13,151 research outputs found
Cosmic Ray acceleration and Balmer emission from SNR 0509-67.5
Context: Observation of Balmer lines from the region around the forward shock
of supernova remnants may provide precious information on the shock dynamics
and on the efficiency of particle acceleration at the shock.
Aims: We calculate the Balmer line emission and the shape of the broad Balmer
line for parameter values suitable for SNR 0509-67.5, as a function of the
cosmic ray acceleration efficiency and of the level of thermal equilibration
between electrons and protons behind the shock. This calculation aims at using
the width of the broad Balmer line emission to infer the cosmic ray
acceleration efficiency in this remnant.
Methods: We use the recently developed non-linear theory of diffusive shock
acceleration in the presence of neutrals. The semi-analytical approach that we
developed includes a description of magnetic field amplification as due to
resonant streaming instability, the dynamical reaction of both accelerated
particles and turbulent magnetic field on the shock, and all channels of
interaction between neutral atoms and background plasma that change the shock
dynamics.
Results: We achieve a quantitative assessment of the CR acceleration
efficiency in SNR 0509-67.5 as a function of the shock velocity and different
levels of electron-proton thermalization in the shock region. If the shock
moves faster than ~4500 km/s, one can conclude that particle acceleration must
be taking place with efficiency of several tens of percent. For lower shock
velocity the evidence for particle acceleration becomes less clear because of
the uncertainty in the electron-ion equilibration downstream. We also discuss
the role of future measurements of the narrow Balmer line.Comment: 7 pages, 5 figure. Accepted for publication in Astronomy &
Astrophysic
Contribution to diffuse gamma-ray emission coming from self-confined CRs around their Galactic sources
Recent observations of the diffuse Galactic gamma-ray emission by the
Fermi-LAT satellite have shown significant deviations from models which assume
the same diffusion properties for cosmic rays (CR) throughout the Galaxy. We
explore the possibility that a fraction of this diffuse Galactic emission could
be due to hadronic interactions of CRs self-confined in the region around their
sources. In fact, freshly accelerated CRs that diffuse away from the
acceleration region can trigger the streaming instability able to amplify
magnetic disturbance and to reduce the particle diffusion. When this happen,
CRs are trapped in the near source region for a time longer than expected and
an extended gamma-ray halo is produces around each source. Here we calculate
the contribution to the diffuse gamma-ray background due to the overlap along
lines of sight of several of these extended halos. We find that if the density
of neutrals is low, the halos can account for a substantial fraction of the
diffuse emission observed by Fermi-LAT, depending on the orientation of the
line of sight with respect to the direction of the galactic center.Comment: 8 pages, 2 figs. Proceeding the 35th International Cosmic Ray
Conference (ICRC2017), Bexco, Busan, Kore
Broad Balmer line emission and cosmic ray acceleration efficiency in supernova remnant shocks
Balmer emission may be a powerful diagnostic tool to test the paradigm of
cosmic ray (CR) acceleration in young supernova remnant (SNR) shocks. The width
of the broad Balmer line is a direct indicator of the downstream plasma
temperature. In case of efficient particle acceleration an appreciable fraction
of the total kinetic energy of the plasma is channeled into CRs, therefore the
downstream temperature decreases and so does the broad Balmer line width. This
width also depends on the level of thermal equilibration between ions and
neutral hydrogen atoms in the downstream. Since in general in young SNR shocks
only a few charge exchange (CE) reactions occur before ionization,
equilibration between ions and neutrals is not reached, and a kinetic
description of the neutrals is required in order to properly compute Balmer
emission.
We provide a method for the calculation of Balmer emission using a
self-consistent description of the shock structure in the presence of neutrals
and CRs. We use a recently developed semi-analytical approach, where neutral
particles, ionized plasma, accelerated particles and magnetic fields are all
coupled together through the mass, momentum and energy flux conservation
equations. The distribution of neutrals is obtained from the full Boltzmann
equation in velocity space, coupled to Maxwellian ions through ionization and
CE processes. The computation is also improved with respect to previous work
thanks to a better approximation for the atomic interaction rates. We find that
for shock speeds >2500km/s the distribution of broad neutrals never approaches
a Maxwellian and its moments differ from those of the ionized component. These
differences reflect into a smaller FWHM than predicted in previous
calculations, where thermalization was assumed. The method presented here
provides a realistic estimate of particle acceleration efficiency in Balmer
dominated shocks.Comment: 6 pages, 3 figures. Accepted for publication in Astronomy &
Astrophysic
Cosmic Ray acceleration and Balmer emission from RCW 86 (G315.4-2.3)
Context. Observation of Balmer lines from the region around the forward shock
of supernova remnants (SNR) may provide valuable information on the shock
dynamics and the efficiency of particle acceleration at the shock.
