9,183 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
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
Evidence for Cooper Pair Diffraction on the Vortex Lattice of Superconducting Niobium
We investigated the Abrikosov vortex lattice (VL) of a pure Niobium single
crystal with the muon spin rotation (\mu SR) technique. Analysis of the \mu SR
data in the framework of the BCS-Gor'kov theory allowed us to determine
microscopic parameters and the limitations of the theory. With decreasing
temperature the field variation around the vortex cores deviates substantially
from the predictions of the Ginzburg-Landau theory and adopts a pronounced
conical shape. This is evidence of partial diffraction of Cooper pairs on the
VL predicted by Delrieu for clean superconductors.Comment: 9 pages, 6 figure
High pressure magnetic state of MnP probed by means of muon-spin rotation
We report a detailed SR study of the pressure evolution of the magnetic
order in the manganese based pnictide MnP, which has been recently found to
undergo a superconducting transition under pressure once the magnetic ground
state is suppressed. Using the muon as a volume sensitive local magnetic probe,
we identify a ferromagnetic state as well as two incommensurate helical states
(with propagation vectors aligned along the crystallographic and
directions, respectively) which transform into each other through first
order phase transitions as a function of pressure and temperature. Our data
appear to support that the magnetic state from which superconductivity develops
at higher pressures is an incommensurate helical phase.Comment: 11 pages, 9 figure
Superconducting and magnetic properties of Sr3Ir4Sn13
Magnetization and muon spin relaxation or rotation (muSR) measurements have
been performed to study the superconducting and magnetic properties of
Sr3Ir4Sn13. From magnetization measurements the lower and upper critical fields
of Sr3Ir4Sn13 are found to be 81(1) Oe and 14.4(2) kOe, respectively.
Zero-field muSR data show no sign of any magnetic ordering or weak magnetism in
Sr3Ir4Sn13. Transverse-field muSR measurements in the vortex state provided the
temperature dependence of the magnetic penetration depth. The dependence of
penetration depth with temperature is consistent with the existence of single
s-wave energy gap in the superconducting state of Sr3Ir4Sn13 with a gap value
of 0.82(2) meV at absolute zero temperature. The magnetic penetration depth at
zero temperature is 291(3) nm. The gap to Tc ratio is 2.1(1), indicates that
Sr3Ir4Sn13 should be considered as a strong-coupling superconductor.Comment: 6 pages, 5 figure
Evidence for time-reversal symmetry breaking in superconducting PrPt4Ge12
Zero and longitudinal field muon spin rotation (muSR) experiments were
performed on the superconductors PrPt4Ge12 and LaPt4Ge12. In PrPt4Ge12 below Tc
a spontaneous magnetization with a temperature variation resembling that of the
superfluid density appears. This observation implies time-reversal symmetry
(TRS) breaking in PrPt4Ge12 below Tc = 7.9 K. This remarkably high Tc for an
anomalous superconductor and the weak and gradual change of Tc and of the
related specific heat anomaly upon La substitution in La_(1-x)Pr_xPt_4Ge_(12)
suggests that the TRS breaking is due to orbital degrees of freedom of the
Cooper pairs.Comment: To appear in Phys. Rev. B. 5 pages, 3 figure
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