| 1. |
- Di Bernardo, Giuseppe, et al.
(författare)
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“Cosmic Ray Electrons, Positrons and the Synchrotron emission of the Galaxy: consistent analysis and implications”
- 2013
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Ingår i: Journal of Cosmology and Astroparticle Physics. - : IOP Publishing. - 1475-7516 .- 1475-7516. ; 2013:3
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 2. |
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| 3. |
- Di Bernardo, Giuseppe
(författare)
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Transport, Turbulence and Instabilities in Cosmic Magnetic Fields
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This decade has seen a large number of space missions, which, alongside ground-based radio, optical and gamma-ray telescopes, have enabled a deep insight into the non-thermal astrophysical environments. Interstellar Medium (ISM), Supernovae Remnants (SNRs) and Black-Hole (BH) accretion discs (ADs) are only a few examples of natural habitat of interaction of relativistic particles and magnetic fields, largely mediated by the action of the turbulence. In spite of many efforts, and the recent progress in this field, we are still missing a fully comprehension of the nature of the problem. Throughout the Thesis, the key science driver concept is the transport in magnetic turbulent fields. The aims of the work here presented are meant to be a step in that direction. They can be precisely grouped into two main themes: (i) understanding the transport of Cosmic Rays (Crs), and their dynamical role in the Milky Way; (ii) understanding the physics of ADs, with special attention on the magnetic, turbulent environment around compact objects responsible of driving inflow material through the discs. In this regard, I will firstly give a review intended to cover the main the- oretical aspects involved in the astrophysics of CRs. A section will be dedicated to the presentation of preliminary results accomplished in the context of the magneto- hydrodynamics (MHD) shearing box numerical simulations of turbulence in ADs. I will move on by introducing the main achievements of my scientific activity, as reported in the following Thesis. A detailed cosmic ray transport description in the Galaxy has been implemented in the DRAGON code, a numerical tool used to simulate the local interstellar spectra (LIS) of CRs. There is by now compelling evidence of an anomalous rise with energy of the cosmic ray positron fraction. Conversely to the standard picture of a pure secondary positron production, the data strengthen the evidence for the presence of two distinct electron and positron spectral components. Given the cosmic ray transport model, I will show that nearby pulsars are viable source candidates of the required e± extra-component. In a multichannel analysis of cosmic ray electron and positron spectra, I will present the results of our recent study on the diffuse synchrotron emission of the Galaxy. At low energies - roughly below 4 GeV - we find that the electron primary spectrum is significantly suppressed so that the low-energy total spectrum will turn out to be dominated by secondary particles. Comparing the computed synchrotron emission intensity with the radio data, we placed a constraint on the diffusive magnetic halo scale height, of relevant importance especially for indirect Dark Matter searches. Fairly poor knowledge is still present about the cosmic ray spectra at low energies, due to the distortion produced by the solar wind on the particle fluxes. Going beyond the standard force-field solar modulation, I will show the results of a self- consistent galactic-plus-solar transport model, where charge-sign dependent motion effects are taken in account. Lately, I will discuss the impact of a realistic spiral arm distribution of CRs source in the Galaxy, modelling the e± spectra measured by Pamela and Ams-02 by running DRAGON in a full three-dimensional version.
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| 4. |
- Di Bernardo, Giuseppe, et al.
(författare)
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Wave modes from the magnetorotational instability in accretion discs
- 2013
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Ingår i: IAU Symp. 290: Feeding compact objects: Accretion on all scales, C. Zhang, T. Belloni, M. Méndez & S. Zhang (eds.). - 1743-9213. - 9781107033795 ; , s. 201-202
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The magnetorotational instability (MRI) is widely believed to be the source of turbulence in accretion discs. This turbulence is responsible for the anomalous angular momentum transport in accretion discs. The turbulence will affect other aspects of the dynamics of the disc as well, and we will concentrate on two such issues: a) what kind of oscillations can be excited by the turbulence itself, and b) how the turbulence is interacting with modes that have been excited by some other agent. This is of interest in understanding the quasi-periodic oscillations (QPOs) that have been observed in the X-ray light curves of accreting neutron star and black hole binaries. We carry out local three dimensional (3D) magnetohydrodynamic simulations of a keplerian differentially rotating accretion disc, using a shearing box configuration taking in account the effects of the vertical stratification.
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| 5. |
- Gaggero, Daniele, 1982, et al.
(författare)
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PAMELA and AMS-02 positron and electron spectra are reproduced by three-dimensional cosmic-ray modeling
- 2014
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Ingår i: Physical Review D. Particles and fields. - 0556-2821. ; 89:8
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Tidskriftsartikel (refereegranskat)abstract
- 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 ∼1–10TeV 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 ∼10TeV 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 (e.g., one or few pulsars) is invoked.
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| 6. |
- Gaggero, Daniele, 1981, et al.
(författare)
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“Three-Dimensional Model of Cosmic-Ray Lepton Propagation Reproduces Data from the Alpha Magnetic Spectrometer on the International Space Station”
- 2013
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 111:2
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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