5,214 research outputs found
Comptonization and the Spectra of Accretion-Powered X-Ray Pulsars
Accretion-powered X-ray pulsars are among the most luminous X-ray sources in
the Galaxy. However, despite decades of theoretical and observational work
since their discovery, no satisfactory model for the formation of the observed
X-ray spectra has emerged. In this paper, we report on a self-consistent
calculation of the spectrum emerging from a pulsar accretion column that
includes an explicit treatment of the bulk and thermal Comptonization occurring
in the radiation-dominated shocks that form in the accretion flows. Using a
rigorous eigenfunction expansion method, we obtain a closed-form expression for
the Green's function describing the upscattering of monochromatic radiation
injected into the column. The Green's function is convolved with
bremsstrahlung, cyclotron, and blackbody source terms to calculate the emergent
photon spectrum. We show that energization of photons in the shock naturally
produces an X-ray spectrum with a relatively flat continuum and a high-energy
exponential cutoff. Finally, we demonstrate that our model yields good
agreement with the spectra of the bright pulsar Her X-1 and the low luminosity
pulsar X Per.Comment: 6 Pages, 2 Figures, To appear in "The Multicoloured Landscape of
Compact Objects and their Explosive Progenitors" (Cefalu, Sicily, June 2006).
Eds. L. Burderi et al. (New York: AIP
Iron Abundance Diagnostics in High-Redshift QSOs
The abundance of alpha-process elements such as magnesium and carbon relative
to iron measured from the broad emission lines of QSOs can serve as a
diagnostic of the star formation and chemical enrichment histories of their
host galaxies.
We investigate the relationship between Fe/Mg and Fe/C abundance ratios and
the resulting Fe II / Mg II 2800A and Fe II / 1900A-blend flux ratios, both of
which have been measured in QSOs out to redshifts of approximately six. Using a
galactic chemical evolution model based on a starburst in a giant elliptical
galaxy, we find that these flux ratios are good tracers of the chemical
enrichment of the nuclei. However, the values of these ratios measured in
objects at redshifts of approximately six suggest that iron enrichment has
occurred more rapidly in these objects than predicted by the assumed elliptical
starburst model, under currently favored cosmologies.Comment: 2 pages, to appear in proceedings of IAU Symposium No. 222, The
Interplay Among Black Holes, Stars and ISM in Galacti Nucle
Calculating the Fierz Transformation for Higher Orders
We consider the higher-order Fierz transformation, which corresponds to
expanding a product of terms into a sum of products of
Dirac densities and currents. It is shown that the Fierz transformation can be
obtained by solving a large system of linear equations with fractional complex
coefficients, which is practical at least up to fourth power.Comment: 6 pages, 3 table
Application of Inelastic Electron Tunneling to the Study of Adhesion
The problem of devising meaningful and reliable methods for nondestructive evaluation of adhesively bonded joints has been the subject of considerable effort for many years. It remains an important and unsolved problem. The use of conventional NOT methods involving ultrasonics is not entirely satisfactory because the thickness of a bond line is small compared with typical acoustic wavelengths and, perhaps more importantly, because the strength and service life of an adhesive bond is dependent on its chemical as well as mechanical properties. That is, an adhesive bond that is mechanically perfect may fail because of chemical degradation initiated at the adhesive/adhered interface
Band Gap and Edge Engineering via Ferroic Distortion and Anisotropic Strain: The Case of SrTiO
The effects of ferroic distortion and biaxial strain on the band gap and band
edges of SrTiO (STO) are calculated using density functional theory and
many-body perturbation theory. Anisotropic strains are shown to reduce the gap
by breaking degeneracies at the band edges. Ferroic distortions are shown to
widen the gap by allowing new band edge orbital mixings. Compressive biaxial
strains raise band edge energies, while tensile strains lower them. To reduce
the STO gap, one must lower the symmetry from cubic while suppressing ferroic
distortions. Our calculations indicate that for engineered orientation of the
growth direction along [111], the STO gap can be controllably and considerably
reduced at room temperature.Comment: 5 pages, 5 figures. To be published in Phys. Rev. Let
Dynamics of Vacillating Voters
We introduce the vacillating voter model in which each voter consults two
neighbors to decide its state, and changes opinion if it disagrees with either
neighbor. This irresolution leads to a global bias toward zero magnetization.
In spatial dimension d>1, anti-coarsening arises in which the linear dimension
L of minority domains grows as t^{1/(d+1)}. One consequence is that the time to
reach consensus scales exponentially with the number of voters.Comment: 4 pages, 6 figures, 2-column revtex4 forma
Expansion for -Core Percolation
The physics of -core percolation pertains to those systems whose
constituents require a minimum number of connections to each other in order
to participate in any clustering phenomenon. Examples of such a phenomenon
range from orientational ordering in solid ortho-para mixtures to
the onset of rigidity in bar-joint networks to dynamical arrest in
glass-forming liquids. Unlike ordinary () and biconnected ()
percolation, the mean field -core percolation transition is both
continuous and discontinuous, i.e. there is a jump in the order parameter
accompanied with a diverging length scale. To determine whether or not this
hybrid transition survives in finite dimensions, we present a expansion
for -core percolation on the -dimensional hypercubic lattice. We show
that to order the singularity in the order parameter and in the
susceptibility occur at the same value of the occupation probability. This
result suggests that the unusual hybrid nature of the mean field -core
transition survives in high dimensions.Comment: 47 pages, 26 figures, revtex
d0 Ferromagnetic Interface Between Non-magnetic Perovskites
We use computational and experimental methods to study d0 ferromagnetism at a
charge- imbalanced interface between two perovskites. In SrTiO3/KTaO3
superlattice calculations, the charge imbalance introduces holes in the SrTiO3
layer, inducing a d0 ferromagnetic half-metallic 2D electron gas at the
interface oxygen 2p orbitals. The charge imbalance overrides doping by
vacancies at realistic concentrations. Varying the constituent materials shows
ferromagnetism to be a gen- eral property of hole-type d0 perovskite
interfaces. Atomically sharp epitaxial d0 SrTiO3/KTaO3, SrTiO3 /KNbO3 and
SrTiO3 /NaNbO3 interfaces are found to exhibit ferromagnetic hysteresis at room
temperature. We suggest the behavior is due to high density of states and
exchange coupling at the oxygen t1g band in comparison with the more studied d
band t2g symmetry electron gas.Comment: 5 pages, 5 figure
Precise mass-dependent QED contributions to leptonic g-2 at order alpha^2 and alpha^3
Improved values for the two- and three-loop mass-dependent QED contributions
to the anomalous magnetic moments of the electron, muon, and tau lepton are
presented. The Standard Model prediction for the electron (g-2) is compared
with its most precise recent measurement, providing a value of the
fine-structure constant in agreement with a recently published determination.
For the tau lepton, differences with previously published results are found and
discussed. An updated value of the fine-structure constant is presented in
"Note added after publication."Comment: 6 pages, 1 figure. v2: New determination of alpha presented (based on
the recent electron g-2 measurement). v3: New formulae added in Sec.IIB. v4:
Updated value of alpha presente
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