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Digital archive of theses discussed at the University of Pisa


Thesis etd-10092011-000248

Thesis type
Tesi di dottorato di ricerca
email address
daniele.gaggero@pi.infn.it, daniele.gaggero@gmail.com
Thesis title
Cosmic Ray diffusion in the Galaxy and diffuse Gamma emission.
Academic discipline
Course of study
tutor Dott. Grasso, Dario
relatore Prof. Bellazzini, Ronaldo
  • Astroparticle
  • Cosmic Rays
  • Fermi
  • Fermi-LAT
  • Gamma Rays
Graduation session start date
This thesis deals with the problems related to the origin and propagation of Galactic Cosmic Rays (CRs) and to the analysis of the diffuse Gamma emission from the Milky Way.

The thesis is devoted to three main topics: the hadronic component, the leptonic component and the diffuse
gamma-ray emission from the Galaxy.

I will describe in the first part of this thesis how we succeeded in building a comprehensive model which provides
a good fit of most CR light nuclei and antiproton spectra making use of a new numerical code called DRAGON, originally designed by our group to solve the CR diffusion equation and compute all the relevant processes (spallation, energy losses) that are involved in CR propagation.
The model parameters
are determined through a combined maximum likelihood analysis based on light nuclei and antiproton-to-proton ratio data.
The analysis focuses
on the high-energy part of the measurements, following a very different strategy with respect to the previous analyses that can
be found in the literature.

The second part deals with leptons.

Several experiments reported relevant excesses of some components
of leptonic CR fluxes with respect to model predictions: the debate that arose from these measurements
was particularly exciting.

In particular, PAMELA collaboration observed in 2008 a rising positron-to-electron ratio; this rise was recently confirmed by the Fermi collaboration too. This kind of behaviour is
in strong contrast with models: in fact, in a scenario in which positrons are created as secondary products by
the interaction of primary CR nuclei with interstellar gas, the positron-to-electron ratio should be decreasing with energy.

This anomaly was interpreted by many authors as an indication of extra primary sources of electrons and/or positrons, either of astrophysical nature (e.g. pulsars) or exotic (Dark Matter annihilation or decay).

I will discuss these different
scenarios with particular emphasis on the interpretation of Fermi-LAT electron+positron dataset which, due to the high statistics and the precise evaluation of systematic effects, appears as the most reliable in the leptonic field.
I will show in detail what we discussed in our publications that
followed Fermi-LAT measurement: we found that a simple phenomenological model, in which a primary extra component of electrons and positrons is added to a diffuse conventional emission, is compatible with all the existing observations (including recent measurements of upper limits on CRE anisotropy reported by Fermi-LAT).
Concerning the nature of this extra-component, the debate is still open. We pointed out that, under simple assumptions, the observed pulsars -- that are known as electron+positron pair emitters -- are natural candidates; on the other hand, the hypothesis of a Dark Matter origin for the extra electrons and positrons is losing part of its appeal for the reasons I will explain in detail in this work, but it remains a fascinating possibility which must not be considered ruled out yet.

The last Chapter of the thesis deals with
gamma-rays and pays particular attention to the gradient
problem: the computed
gamma-ray profile along the Galactic plane appears steeper than the observed
one. This problem has been known since the EGRET era and was recently confirmed by Fermi-LAT.

I will show how the gradient problem may be
solved introducing of a spatial dependent diffusion coefficient, which is able to trace
the physical distribution of sources in the galactic disk.