arXiv subjects
Publications and source records attributed to Roberto Aloisio.
We discuss the basic features of the propagation of Ultra High Energy Cosmic Rays in astrophysical backgrounds, comparing two alternative computation schemes to compute the expected fluxes. We also discuss the issue of the transition among galactic and extra-galactic cosmic rays using theoretical results on fluxes to compare different models.
The status of the observations of Ultra High Energy Cosmic Rays will be reviewed, focusing on the the latest results of HiRes and Auger observatories. A comprehensive analytical computation scheme to compute the expected UHECR spectrum on earth will be presented, applying such scheme to interpret the experimental results. The phenomenological scenarios favored by HiRes and Auger in terms of chemical composition and spectrum will be also presented.
We discuss the problem of ultra high energy nuclei propagation in astrophysical backgrounds. We present a new analytical computation scheme based on the hypothesis of continuos energy losses in a kinetic formulation of the particles propagation. This scheme enables the computation of the fluxes of ultra high energy nuclei as well as the fluxes of secondaries (nuclei and nucleons) produced by the process of photo-disintegration suffered by nuclei.
Extragalactic cosmic ray protons with an injection spectrum of the type $E^{-2.7}$ show a spectrum on earth with a dip due to the Bethe-Heitler pair production against the photons of the cosmic microwave background. The dip is produced in the energy region $10^{18} - 4\times 10^{19}$ eV with position and shape that reproduce with high accuracy the spectrum observed experimentally. This interpretation of the observed data predicts the existence of an energy scale that signals a possible transition from galactic to extragalactic cosmic rays. In fact, at energies lower than a characteristic value $E_c\approx 1\times 10^{18}$ eV, determined by the equality between the rate of energy losses due to pair production and adiabatic losses, the spectrum of cosmic rays flattens in all cases of interest. In this model, the transition from galactic to extragalactic cosmic rays occurs at some energy below $E_c$, corresponding to the position of the so-called second knee. Another viable explanation of the observed data is based on a completely different approach assuming a mixed composition with protons and nuclei at energies $E\ge 10^{19}$ eV. This scenario implies a transition from galactic to extragalactic cosmic rays at the ankle energies ($\sim 10^{19}$ eV). In the present paper we will review the main features of the dip model comparing it with the model of transition at the ankle.
Neutrinos are the best candidates to test the extreme Universe and ideas beyond the Standard Model of particle Physics. Once produced, neutrinos do not suffer any kind of attenuation by intervening radiation fields like the Cosmic Microwave Background and are not affected by magnetic fields. In this sense neutrinos are useful messengers from the far and young Universe. In the present paper we will discuss a particular class of sources of Ultra High Energy Cosmic Rays introduced to explain the possible excess of events with energy larger than the Graisen-Zatsepin-Kuzmin cut-off. These sources, collectively called top-down, share a common feature: UHE particles are produced in the decay or annihilation of superheavy, exotic, particles. As we will review in the present paper, the largest fraction of Ultra High Energy particles produced in the top-down scenario are neutrinos. The study of these radiation offers us a unique opportunity to test the exotic mechanisms of the top-down scenario.
In this paper we review the main features of the observed Cosmic Rays spectrum in the energy range $10^{17} {\rm eV}~÷~10^{20} {\rm eV}$. We present a theoretical model that explains the main observed features of the spectrum, namely the second Knee and Dip, and implies a transition from Galactic to Extra-Galactic cosmic rays at energy $E\simeq 10^{18}$ eV, with a proton dominated Extra-Galactic spectrum.
