explained by charge-sign dependent solar modulation

We compute cosmic ray (CR) nuclei, proton, antiproton, electron and positron spectra below 1 TeV at Earth by means of a detailed transport description in the galaxy and in the solar system. CR spectra below 10 GeV are strongly modified by charge-sign dependent propagation effects. These depend on the polarity of the solar magnetic field and therefore vary with the solar cycle. The puzzling discrepancy between the low-energy positron fraction measured by PAMELA and AMS-01 is then easily explained by their different data-taking epochs. We reproduce the observed spectra of CR light nuclei within the same galactic and solar-system propagation model.

astro-ph.HE↗

arXiv2012

Scale hierarchy in Horava-Lifshitz gravity: a strong constraint from synchrotron radiation in the Crab nebula

Horava-Lifshitz gravity models contain higher order operators suppressed by a characteristic scale, which is required to be parametrically smaller than the Planck scale. We show that recomputed synchrotron radiation constraints from the Crab nebula suffice to exclude the possibility that this scale is of the same order of magnitude as the Lorentz breaking scale in the matter sector. This highlights the need for a mechanism that suppresses the percolation of Lorentz violation in the matter sector and is effective for higher order operators as well.

gr-qc↗

arXiv2012

The effect of photon-axion-like particle conversions in galaxy clusters on very high energy γ-ray spectra

Very high energy (VHE, energy >~ 100 GeV) γ-rays originating from extragalactic sources interact with low energy photons of background radiation fields and produce electron-positron pairs. Alternatively, in the presence of ambient magnetic fields, they can convert into hypothetical axion-like particles (ALPs), pseudo-scalar spin-0 bosons, predicted by extensions of the standard model. These particles propagate unimpeded over cosmological distances. Here, the effect of photon-ALP oscillations in magnetic fields of galaxy clusters and the Milky Way on VHE γ-ray spectra is studied. It is shown that this mechanism can lead to a substantial enhancement of the VHE flux and a spectral hardening, thus effectively reducing the opacity of the Universe to VHE γ-rays.

astro-ph.HE↗

arXiv2012

Hardening of TeV gamma spectrum of AGNs in galaxy clusters by conversions of photons into axion-like particles

A fraction of AGN producing VHE gamma-rays are located in galaxy clusters. The magnetic field present in the intra-cluster medium would lead to conversions of VHE photons into axion-like particles (ALPs), which are a generic prediction of several extensions of the Standard Model. ALPs produced in this way would traverse cosmological distances unaffected by the extragalactic background light at variance with VHE photons which undergo a substantial absorption. Eventually, a nontrivial fraction of ALPs would re-convert into VHE photons in the magnetic field of the Milky Way. This mechanism produces a significant hardening of the VHE spectrum of AGN in galaxy clusters. As a specific example we consider the energy spectra of two observed VHE gamma-ray sources located in galaxy clusters, namely 1ES 0414+009 at redshift z=0.287 and Mkn 501 at z=0.034. We find that the hardening in the observed spectra becomes relevant at E > 1 TeV. The detection of this signature would allow to indirectly probe the existence of ultra-light ALPs with mass m_a < 10^{-8} eV and photon-ALP coupling g_{a gamma} < 10^{-10} GeV^{-1} with the presently operating Imaging Atmospheric Cherenkov Telescopes like H.E.S.S., MAGIC, VERITAS and CANGAROO-III and even more likely with the planned detectors like CTA, HAWC and HiSCORE. An independent laboratory check of ultra-light ALPs invoked in this mechanism can be performed with the planned upgrade of the photon regeneration experiment ALPS at DESY and with the next generation solar axion detector IAXO.

astro-ph.HE↗

arXiv2012

Antiprotons from dark matter annihilation in the Galaxy: astrophysical uncertainties

