372 research outputs found
Acceleration Rates and Injection Efficiencies in Oblique Shocks
The rate at which particles are accelerated by the first-order Fermi
mechanism in shocks depends on the angle, \teq{\Tbone}, that the upstream
magnetic field makes with the shock normal. The greater the obliquity the
greater the rate, and in quasi-perpendicular shocks rates can be hundreds of
times higher than those seen in parallel shocks. In many circumstances
pertaining to evolving shocks (\eg, supernova blast waves and interplanetary
traveling shocks), high acceleration rates imply high maximum particle energies
and obliquity effects may have important astrophysical consequences. However,
as is demonstrated here, the efficiency for injecting thermal particles into
the acceleration mechanism also depends strongly on obliquity and, in general,
varies inversely with \teq{\Tbone}. The degree of turbulence and the resulting
cross-field diffusion strongly influences both injection efficiency and
acceleration rates. The test particle \mc simulation of shock acceleration used
here assumes large-angle scattering, computes particle orbits exactly in
shocked, laminar, non-relativistic flows, and calculates the injection
efficiency as a function of obliquity, Mach number, and degree of turbulence.
We find that turbulence must be quite strong for high Mach number, highly
oblique shocks to inject significant numbers of thermal particles and that only
modest gains in acceleration rates can be expected for strong oblique shocks
over parallel ones if the only source of seed particles is the thermal
background.Comment: 24 pages including 6 encapsulated figures, as a compressed,
uuencoded, Postscript file. Accepted for publication in the Astrophysical
Journa
Local and Global Superconductivity in Bismuth
We performed magnetization M(H,T) and magnetoresistance R(T,H) measurements
on powdered (grain size ~ 149 micrometers) as well as highly oriented
rhombohedral (A7) bismuth (Bi) samples consisting of single crystalline blocks
of size ~ 1x1 mm2 in the plane perpendicular to the trigonal c-axis. The
obtained results revealed the occurrence of (1) local superconductivity in
powdered samples with Tc(0) = 8.75 \pm 0.05 K, and (2) global superconductivity
at Tc(0) = 7.3 \pm 0.1 K in polycrystalline Bi triggered by low-resistance
Ohmic contacts with silver (Ag) normal metal. The results provide evidence that
the superconductivity in Bi is localized in a tiny volume fraction, probably at
intergrain or Ag/Bi interfaces. On the other hand, the occurrence of global
superconductivity observed for polycrystalline Bi can be accounted for by
enhancement of the superconducting order parameter phase stiffness induced by
the normal metal contacts, the scenario proposed in the context of "pseudogap
regime" in cuprates [E. Berg et al., PRB 78, 094509 (2008)].Comment: 12 pages including 9 figures and 1 table, Special Issue to the 80th
birthday anniversary of V. G. Peschansky, Electronic Properties of Conducting
System
Pair Production Absorption Troughs in Gamma-Ray Burst Spectra: A Potential Distance Discriminator
Relativistic bulk motion with large Lorentz factors has recently been
inferred for gamma-ray bursts regardless of whether they are of galactic or
cosmological origin. This conclusion results from calculations of internal pair
production transparency in bursts that usually assume an infinite power-law
source spectrum for simplicity, an approximation that is quite adequate for
some bursts detected by EGRET. However, for a given bulk Lorentz factor
\teq{\Gamma}, photons above the EGRET range can potentially interact with
sub-MeV photons in such calculations. Hence it is essential to accurately
address the spectral curvature in bursts seen by BATSE. In this paper we
present the major properties induced in photon-photon opacity considerations by
such spectral curvature. The observed spectral breaks around 1 MeV turn out to
be irrelevant to opacity in cosmological bursts, but are crucial to estimates
of source transparency in the 1 GeV -- 1 TeV range for sources located in the
galactic halo. We find that broad absorption troughs can arise at these
energies for suitable bulk motion parameters \teq{\Gamma}. Such troughs are
probably an unambiguous signature of a galactic halo population, and if
observed by experiments such as Whipple, MILAGRO and GLAST, would provide
powerful evidence that such bursts are not at cosmological distances.Comment: 10 pages, AASTeX format, including 2 eps figures, ApJLett in pres
A retrospective review and multi-specialty, evidence-based guideline for the management of necrotising otitis externa
The Escape of High-Energy Photons from Gamma-Ray Bursts
Eleven bright gamma-ray bursts (GRBs) detected by BATSE have also been seen
at much higher energies by EGRET, six at energies above 10 MeV. Such
observations imply that these bursts are optically thin to photon-photon pair
production at all observed energies. For bursts more than about 30pc away,
internal transparency can be achieved only if the source is moving with a
relativistic bulk Lorentz factor , or if the radiation is highly
beamed. Early calculations of considerations for GRBs
were limited to cases of a beam with opening half-angle \Thetab\sim 1/\Gamma,
or expansions of infinitely thin spherical shells. This paper presents our
extension of pair production optical depth calculations in relativistically
expanding sources to more general geometries, including shells of finite
thickness and arbitrary opening angle. The problem is reduced analytically to a
single integral in the broadly applicable case of observing photons along the
axis of the expansion. We find that the minimum bulk Lorentz factor for the
EGRET sources to be optically thin is only moderately dependent on the shell
thickness and virtually independent of its opening solid angle if
\Thetab\gtrsim 1/\Gamma. This insensitivity to \Thetab relieves the
