477 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
Magnetic Photon Splitting: the S-Matrix Formulation in the Landau Representation
Calculations of reaction rates for the third-order QED process of photon
splitting in strong magnetic fields traditionally have employed either the
effective Lagrangian method or variants of Schwinger's proper-time technique.
Recently, Mentzel, Berg and Wunner (1994) presented an alternative derivation
via an S-matrix formulation in the Landau representation. Advantages of such a
formulation include the ability to compute rates near pair resonances above
pair threshold. This paper presents new developments of the Landau
representation formalism as applied to photon splitting, providing significant
advances beyond the work of Mentzel et al. by summing over the spin quantum
numbers of the electron propagators, and analytically integrating over the
component of momentum of the intermediate states that is parallel to field. The
ensuing tractable expressions for the scattering amplitudes are satisfyingly
compact, and of an appearance familiar to S-matrix theory applications. Such
developments can facilitate numerical computations of splitting considerably
both below and above pair threshold. Specializations to two regimes of interest
are obtained, namely the limit of highly supercritical fields and the domain
where photon energies are far inferior to that for the threshold of
single-photon pair creation. In particular, for the first time the
low-frequency amplitudes are simply expressed in terms of the Gamma function,
its integral and its derivatives. In addition, the equivalence of the
asymptotic forms in these two domains to extant results from effective
Lagrangian/proper-time formulations is demonstrated.Comment: 19 pages, 3 figures, REVTeX; accepted for publication in Phys. Rev.
Photon Splitting Cascades in Gamma-Ray Pulsars and the Spectrum of PSR1509-58
Magnetic photon splitting, a QED process that becomes important only in
magnetic fields approaching the quantum critical value, B_cr = 4.41 X 10^13
Gauss, is investigated as a mechanism for attenuation of gamma-rays emitted
near the surface of strongly-magnetized pulsars. We model photon splitting
attenuation and subsequent splitting cascades in gamma-ray pulsars, including
the dipole field and curved spacetime geometry of the neutron star
magnetosphere. We focus specifically on PSR1509-58, which has the highest
surface magnetic field of all the gamma-ray pulsars (B_0 = 3 X 10^13 Gauss). We
find that splitting will not be important for most gamma-ray pulsars, i.e.
those with B_0 <~ 0.2 B_cr, but will be important for gamma-ray pulsars having
B_0 >~ 0.3 B_cr, where the splitting attenuation lengths and escape energies
become comparable to or less than those for pair production. We compute Monte
Carlo spectral models for PSR1509-58. We find that photon splitting, or
combined splitting and pair production, can explain the unusually low cutoff
energy (between 2 and 30 MeV) of PSR1509-58, and that the model cascade
spectra, which display strong polarization, are consistent with the observed
spectral points and upper limits for polar cap emission at a range of magnetic
colatitudes up to ~ 25 degrees.Comment: 39 pages, 14 embedded figures, AASTEX To appear in ApJ, January 20,
199
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Status of Wind-Diesel Applications in Arctic Climates: Preprint
The rising cost of diesel fuel and the environmental regulation for its transportation, use, and storage, combined with the clear impacts of increased arctic temperatures, is driving remote communities to examine alternative methods of providing power. Over the past few years, wind energy has been increasingly used to reduce diesel fuel consumption, providing economic, environmental, and security benefits to the energy supply of communities from Alaska to Antarctica. This summary paper describes the current state of wind-diesel systems, reviews the operation of wind-diesel plants in cold climates, discusses current research activities pertaining to these systems, and addresses their technical and commercial challenges. System architectures, dispatch strategies, and operating experience from a variety of wind-diesel systems in Alaska will be reviewed. Specific focus will also be given to the control of power systems with large amounts of wind generation and the complexities of replacing diesel engine waste heat with excess wind energy, a key factor in assessing power plants for retrofit. A brief overview of steps for assessing the viability of retrofitting diesel power systems with wind technologies will also be provided. Because of the large number of isolated diesel minigrids, the market for adding wind to these systems is substantial, specifically in arctic climates and on islands that rely on diesel-only power generation
Compton Scattering in Ultra-Strong Magnetic Fields: Numerical and Analytical Behavior in the Relativistic Regime
This paper explores the effects of strong magnetic fields on the Compton
scattering of relativistic electrons. Recent studies of upscattering and energy
loss by relativistic electrons that have used the non-relativistic, magnetic
Thomson cross section for resonant scattering or the Klein-Nishina cross
section for non-resonant scattering do not account for the relativistic quantum
effects of strong fields ( G). We have derived a
simplified expression for the exact QED scattering cross section for the
broadly-applicable case where relativistic electrons move along the magnetic
field. To facilitate applications to astrophysical models, we have also
