8,297 research outputs found
The galactic gamma-ray distribution: Implications for galactic structure and the radial cosmic ray gradient
The radial distribution of gamma ray emissivity in the Galaxy was derived from flux longitude profiles, using both the final SAS-2 results and the recently corrected COS-B results and analyzing the northern and southern galactic regions separately. The recent CO surveys of the Southern Hemisphere, were used in conjunction with the Northern Hemisphere data, to derive the radial distribution of cosmic rays on both sides of the galactic plane. In addition to the 5 kpc ring, there is evidence from the radial asymmetry for spiral features which are consistent with those derived from the distribution of bright HII regions. Positive evidence was also found for a strong increase in the cosmic ray flux in the inner Galaxy, particularly in the 5 kpc region in both halves of the plane
Pulsar and diffuse contributions to the observed galactic gamma radiation
With the acquisition of satellite data on the energy spectrum of galactic gamma-radiation, it is clear that such radiation has a multicomponent nature. A calculation of the pulsar gamma ray emission spectrum is used together with a statistical analysis of recent data on 328 known pulsars to make a new determination of the pulsar contribution to galactic gamma ray emission. The contributions from diffuse interstellar cosmic ray induced production mechanisms to the total emission are then reexamined. It is concluded that pulsars may account for a significant fraction of galactic gamma ray emission
Pulsar Polar Cap Heating and Surface Thermal X-Ray Emission I. Curvature Radiation Pair Fronts
We investigate the effect of pulsar polar cap (PC) heating produced by
positrons returning from the upper pair formation front. Our calculations are
based on a self-consistent treatment of the pair dynamics and the effect of
electric field screening by the returning positrons. We calculate the resultant
X-ray luminosities, and discuss the dependence of the PC heating efficiencies
on pulsar parameters, such as characteristic spin-down age, spin period, and
surface magnetic field strength. In this study we concentrate on the regime
where the pairs are produced in a magnetic field by curvature photons emitted
by accelerating electrons. Our theoretical results are not in conflict with the
available observational X-ray data and suggest that the effect of PC heating
should significantly contribute to the thermal X-ray fluxes from middle-aged
and old pulsars. The implications for current and future X-ray observations of
pulsars are briefly outlined.Comment: 28 pages, 7 figures, accepted for publication in Ap
Resistance of superconducting nanowires connected to normal metal leads
We study experimentally the low temperature resistance of superconducting
nanowires connected to normal metal reservoirs. We find that a substantial
fraction of the nanowires is resistive, down to the lowest temperature
measured, indicative of an intrinsic boundary resistance due to the
Andreev-conversion of normal current to supercurrent. The results are
successfully analyzed in terms of the kinetic equations for diffusive
superconductors
Key dating features for timber-framed dwellings in Surrey
This article is made available through the Brunel Open Access Publishing Fund. Copyright @ The Vernacular Architecture Group 2013. MORE OpenChoice articles are open access and distributed under the terms of the Creative Commons Attribution License 3.0.The main component of the Surrey Dendrochronology Project is the accurate dating of 177 ‘dwellings’, nearly all by tree-ring analysis. The dates are used to establish date ranges for 52 ‘key features’, which cover many aspects of timber-framing from building type to details of carpentry. It is shown that changes of method and fashion were in many cases surprisingly rapid, almost abrupt in historical terms. Previous dating criteria for timber-framed dwellings in the county have been refined and new criteria introduced. Clusters of change from the 1440s and the 1540s are shown and some possible historical links suggested.The Heritage Lottery Fund, the Domestic Buildings Research Group (Surrey), the Surrey Archaeological Society and the historical societies of Charlwood, Farnham and Nutfield
Production of Neutral Fermion in Linear Magnetic Field through Pauli Interaction
We calculate the production rate of neutral fermions in linear magnetic
fields through the Pauli interaction. It is found that the production rate is
exponentially decreasing function with respect to the inverse of the magnetic
field gradient, which shows the non-perturbative characteristics analogous to
the Schwinger process. It turns out that the production rate density depends on
both the gradient and the strength of magnetic fields in 3+1 dimension. It is
quite different from the result in 2+1 dimension, where the production rate
depends only on the gradient of the magnetic fields, not on the strength of the
magnetic fields. It is also found that the production of neutral fermions