commonly-perceived number problem for non-repeating sources at cosmological
distances, i.e. it is not necessary to invoke small \Thetab to effect photon
escape.Comment: 51 pages, including 8 eps figures, to appear in ApJ, December 20 199
Acceleration of Solar Wind Ions by Nearby Interplanetary Shocks: Comparison of Monte Carlo Simulations with Ulysses Observations
The most stringent test of theoretical models of the first-order Fermi
mechanism at collisionless astrophysical shocks is a comparison of the
theoretical predictions with observational data on particle populations. Such
comparisons have yielded good agreement between observations at the
quasi-parallel portion of the Earth's bow shock and three theoretical
approaches, including Monte Carlo kinetic simulations. This paper extends such
model testing to the realm of oblique interplanetary shocks: here observations
of proton and alpha particle distributions made by the SWICS ion mass
spectrometer on Ulysses at nearby interplanetary shocks are compared with test
particle Monte Carlo simulation predictions of accelerated populations. The
plasma parameters used in the simulation are obtained from measurements of
solar wind particles and the magnetic field upstream of individual shocks. Good
agreement between downstream spectral measurements and the simulation
predictions are obtained for two shocks by allowing the the ratio of the
mean-free scattering length to the ionic gyroradius, to vary in an optimization
of the fit to the data. Generally small values of this ratio are obtained,
corresponding to the case of strong scattering. The acceleration process
appears to be roughly independent of the mass or charge of the species.Comment: 26 pages, 6 figures, AASTeX format, to appear in the Astrophysical
Journal, February 20, 199
Prompt high-energy emission from gamma-ray bursts in the internal shock model
The prompt GRB emission is thought to arise from electrons accelerated in
internal shocks propagating within a highly relativistic outflow. The launch of
Fermi offers the prospect of observations with unprecedented sensitivity in
high-energy (>100 MeV) gamma-rays. The aim is to explore the predictions for HE
emission from internal shocks, taking into account both dynamical and radiative
aspects, and to deduce how HE observations constrain the properties of the
relativistic outflow. The emission is modeled by combining a time-dependent
radiative code with a dynamical code giving the evolution of the physical
conditions in the shocked regions.Synthetic lightcurves and spectra are
compared to observations. The HE emission deviates significantly from
analytical estimates, which tend to overpredict the IC component, when the time
dependence and full cross-sections are included. The exploration of the
parameter space favors the case where the dominant process in the BATSE range
is synchrotron emission. The HE component becomes stronger for weaker magnetic
fields. The HE lightcurve can display a prolonged pulse duration due to IC
emission, or even a delayed peak compared to the BATSE range.Alternatively,
having dominant IC emission in the BATSE range requires most electrons to be
accelerated into a steep power-law distribution and implies strong 2nd order IC
scattering. In this case, the BATSE and HE lightcurves are very similar. The
combined dynamical and radiative approach allows a firm appraisal of GRB HE
prompt emission. A diagnostic procedure is presented to identify from
observations the dominant emission process and derive constrains on the bulk
Lorentz factor, particle density and magnetic field of the outflow.Comment: 28 pages, 20 figures, accepted for publication in A&
On The Origin of Very High Energy Cosmic Rays
We discuss the most recent developments in our understanding of the
acceleration and propagation of cosmic rays up to the highest energies. In
particular we specialize our discussion to three issues: 1) developments in the
theory of particle acceleration at shock waves; 2) the transition from galactic
to extragalactic cosmic rays; 3) implications of up-to-date observations for
the origin of ultra high energy cosmic rays (UHECRs).Comment: Invited Review Article to appear in Modern Physics Letters A, Review
Sectio
Charged-Particle Motion in Electromagnetic Fields Having at Least One Ignorable Spatial Coordinate
We give a rigorous derivation of a theorem showing that charged particles in
an arbitrary electromagnetic field with at least one ignorable spatial
coordinate remain forever tied to a given magnetic-field line. Such a situation
contrasts the significant motions normal to the magnetic field that are
expected in most real three-dimensional systems. It is pointed out that, while
the significance of the theorem has not been widely appreciated, it has
important consequences for a number of problems and is of particular relevance
for the acceleration of cosmic rays by shocks.Comment: 7 pages, emulateapj format, including 1 eps figure, to appear in The
Astrophysical Journal, Dec. 10 1998 issu
Polarization Evolution in Strong Magnetic Fields
Extremely strong magnetic fields change the vacuum index of refraction.
Although this polarization dependent effect is small for typical neutron stars,
it is large enough to decouple the polarization states of photons traveling
within the field. The photon states evolve adiabatically and follow the
changing magnetic field direction. The combination of a rotating magnetosphere
and a frequency dependent state decoupling predicts polarization phase lags
between different wave bands, if the emission process takes place well within
the light cylinder. This QED effect may allow observations to distinguish
between different pulsar emission mechanisms and to reconstruct the structure
of the magnetosphere.Comment: 22 pages, 10 figures, accepted for publication in MNRA
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