12,017 research outputs found
Escape of cosmic rays from the Galaxy and effects on the circumgalactic medium
The escape of cosmic rays from the Galaxy is expected to shape their spectrum
inside the Galaxy. Yet, this phenomenon is very poorly understood and, in the
absence of a physical description, it is usually modelled as free escape from a
given boundary, typically located at a few kpc distance from the Galactic disc.
We show that the assumption of free escape leads to the conclusion that the
cosmic ray current propagating in the circumgalactic medium is responsible for
a non resonant cosmic ray induced instability that in turn leads to the
generation of a magnetic field of strength Gauss on a
scale kpc around our Galaxy. The self-generated diffusion produces
large gradients in the particle pressure that induce a displacement of the
intergalactic medium with velocity km/s. Cosmic rays are then
carried away by advection. If the overdensity of the intergalactic gas in a
region of size kpc around our Galaxy is with respect to
the cosmological baryon density , then the flux of high
energy neutrinos as due to pion production becomes comparable with the flux of
astrophysical neutrinos recently measured by IceCube.Comment: Accepted for publication in Phys. Rev. Letter
High energy cosmic ray self-confinement close to extragalactic sources
The ultra-high energy cosmic rays observed at the Earth are most likely
accelerated in extra-galactic sources. For the typical luminosities invoked for
such sources, the electric current associated to the flux of cosmic rays that
leave them is large. The associated plasma instabilities create magnetic
fluctuations that can efficiently scatter particles. We argue that this
phenomenon forces cosmic rays to be self-confined in the source proximity for
energies , where GeV
for low background magnetic fields (). For larger values of
, cosmic rays are confined close to their sources for energies GeV, where
is the field in units of nG, is its coherence
lengths in units of 10 Mpc and is the source luminosity in units of
erg/s.Comment: To Appear in Physical Review Letter
Non-linear diffusive shock acceleration with free escape boundary
We present here a semi-analytical solution of the problem of particle
acceleration at non-linear shock waves with a free escape boundary at some
location upstream. This solution, besides allowing us to determine the spectrum
of particles accelerated at the shock front, including the shape of the cutoff
at some maximum momentum, also allows us to determine the spectrum of particles
escaping the system from upstream. This latter aspect of the problem is crucial
for establishing a connection between the accelerated particles in
astrophysical sources, such as supernova remnants, and the cosmic rays observed
at the Earth. An excellent approximate solution, which leads to a
computationally fast calculation of the structure of shocks with an arbitrary
level of cosmic ray modification, is also obtained.Comment: 11 pages, 2 figures, Accepted for publication in APh
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
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
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
On the escape of particles from cosmic ray modified shocks
Stationary solutions to the problem of particle acceleration at shock waves
in the non-linear regime, when the dynamical reaction of the accelerated
particles on the shock cannot be neglected, are known to show a prominent
energy flux escaping from the shock towards upstream infinity. On physical
grounds, the escape of particles from the upstream region of a shock has to be
expected in all those situations in which the maximum momentum of accelerated
particles, , decreases with time, as is the case for the Sedov-Taylor
phase of expansion of a shell Supernova Remnant, when both the shock velocity
and the cosmic ray induced magnetization decrease. In this situation, at each
time , particles with momenta larger than leave the system from
upstream, carrying away a large fraction of the energy if the shock is strongly
modified by the presence of cosmic rays. This phenomenon is of crucial
importance for explaining the cosmic ray spectrum detected at Earth. In this
paper we discuss how this escape flux appears in the different approaches to
non-linear diffusive shock acceleration, and especially in the quasi-stationary
semi-analytical kinetic ones. We apply our calculations to the Sedov-Taylor
phase of a typical supernova remnant, including in a self-consistent way
particle acceleration, magnetic field amplification and the dynamical reaction
on the shock structure of both particles and fields. Within this framework we
calculate the temporal evolution of the maximum energy reached by the
accelerated particles and of the escape flux towards upstream infinity. The
latter quantity is directly related to the cosmic ray spectrum detected at
Earth.Comment: Version accepted for publication in MNRA
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