1,602 research outputs found
New insights on hadron acceleration at supernova remnant shocks
We outline the main features of nuclei acceleration at supernova remnant
forward shocks, stressing the crucial role played by self-amplified magnetic
fields in determining the energy spectrum observed in this class of sources. In
particular, we show how the standard predictions of the non-linear theory of
diffusive shock acceleration has to be completed with an additional ingredient,
which we propose to be the enhanced velocity of the magnetic irregularities
particles scatter against, to reconcile the theory of efficient particle
acceleration with recent observations of gamma-ray bright supernova remnants.Comment: 7 pages, 2 figures. To apper in "Cosmic-ray induced phenomenology in
star-forming environments: Proceedings of the 2nd Session of the Sant Cugat
Forum of Astrophysics" (April 16-19, 2012), Olaf Reimer and Diego F. Torres
(eds.
Strong evidences of hadron acceleration in Tycho's Supernova Remnant
Very recent gamma-ray observations of G120.1+1.4 (Tycho's) supernova remnant
(SNR) by Fermi-LAT and VERITAS provided new fundamental pieces of information
for understanding particle acceleration and non-thermal emission in SNRs. We
want to outline a coherent description of Tycho's properties in terms of SNR
evolution, shock hydrodynamics and multi-wavelength emission by accounting for
particle acceleration at the forward shock via first order Fermi mechanism. We
adopt here a quick and reliable semi-analytical approach to non-linear
diffusive shock acceleration which includes magnetic field amplification due to
resonant streaming instability and the dynamical backreaction on the shock of
both cosmic rays (CRs) and self-generated magnetic turbulence. We find that
Tycho's forward shock is accelerating protons up to at least 500 TeV,
channelling into CRs about the 10 per cent of its kinetic energy. Moreover, the
CR-induced streaming instability is consistent with all the observational
evidences indicating a very efficient magnetic field amplification (up to ~300
micro Gauss). In such a strong magnetic field the velocity of the Alfv\'en
waves scattering CRs in the upstream is expected to be enhanced and to make
accelerated particles feel an effective compression factor lower than 4, in
turn leading to an energy spectrum steeper than the standard prediction
{\propto} E^-2. This latter effect is crucial to explain the GeV-to-TeV
gamma-ray spectrum as due to the decay of neutral pions produced in nuclear
collisions between accelerated nuclei and the background gas. The
self-consistency of such an hadronic scenario, along with the fact that the
concurrent leptonic mechanism cannot reproduce both the shape and the
normalization of the detected the gamma-ray emission, represents the first
clear and direct radiative evidence that hadron acceleration occurs efficiently
in young Galactic SNRs.Comment: Minor changes. Accepted for publication in Astronomy & Astrophysic
Gamma Rays and Neutrinos from SNR RX J1713.7-3946
The supernova paradigm for the origin of galactic cosmic rays can be tested
using multifrequency observations of both non-thermal and thermal emission from
supernova remnants. The smoking gun of hadronic acceleration in these sources
can, however, only be provided by the detection of a high energy neutrino
signal. Here we apply the theory of non-linear particle acceleration at
supernova shocks to the case of the supernova remnant RX J1713.7-3946, which is
becoming the stereotypical example of a possible hadronic accelerator after the
detection of high energy gamma rays by the HESS telescope. Our aim is twofold:
on one hand we want to address the uncertainties in the discrimination between
a hadronic and a leptonic interpretation of the gamma ray emission, mainly
related to the possibility of a statistical uncertainty in the energy
determination of the gamma ray photons in the TeV region. On the other we want
to stress how a km cube neutrino telescope would break the degeneracy and
provide evidence for efficient cosmic ray acceleration in RX J1713.7-3946. A 3
sigma evidence would require about two years of observation.Comment: 11 pages, 3 figures, accepted for publication in Astropaticle Physic
Yale School of Public Health Symposium on tissue imaging mass spectrometry: illuminating phenotypic heterogeneity and drug disposition at the molecular level.
Antibiotic resistance patterns of faecal indicator organisms and occurrence of Salmonella spp. in wild boar (Sus scrofa scrofa) in Italy.
In order to monitor antibiotic resistance in faecal indicator organisms and evaluate the occurrence of Salmonella spp., faeces from 110 wild boars (Sus scrota scrota), killed during a demographic control program in two different regional parks in Bologna province, were collected from September 2002 to June 2003. A single isolate of Escherichia coli, Enterococcus faecalis and Enterococcus faecium from each sample was tested for antibiotic susceptibility using the agar diffusion method recommended by CLSI (formerly, NCCLS). A total of 110 E. coli, 48 E. faecium and 5 E. faecalis strains were isolated and submitted to antibiotic susceptibility tests. Antibiotic resistance patterns were similar in wild boar populations from both parks. Multiple antibiotic-resistance to ciprofloxacin, rifampin and erythromycin was found with h1gh frequency in Enterococcus spp strains. E. coli isolates showed a low antibiotic resistance level. Two Salmonella arizonae and one Salmonella spp. strains, isolated from wild boars of one park, didn\u27t show any resistance concerning the antibiotics tested
Gamma ray emission from SNR RX J1713.7-3946 and the origin of galactic cosmic rays
We calculate the flux of non-thermal radiations from the supernova remnant RX
J1713.7-3946 in the context of the non-linear theory of particle acceleration
at shocks, which allows us to take into account self-consistently the dynamical
reaction of the accelerated particles, the generation of magnetic fields in the
shock proximity and the dynamical reaction of the magnetic field on the plasma.
When the fraction of particles which get accelerated is of order , we find that the strength of the magnetic field obtained as a result
of streaming instability induced by cosmic rays is compatible with the
interpretation of the X-ray emitting filaments being produced by strong
synchrotron losses in magnetic fields. If the X-ray filaments
are explained in alternative ways, the constraint on the magnetic field
downstream of the shock disappears and the HESS data can be marginally fit with
ICS of relativistic electrons off a complex population of photons, tailored to
comprise CMB and ambient IR/Optical photons. The fit, typically poor at the
highest energies, requires a large density of target photons within the
remnant; only a fraction of order of the background particles
gets accelerated; the local magnetic field is of order and the
maximum energy of protons is much lower than the knee energy. Current HESS
gamma ray observations combined with recent X-ray observations by Suzaku do not
allow as yet to draw a definitive conclusion on whether RX J1713.7-3946 is an
efficient cosmic ray accelerator, although at the present time a hadronic
interpretation of HESS data seems more likely. We discuss the implications of
our results for the GLAST gamma ray telescope, which should be able to
discriminate the two scenarios discussed above.Comment: Accepted for Publication in MNRA
Short communication: Isolation of Shiga toxin-producing Escherichia coli in raw milk and mozzarella cheese in southern Italy
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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
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