416 research outputs found
Spectra of accelerated particles at supernova shocks in the presence of neutral hydrogen: the case of Tycho
The presence of neutral hydrogen in the shock proximity changes the structure
of the shock and affects the spectra of particles accelerated through the
first-order Fermi mechanism. This phenomenon has profound implications for the
interpretation of the multifrequency spectra of radiation from supernova
remnants. Neutrals that undergo charge exchange with hot ions downstream of the
shock may result in fast neutrals moving towards the upstream gas, where they
can suffer additional charge exchange or ionisation reactions, thereby
depositing energy and momentum upstream. Here we discuss the implications of
this neutral return flux, which was already predicted in our previous work on
neutral mediated supernova shocks, and show how the spectra of accelerated
particles turn out to be appreciably steeper than , thereby affecting
the gamma ray spectra from supernova remnants in general and from Tycho
specifically. The theory that describes non-linear diffusive shock acceleration
in the presence of neutral hydrogen has been developed in recent years. Here we
use a semi-analytical theory developed in previous work and specialise our
predictions to the case of the Tycho supernova shock, where there is evidence
that the spectrum of the accelerated cosmic rays is steeper than expected from
the traditional theory of diffusive shock acceleration. We show that, if the
fraction of neutral hydrogen in the vicinity of the Tycho supernova shock is,
as suggested by observations, ~70-90, then spectra of accelerated protons
steeper than may be a natural consequence of charge exchange reactions
and the associated neutral return flux. The spectral shape is affected by this
phenomenon for particles with energies below ~100-1000 GeV, for which the
diffusion length is less than or at most comparable to the pathlength of charge
exchange and ionisation upstream of the shock.Comment: 6 pages, 3 figures. Accepted for publication by A&
Cosmic ray penetration in diffuse clouds
Cosmic rays are a fundamental source of ionization for molecular and diffuse
clouds, influencing their chemical, thermal, and dynamical evolution. The
amount of cosmic rays inside a cloud also determines the -ray flux
produced by hadronic collisions between cosmic rays and cloud material. We
study the spectrum of cosmic rays inside and outside of a diffuse cloud, by
solving the stationary transport equation for cosmic rays including diffusion,
advection and energy losses due to ionization of neutral hydrogen atoms. We
found that the cosmic ray spectrum inside a diffuse cloud differs from the one
in the interstellar medium for energies smaller than MeV,
irrespective of the model details. Below , the spectrum is harder
(softer) than that in the interstellar medium if the latter is a power law
with larger (smaller) than .Comment: 5 pages, 4 figures. Published in MNRAS Letters. Minor changes to
match the published versio
Mass-loading of bow shock pulsar wind nebulae
We investigate the dynamics of bow shock nebulae created by pulsars moving
supersonically through a partially ionized interstellar medium. A fraction of
interstellar neutral hydrogen atoms penetrating into the tail region of a
pulsar wind will undergo photo-ionization due to the UV light emitted by the
nebula, with the resulting mass loading dramatically changing the flow dynamics
of the light leptonic pulsar wind. Using a quasi 1-D hydrodynamic model of
relativistic flow we find that if a relatively small density of neutral
hydrogen, as low as cm, penetrate inside the pulsar wind, this
is sufficient to strongly affect the tail flow. Mass loading leads to the fast
expansion of the pulsar wind tail, making the tail flow intrinsically
non-stationary. The shapes predicted for the bow shock nebulae compare well
with observations, both in H and X-rays.Comment: 7 pages, 2 figures. Proceeding to the conference "High Energy
Phenomena in Relativistic Outflow V", La Plata 2015, AAA Workshop Series 8,
201
On the radial distribution of Galactic cosmic rays
The spectrum and morphology of the diffuse Galactic gamma-ray emission
carries valuable information on cosmic ray (CR) propagation. Recent results
obtained by analyzing Fermi-LAT data accumulated over seven years of
observation show a substantial variation of the CR spectrum as a function of
the distance from the Galactic Center. The spatial distribution of the CR
density in the outer Galaxy appears to be weakly dependent upon the
galactocentric distance, as found in previous studies as well, while the
density in the central region of the Galaxy was found to exceed the value
measured in the outer Galaxy. At the same time, Fermi-LAT data suggest a
gradual spectral softening while moving outward from the center of the Galaxy
to its outskirts. These findings represent a challenge for standard
calculations of CR propagation based on assuming a uniform diffusion
coefficient within the Galactic volume. Here we present a model of non-linear
CR propagation in which transport is due to particle scattering and advection
off self-generated turbulence. We find that for a realistic distribution of CR
sources following the spatial distribution of supernova remnants and the space
dependence of the magnetic field on galactocentric distance, both the spatial
profile of CR density and the spectral softening can easily be accounted for.Comment: 6 pages, 3 figures. Accepted for publivation to MNRAS letter
Cosmic ray driven Galactic winds
The escape of cosmic rays from the Galaxy leads to a gradient in the cosmic
ray pressure that acts as a force on the background plasma, in the direction
opposite to the gravitational pull. If this force is large enough to win
against gravity, a wind can be launched that removes gas from the Galaxy,
thereby regulating several physical processes, including star formation. The
dynamics of these cosmic ray driven winds is intrinsically non-linear in that
the spectrum of cosmic rays determines the characteristics of the wind
(velocity, pressure, magnetic field) and in turn the wind dynamics affects the
cosmic ray spectrum. Moreover, the gradient of the cosmic ray distribution
function causes excitation of Alfven waves, that in turn determine the
scattering properties of cosmic rays, namely their diffusive transport. These
effects all feed into each other so that what we see at the Earth is the result
of these non-linear effects. Here we investigate the launch and evolution of
such winds, and we determine the implications for the spectrum of cosmic rays
by solving together the hydrodynamical equations for the wind and the transport
equation for cosmic rays under the action of self-generated diffusion and
advection with the wind and the self-excited Alfven waves.Comment: 14 pages, 15 figures. Accepted for publication to MNRAS main journa
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
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