103 research outputs found
High-energy astrophysics, cosmic rays and fundamental physics
This thesis is devoted to the study of phenomenological consequences of theoretical models of Quantum Gravity. In particular, this work is focused on the study of possible violations of Lorentz invariance, which may arise if, owing to quantum gravity effects, the high-energy structure of the spacetime is different from the smooth, continuous one we are used to in our low-energy world. After a brief description of the most widely known models accounting for Lorentz invariance violations, particular focus will be given to astrophysical tests of Lorentz invariance. These are motivated by the fact that some astrophysical objects are able to accelerate particles to extremely high energies, unreachable to terrestrial experiments. This consideration naturally leads us to look at the radiation of the Crab Nebula, one of the most powerful objects in our Galaxy. We first understand how the violation of Lorentz invariance affects the physical processes at the basis of the production of electromagnetic radiation by this object. Then, we compare our prediction for the Lorentz violating spectrum to observational data, exploiting the vast multi-wavelength information on the Crab Nebula radiation. Furthermore, we take advantage of the recent development of new technology to improve on our analysis of the Crab Nebula radiation by extending our research to the effects of Lorentz violation onto hard X-ray polarization. After this investigation we shall move to study the physics of cosmic rays, the most energetic particles ever experienced on Earth. Our interest in this physics is twofold: on the one hand, we want to understand more about their properties and their propagation. To this aim, we develop a new model of propagation for cosmic rays in our Galaxy, exploiting as much as possible of the multi-channel information available at present. On the other hand, according to the multi-channel perspective, we try to understand the consequences of Lorentz symmetry violation on the properties of ultra-highenergy cosmic rays
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.Comment: 4 page, 2 figures; v2: minor changes to match published versio
Probing axion-like particles with the ultraviolet photon polarization from active galactic nuclei in radio galaxies
The mixing of photons with axion-like particles (ALPs) in the large-scale
magnetic field changes the polarization angle of a linearly polarized
photon beam from active galactic nuclei in radio galaxies as it propagates over
cosmological distances. Using available ultraviolet polarization data
concerning these sources we derive a new bound on the product of the photon-ALP
coupling times . We find
GeV nG for ultralight ALPs with eV. We compare
our new bound with the ones present in the literature and we comment about
possible improvements with observations of more sources.Comment: v2: one typo corrected. Added a few comments, matches published
versio
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 -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 -ray
longitude distribution recently measured by Fermi-LAT.Comment: 4 pages, 3 figures. v2: Accepted in Phys. Rev. Let
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