The annihilation of neutralino dark matter in the Galactic Center (GC) may result in radio signals that can be used to detect or constrain the dark matter halo density profile or dark matter particle properties. At the Galactic Center, the accretion flow onto the central Black Hole (BH) sustains strong magnetic fields that can induce synchrotron emission by electrons and positrons generated in neutralino annihilations during advection onto the BH. Here we reanalyze the radiative processes relevant for the neutralino annihilation signal at the GC, with realistic assumptions about the accretion flow and its magnetic properties. We find that neglecting these effects, as done in previous papers, leads to the incorrent electron and photon spectra. We find that the magnetic fields associated with the flow are significantly stronger than previously estimated. We derive the appropriate equilibrium distribution of electrons and positron and the resulting radiation, considering adiabatic compression in the accretion flow, inverse Compton scattering off synchrotron photons (synchrotron self-Compton scattering), and synchrotron self-absorption of the emitted radiation. We derive the signal for a Navarro-Frenk-White (NFW) dark matter halo profile and a NFW profile with a dark matter spike due to the central BH. We find that the observed radio emission from the GC is inconsistent with the scenario in which a spiky distribution of neutralinos is present. We discuss several important differences between our calculations and those previously presented in the literature.
The annihilation of neutralino dark matter may result in observable signals in different wavelength. In the present paper we will discuss the effect of neutralino annihilation in the halo of our Galaxy and in its center. According to high resolution cold dark matter simulations, large virialized halos are formed through the constant merging of smaller halos appeared at previous times. At each epoch, dark matter halos have then a clumpy component which is made of these merging subhalos. The annihilation of dark matter in these clumps, always present in the halo of our Galaxy, may be responsible for appreciable fluxes of $γ$-rays, potentially detectable. We find that, depending on the fundamental parameters of the clump density profile and on the distribution of clumps in the Galactic halo, the contribution to the diffuse $γ$-ray background from clumps could be used to obtain constraints on the neutralino properties such as mass and annihilation cross section. On the other hand the annihilation of neutralino dark matter in the galactic center may result in radio signals. At the galactic center, infact, the accretion flow onto the central black hole sustains strong magnetic fields that can induce synchrotron emission, in the radio wavelength, by electrons and positrons generated in neutralino annihilations during advection onto the black hole. We find that the observed emission from the galactic center is consistent with neutralinos following a Navarro Frenk and White density profile at the galactic center while it is inconsistent with the presence of a spike density profile, supposed to be generated by the formation history of the central black hole.
The standard calculations of the synchrotron emission from charged particles in magnetic fields does not apply when the energy losses of the particles are so severe that their energy is appreciably degraded during one Larmor rotation. In these conditions, the intensity and spectrum of the emitted radiation depend on the observation time $T_{obs}$: the standard result is recovered only in the limit $T_{obs}\ll T_{loss}$, where $T_{loss}$ is the time for synchrotron losses. In this case the effects of the radiation backreaction cannot be detected by the observer. We calculate the emitted power of the radiation in the most general case, naturally including both the cases in which the backreaction is relevant and the standard case, where the usual result is recovered. Finally we propose several scenarios of astrophysical interest in which the effects of the backreaction cannot and should not be ignored.
Synchrotron emission of relativistic particles in magnetic fields is a process of paramount importance in astrophysics. Although known for over thirty years, there are still aspects of this radiative process that have received little attention, mainly because they appear only in extreme conditions. In the present paper, we first provide a general introduction to synchrotron emission, using a formalism that represents a generalization of the standard calculations. The use of this formalism allows us to discuss situations in which charged particles can radiate coherently, with special attention for the cases in which the production occurs in the form of a bunch of particles created in a pulse of very short duration. We calculate the spectra of the radiation for both monoenergetic particles and distributions of particles with different Lorentz factors. For both cases we study the conditions for the coherent effects to appear, and demonstrate that in the limit of incoherent emission we reobtain the well known results.
Even a fundamental symmetry like Lorentz Invariance is an experimental fact and must be experimentally verified. We show that the study of the interactions of Cosmic Rays with universal diffuse background radiation can provide very stringent tests of this symmetry. The interactions we consider are the ones characterized by well defined energy thresholds whose energy position can be predicted on the basis of Special Relativity. We argue that the experimental verification of these thresholds can address the physics of supra-Planckian scales.