through the Pauli interaction is a magnetic effect whereas the production of
charged particles through minimal coupling is an electric effect.Comment: 11 pages, 2 figure
Altitude Limits for Rotating Vector Model Fitting of Pulsar Polarization
Traditional pulsar polarization sweep analysis starts from the point dipole
rotating vector model (RVM) approximation. If augmented by a measurement of the
sweep phase shift, one obtains an estimate of the emission altitude
(Blaskiewicz, Cordes, & Wasserman). However, a more realistic treatment of
field line sweepback and finite altitude effects shows that this estimate
breaks down at modest altitude ~ 0.1R_{LC}. Such radio emission altitudes turn
out to be relevant to the young energetic and millisecond pulsars that dominate
the \gamma-ray population. We quantify the breakdown height as a function of
viewing geometry and provide simple fitting formulae that allow observers to
correct RVM-based height estimates, preserving reasonable accuracy to R ~
0.3R_{LC}. We discuss briefly other observables that can check and improve
height estimates
Full polar cap cascade scenario: -ray and X-ray luminosities from spin-powered pulsars
We modify polar cap cascade picture to include the ICS of the higher
generation pairs. In such a ``full-cascade'' scenario, not only the
perpendicular portion of the energy of the pairs goes to high energy radiation
via SR, but the parallel portion of the energy of the pairs can also contribute
to high energy emission via ICS with the soft thermal photons from either the
full neutron star surface or the hot polar cap. An important output of such a
scenario is that the soft tail of the ICS spectrum can naturally result in a
non-thermal X-ray component which can contribute to the luminosities observed
by ROSAT and ASCA. Here we present an analytic description of such a full polar
cap cascade scenario within the framework of Harding & Muslimov acceleration
model. We present the theoretical predictions of the -ray luminosities,
the thermal and non-thermal X-ray luminosities for the known spin-powered X-ray
pulsars. Our results show that the observed different dependences of the high
energy luminosities on the pulsar spin-down luminosities, i.e., and , are well
reproduced. Our model predicts that the {\em pulsed} soft X-rays in the ROSAT
band from most of the millisecond pulsars might be of thermal origin if there
is no strong multipole field components near the surfaces of these pulsars.Comment: 23 pages, emulateapj style, final version to appear in the
Astrophysical Journa
Comment on ``Cosmological Gamma Ray Bursts and the Highest Energy Cosmic Rays''
In a letter with the above title, published some time ago in PRL, Waxman made
the interesting suggestion that cosmological gamma ray bursts (GRBs) are the
source of the ultra high energy cosmic rays (UHECR). This has also been
proposed independently by Milgrom and Usov and by Vietri. However, recent
observations of GRBs and their afterglows and in particular recent data from
the Akeno Great Air Shwoer Array (AGASA) on UHECR rule out extragalactic GRBs
as the source of UHECR.Comment: Comment on a letter with the above title published by E. Waxman in
PRL 75, 386 (1995). Submitted for publication in PRL/Comment
The implications of climate change scenario selection for future streamflow projection in the Upper Colorado River Basin
The impact of projected 21st century climate conditions on streamflow in the Upper Colorado River Basin was estimated using a multi-model ensemble approach wherein the downscaled outputs of 112 future climate projections from 16 global climate models (GCMs) were used to drive a macroscale hydrology model. By the middle of the century, the impacts on streamflow range, over the entire ensemble, from a decrease of approximately 30% to an increase of approximately the same magnitude. Although prior studies and associated media coverage have focused heavily on the likelihood of a drier future for the Colorado River Basin, approximately 25 to 35% of the ensemble of runs, by 2099 and 2039, respectively, result in no change or increases in streamflow. The broad range of projected impacts is primarily the result of uncertainty in projections of future precipitation, and a relatively small part of the variability of precipitation across the projections can be attributed to the effect of emissions pathways. The simulated evolution of future temperature is strongly influenced by emissions, but temperature has a smaller influence than precipitation on flow. Period change statistics (i.e., the change in flow from one 30-yr period to another) vary as much within a model ensemble as between models and emissions pathways. Even by the end of the current century, the variability across the projections is much greater than changes in the ensemble mean. The relatively large ensemble analysis described herein provides perspective on earlier studies that have used fewer scenarios, and suggests that impact analyses relying on one or a few climate scenarios are unacceptably influenced by the choice of projections
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