Aims. We calculated the Balmer line emission and the shape of the broad
Balmer line for parameter values suitable for SNR RCW 86 (G315.4-2.3) as a
function of the cosmic-ray (CR) acceleration efficiency and of the level of
thermal equilibration between electrons and protons behind the shock. This
calculation aims at using the width of the broad Balmer-line emission to infer
the CR acceleration efficiency in this remnant.
Methods. We used the recently developed nonlinear theory of diffusive
shock-acceleration in the presence of neutrals. The semianalytical approach we
developed includes a description of magnetic field amplification as due to
resonant streaming instability, the dynamical reaction of accelerated particles
and the turbulent magnetic field on the shock, and all channels of interaction
between neutral hydrogen atoms and background ions that are relevant for the
shock dynamics.
Results. We derive the CR acceleration efficiency in the SNR RCW 86 from the
Balmer emission. Since our calculation used recent measurements of the shock
proper motion, the results depend on the assumed distance to Earth. For a
distance of 2 kpc the measured width of the broad Balmer line is compatible
with the absence of CR acceleration. For a distance of 2.5 kpc, which is a
widely used value in current literature, a CR acceleration efficiency of 5-30%
is obtained, depending upon the electron-ion equilibration and the ionization
fraction upstream of the shock. By combining information on Balmer emission
with the measured value of the downstream electron temperature, we constrain
the CR acceleration efficiency to be ~20%.Comment: 7 pages, 6 figures. Accepted for publication in A&A (minor changes to
match the published version
Relativistic MHD modeling of magnetized neutron stars, pulsar winds, and their nebulae
Neutron stars are among the most fascinating astrophysical sources, being
characterized by strong gravity, densities about the nuclear one or even above,
and huge magnetic fields. Their observational signatures can be extremely
diverse across the electromagnetic spectrum, ranging from the periodic and
low-frequency signals of radio pulsars, up to the abrupt high-energy gamma-ray
flares of magnetars, where energies of ~10^46 erg are released in a few
seconds. Fast-rotating and highly magnetized neutron stars are expected to
launch powerful relativistic winds, whose interaction with the supernova
remnants gives rise to the non-thermal emission of pulsar wind nebulae, which
are known cosmic accelerators of electrons and positrons up to PeV energies. In
the extreme cases of proto-magnetars (magnetic fields of ~10^15 G and
millisecond periods), a similar mechanism is likely to provide a viable engine
for the still mysterious gamma-ray bursts. The key ingredient in all these
spectacular manifestations of neutron stars is the presence of strong magnetic
fields in their constituent plasma. Here we will present recent updates of a
couple of state-of-the-art numerical investigations by the high-energy
astrophysics group in Arcetri: a comprehensive modeling of the steady-state
axisymmetric structure of rotating magnetized neutron stars in general
relativity, and dynamical 3-D MHD simulations of relativistic pulsar winds and
their associated nebulae.Comment: EPS 44th Conference on Plasma Physics (June 2017, Belfast), paper
accepted for publication on Plasma Physics and Controlled Fusio
Bus speed estimation by neural networks to improve the automatic fleet management
In the urban areas, public transport service interacts with the private mobility. Moreover, on each link
of the urban public transport network, the bus speed is affected by a high variability over time. It depends
on the congestion level and the presence of bus way or no. The scheduling reliability of the public
transport service is crucial to increase attractiveness against private car use. A comparison between a
Radial Basis Function network (RBF) and Multi layer Perceptron (MLP) was carried out to estimate the
average speed, analysing the dynamic bus location data achieved by an AVMS (Automatic Vehicle
Monitoring System). Collected data concern bus location, geometrical parameters and traffic conditions.
Public Transport Company of Palermo provided these data
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