developed compact approximate expressions for both the differential and total
polarization-dependent cross sections, with the latter representing well the
exact total QED cross section even at the high fields believed to be present in
environments near the stellar surfaces of Soft Gamma-Ray Repeaters and
Anomalous X-Ray Pulsars. We find that strong magnetic fields significantly
lower the Compton scattering cross section below and at the resonance, when the
incident photon energy exceeds in the electron rest frame. The cross
section is strongly dependent on the polarization of the final scattered
photon. Below the cyclotron fundamental, mostly photons of perpendicular
polarization are produced in scatterings, a situation that also arises above
this resonance for sub-critical fields. However, an interesting discovery is
that for super-critical fields, a preponderance of photons of parallel
polarization results from scatterings above the cyclotron fundamental. This
characteristic is both a relativistic and magnetic effect not present in the
Thomson or Klein-Nishina limits.Comment: AASTeX format, 31 pages included 7 embedded figures, accepted for
publication in The Astrophysical Journa
Magnetic Photon Splitting: Computations of Proper-time Rates and Spectra
The splitting of photons in the presence of an intense magnetic field has
recently found astrophysical applications in polar cap models of gamma-ray
pulsars and in magnetar scenarios for soft gamma repeaters. Numerical
computation of the polarization-dependent rates of this third order QED process
for arbitrary field strengths and energies below pair creation threshold is
difficult: thus early analyses focused on analytic developments and simpler
asymptotic forms. The recent astrophysical interest spurred the use of the
S-matrix approach by Mentzel, Berg and Wunner to determine splitting rates. In
this paper, we present numerical computations of a full proper-time expression
for the rate of splitting that was obtained by Stoneham, and is exact up to the
pair creation threshold. While the numerical results derived here are in accord
with the earlier asymptotic forms due to Adler, our computed rates still differ
by as much as factors of 3 from the S-matrix re-evaluation of Wilke and Wunner,
reflecting the extreme difficulty of generating accurate S-matrix numerics for
fields below about \teq{4.4\times 10^{13}}Gauss. We find that our proper-time
rates appear very accurate, and exceed Adler's asymptotic specializations
significantly only for photon energies just below pair threshold and for
supercritical fields, but always by less than a factor of around 2.6. We also
provide a useful analytic series expansion for the scattering amplitude valid
at low energies.Comment: 13 pages, AASTeX format, including 3 eps figures, ApJ in pres
Magnetars and pulsars: a missing link
There is growing evidence that soft gamma-ray repeaters (SGRs) and anomalous
X-ray pulsars (AXPs) are isolated neutron stars with superstrong magnetic
fields, i.e., magnetars, marking them a distinguished species from the
conventional species of spindown-powered isolated neutron stars, i.e., radio
pulsars. The current arguments in favor of the magnetar interpretation of
SGR/AXP phenomenology will be outlined, and the two energy sources in
magnetars, i.e. a magnetic dissipation energy and a spindown energy, will be
reviewed. I will then discuss a missing link between magnetars and pulsars,
i.e., lack of the observational evidence of the spindown-powered behaviors in
known magnetars. Some recent theoretical efforts in studying such behaviors
will be reviewed along with some predictions testable in the near future.Comment: Invited talk at the Sixth Pacific Rim Conference on Stellar
Astrophysics, a tribute to Helmut A. Abt, July 11-17, 2002, Xi'an. To appear
in the proceedings (eds. K. S. Cheng, K. C. Leung & T. P. Li
The Broadband Spectrum of Galaxy Clusters
We examine whether nonthermal protons energized during a cluster merger are
simultaneously responsible for the Coma cluster's diffuse radio flux (via
secondary decay) and the departure of its intra-cluster medium (ICM) from a
thermal profile via Coulomb collisions between the quasithermal electrons and
the hadrons. Rather than approximating the influence of nonthermal
proton/thermal electron collisions as extremely rare events which cause an
injection of nonthermal, power-law electrons (the `knock-on' approximation), we
self-consistently solve (to our knowledge, for the first time) the covariant
kinetic equations for the two populations. The electron population resulting
from these collisions is out of equilibrium, yet not a power law, and
importantly displays a higher bremsstrahlung radiative efficiency than a pure
power law. Observations with GLAST will test this model directly.Comment: Accepted for publication in Ap
The magnetar emission in the IR band: the role of magnetospheric currents
There is a general consensus about the fact that the magnetar scenario
provides a convincing explanation for several of the observed properties of the
Anomalous X-ray Pulsars and the Soft Gamma Repeaters. However, the origin of
the emission observed at low energies is still an open issue. We present a
quantitative model for the emission in the optical/infrared band produced by
curvature radiation from magnetospheric charges, and compare results with
current magnetars observations.Comment: 6 Pages, 2 Figures. Astrophysics and Space Science, in press.
Proceedings of the ICREA Workshop on The High-Energy Emission from Pulsars
and their Systems, Sant Cugat, April 12-16 201
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