276 research outputs found
Particle acceleration at shock waves: particle spectrum as a function of the equation of state of the shocked plasma
We determine the spectrum of particles accelerated at shocks with arbitrary
speed and arbitrary scattering properties for different choices of the equation
of state of the downstream plasma. More specifically we consider the effect of
energy exchange between the electron and proton thermal components downstream,
and the effect of generation of a turbulent magnetic field in the downstream
plasma. The slope of the spectrum turns out to be appreciably affected by all
these phenomena, especially in the Newtonian and trans-relativistic regime,
while in the ultra-relativistic limit the universal spectrum
seems to be a very solid prediction.Comment: 21 pages, 8 fig
The origin of ultra high energy cosmic rays
We briefly discuss some open problems and recent developments in the
investigation of the origin and propagation of ultra high energy cosmic rays
(UHECRs).Comment: Invited Review Talk at TAUP 2005 (Zaragoza - September 10-14, 2005).
7 page
Diffusive propagation of cosmic rays from supernova remnants in the Galaxy. I: spectrum and chemical composition
In this paper we investigate the effect of stochasticity in the spatial and
temporal distribution of supernova remnants on the spectrum and chemical
composition of cosmic rays observed at Earth. The calculations are carried out
for different choices of the diffusion coefficient D(E) experienced by cosmic
rays during propagation in the Galaxy. In particular, at high energies we
assume that D(E)\sim E^{\delta}, with and being the
reference scenarios. The large scale distribution of supernova remnants in the
Galaxy is modeled following the distribution of pulsars, with and without
accounting for the spiral structure of the Galaxy. We find that the stochastic
fluctuations induced by the spatial and temporal distribution of supernovae,
together with the effect of spallation of nuclei, lead to mild but sensible
violations of the simple, leaky-box-inspired rule that the spectrum observed at
Earth is with , where
is the slope of the cosmic ray injection spectrum at the sources. Spallation of
nuclei, even with the small rates appropriate for He, may account for slight
differences in spectral slopes between different nuclei, providing a possible
explanation for the recent CREAM observations. For we find that
the slope of the proton and helium spectra are and
respectively at energies above 1 TeV (to be compared with the measured values
of and ). For the hardening of the He
spectra is not observed. We also comment on the effect of time dependence of
the escape of cosmic rays from supernova remnants, and of a possible clustering
of the sources in superbubbles. In a second paper we will discuss the
implications of these different scenarios for the anisotropy of cosmic rays.Comment: 28 pages, To appear in JCA
Expected performance of the ASTRI-SST-2M telescope prototype
ASTRI (Astrofisica con Specchi a Tecnologia Replicante Italiana) is an
Italian flagship project pursued by INAF (Istituto Nazionale di Astrofisica)
strictly linked to the development of the Cherenkov Telescope Array, CTA.
Primary goal of the ASTRI program is the design and production of an end-to-end
prototype of a Small Size Telescope for the CTA sub-array devoted to the
highest gamma-ray energy region. The prototype, named ASTRI SST-2M, will be
tested on field in Italy during 2014. This telescope will be the first
Cherenkov telescope adopting the double reflection layout in a
Schwarzschild-Couder configuration with a tessellated primary mirror and a
monolithic secondary mirror. The collected light will be focused on a compact
and light-weight camera based on silicon photo-multipliers covering a 9.6 deg
full field of view. Detailed Monte Carlo simulations have been performed to
estimate the performance of the planned telescope. The results regarding its
energy threshold, sensitivity and angular resolution are shown and discussed.Comment: In Proceedings of the 33rd International Cosmic Ray Conference
(ICRC2013), Rio de Janeiro (Brazil). All CTA contributions at arXiv:1307.223
New high-frequency radio observations of the Cygnus Loop supernova remnant with the Italian radio telescopes
Supernova remnants (SNRs) represent a powerful laboratory to study the
Cosmic-Ray acceleration processes at the shocks, and their relation to the
properties of the circumstellar medium. With the aim of studying the
high-frequency radio emission and investigating the energy distribution of
accelerated electrons and the magnetic field conditions, we performed
single-dish observations of the large and complex Cygnus Loop SNR from 7.0 to
24.8 GHz with the Medicina and the Sardinia Radio Telescope, focusing on the
northern filament (NGC 6992) and the southern shell. Both regions show a
spectrum well fitted by a power-law function (), with
spectral index for NGC 6992 and for
the southern shell and without any indication of a spectral break. The spectra
are significantly flatter than the whole Cygnus Loop spectrum
(), suggesting a departure from the plain shock
acceleration mechanisms, which for NGC 6992 could be related to the ongoing
transition towards a radiative shock. We model the integrated spectrum of the
whole SNR considering the evolution of the maximum energy and magnetic field
amplification. Through the radio spectral parameters, we infer a magnetic field
at the shock of 10 G. This value is compatible with a pure adiabatic
compression of the interstellar magnetic field, suggesting that the
amplification process is currently inefficient.Comment: 19 pages, 13 figure
Modelli di co-governance urbana, sostenibilità, bancabilità ed eleggibilità finanziaria di imprese civiche o di comunità: il partenariato pubblico-comunità e il partenariato pubblico-privato-comunità
Pulsar-wind nebulae and magnetar outflows: observations at radio, X-ray, and gamma-ray wavelengths
We review observations of several classes of neutron-star-powered outflows:
pulsar-wind nebulae (PWNe) inside shell supernova remnants (SNRs), PWNe
interacting directly with interstellar medium (ISM), and magnetar-powered
outflows. We describe radio, X-ray, and gamma-ray observations of PWNe,
focusing first on integrated spectral-energy distributions (SEDs) and global
spectral properties. High-resolution X-ray imaging of PWNe shows a bewildering
array of morphologies, with jets, trails, and other structures. Several of the
23 so far identified magnetars show evidence for continuous or sporadic
emission of material, sometimes associated with giant flares, and a few
possible "magnetar-wind nebulae" have been recently identified.Comment: 61 pages, 44 figures (reduced in quality for size reasons). Published
in Space Science Reviews, "Jets and Winds in Pulsar Wind Nebulae, Gamma-ray
Bursts and Blazars: Physics of Extreme Energy Release
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