The latest years have seen steady progresses in WIMP dark matter (DM) searches, with hints of possible signals suggested by both direct and indirect detection experiments. Antiprotons can play a key role validating those interpretations since they are copiously produced by WIMP annihilations in the Galactic halo, and the secondary antiproton background produced by Cosmic Ray (CR) interactions is predicted with fair accuracy and matches the observed spectrum very well. Using the publicly available numerical DRAGON code, we reconsider antiprotons as a tool to constrain DM models discussing its power and limitations. We provide updated constraints on a wide class of annihilating DM models by comparing our predictions against the most up-to-date ap measurements, taking also into account the latest spectral information on the p, He and other CR nuclei fluxes. Doing that, we probe carefully the uncertainties associated to both secondary and DM originated antiprotons, by using a variety of distinctively different assumptions for the propagation of CRs and for the DM distribution in the Galaxy. We find that the impact of the astrophysical uncertainties on constraining the DM properties can be much stronger, up to a factor of ~50, than the one due to uncertainties on the DM distribution (~2-6). Remarkably, even reducing the uncertainties on the propagation parameters derived by local observables, non-local effects can still change DM model constraints even by 50%. Nevertheless, current ap data place tight constraints on DM models, excluding some of those suggested in connection with indirect and direct searches. Finally we discuss the power of upcoming CR spectral data from the AMS-02 observatory to drastically reduce the uncertainties discussed in this paper and estimate the expected sensitivity of this instrument to some sets of DM models.

astro-ph.HE↗

arXiv2012

CRPropa 2.0 -- a Public Framework for Propagating High Energy Nuclei, Secondary Gamma Rays and Neutrinos

Version 2.0 of CRPropa is public software to model the extra-galactic propagation of ultra-high energy nuclei of atomic number Z<26 through structured magnetic fields and ambient photon backgrounds taking into account all relevant particle interactions. CRPropa covers the energy range 6*10^16 < E/eV < A*10^22 where A is the nuclear mass number. CRPropa can also be used to track secondary γ-rays and neutrinos which allows the study of their link with the charged primary nuclei -- the so called multi-messenger connection. After a general introduction we present several sample applications of current interest concerning the physics of extragalactic ultra-high energy radiation.

astro-ph.IM↗

arXiv2012

A common solution to the cosmic ray anisotropy and gradient problems

Multichannel Cosmic Ray (CR) spectra and the large scale CR anisotropy can hardly be made compatible in the framework of conventional isotropic and homogeneous propagation models. These models also have problems explaining the longitude distribution and the radial emissivity gradient of the $γ$-ray galactic interstellar emission. We argue here that accounting for a well physically motivated correlation between the CR escape time and the spatially dependent magnetic turbulence power can naturally solve both problems. Indeed, by exploiting this correlation we find propagation models that fit a wide set of CR primary and secondary spectra, and consistently reproduce the CR anisotropy in the energy range $10^2 - 10^4 \GeV$ and the $γ$-ray longitude distribution recently measured by Fermi-LAT.

astro-ph.HE↗

arXiv2011

Dark matter electron anisotropy: a universal upper limit

We study the dipole anisotropy in the arrival directions of high energy CR electrons and positrons (CRE) of Dark Matter (DM) origin. We show that this quantity is very weakly model dependent and offers a viable criterion to discriminate among CRE from DM or from local discrete sources, like e.g. pulsars. In particular, we find that the maximum anisotropy which DM can provide is to a very good approximation a universal quantity and, as a consequence, if a larger anisotropy is detected, this would constitute a strong evidence for the presence of astrophysical local discrete CRE sources, whose anisotropy, instead, can be naturally larger than the DM upper limit. We further find that the main source of anisotropy from DM is given by the fluctuation in the number density of DM sub-structures in the vicinity of the observer and we thus devote special attention to the study of the variance in the sub-structures realization implementing a dedicated Montecarlo simulation. Such scenarios will be probed in the next years by Fermi-LAT, providing new hints, or constraints, about the nature of DM.

astro-ph.HE↗

arXiv2011

An alternative solution to the gamma-ray Gradient problem

The Fermi-LAT collaboration recently confirmed EGRET finding of a discrepancy between the observed longitudinal profile of gamma-ray diffuse emission from the Galaxy and that computed with GALPROP assuming that cosmic rays are produced by Galactic supernova remnants. The accurate Fermi-LAT measurements make this anomaly hardly explainable in terms of conventional diffusion schemes. Here we use DRAGON numerical diffusion code to implement a physically motivated scenario in which the diffusion coefficient is spatially correlated to the source density. We show that under those conditions we are able to reproduce the observed flat emissivity profile in the outer Galaxy with no need to change the source term, the diffusion halo height, or the CO-H2 conversion factor (XCO) with respect to their preferred values/distributions. We also show that our models are compatible with gamma-ray longitudinal profiles measured by Fermi-LAT, and still provide a satisfactory fit of all observed secondary-to-primary ratios, such as B/C and antiprotons/protons.

astro-ph.HE↗