13,745 research outputs found
Electron-Positron Pair Production in Space- or Time-Dependent Electric Fields
Treating the production of electron and positron pairs by a strong electric
field from the vacuum as a quantum tunneling process we derive, in
semiclassical approximation, a general expression for the pair production rate
in a -dependent electric field pointing in the -direction. We also
allow for a smoothly varying magnetic field parallel to . The result is
applied to a confined field for , a
semi-confined field for , and a linearly increasing
field . The boundary effects of the confined fields on
pair-production rates are exhibited. A simple variable change in all formulas
leads to results for electric fields depending on time rather than space.
In addition, we discuss tunneling processes in which empty atomic bound
states are spontaneously filled by negative-energy electrons from the vacuum
under positron emission. In particular, we calculate the rate at which the
atomic levels of a bare nucleus of finite size and large
are filled by spontaneous pair creation.Comment: 33 pages and 9 figures. to appear in Phys. Rev.
The Zel'dovich effect and evolution of atomic Rydberg spectra along the Periodic Table
In 1959 Ya. B. Zel'dovich predicted that the bound-state spectrum of the
non-relativistic Coulomb problem distorted at small distances by a short-range
potential undergoes a peculiar reconstruction whenever this potential alone
supports a low-energy scattering resonance. However documented experimental
evidence of this effect has been lacking. Previous theoretical studies of this
phenomenon were confined to the regime where the range of the short-ranged
potential is much smaller than Bohr's radius of the Coulomb field. We go beyond
this limitation by restricting ourselves to highly-excited s states. This
allows us to demonstrate that along the Periodic Table of elements the
Zel'dovich effect manifests itself as systematic periodic variation of the
Rydberg spectra with a period proportional to the cubic root of the atomic
number. This dependence, which is supported by analysis of experimental and
numerical data, has its origin in the binding properties of the ionic core of
the atom.Comment: 17 pages, 12 figure
Enhanced four-wave mixing via elimination of inhomogeneous broadening by coherent driving of quantum transition with control fields
We show that atoms from wide velocity interval can be concurrently involved
in Doppler-free two-photon resonant far from frequency degenerate four-wave
mixing with the aid of auxiliary electromagnetic field. This gives rise to
substantial enhancement of the output radiation generated in optically thick
medium. Numerical illustrations addressed to typical experimental conditions
are given.Comment: LaTeX2e, hyperref, 7 pages, 5 figures, to appear in PRA 1 august 200
The Determination of the `Diffusion Coefficients' and the Stellar Wind Velocities for X-Ray Binaries
The distribution of neutron stars (NS's) is determined by stationary solution
of the Fokker-Planck equation. In this work using the observed period changes
for four systems: Vela X-1, GX 301-2, Her X-1 and Cen X-3 we determined D, the
'diffusion coefficient',-parameter from the Fokker-Planck equation. Using
strong dependence of D on the velocity for Vela X-1 and GX 301-2, systems
accreting from a stellar wind, we determined the stellar wind velocity. For
different assumptions for a turbulent velocity we obtained . It is in good agreement with the stellar wind velocity determined by
other methods. We also determined the specific characteristic time scales for
the 'diffusion processes' in X-ray pulsars. It is of order of 200 sec for
wind-fed pulsars and 1000-10000 sec for the disk accreting systems.Comment: 8 pages, Latex, no figures, accepted for publication to Astronomical
and Astrophysical Transactions (1995). Admin note 20Feb2000: original
(broken) version now paper.tex.orig in source; fixed version with two bad
equations set in verbatim used for PS, paper.tex in sourc
Schwinger Pair Creation of Particles and Strings
I shortly review the worldline instanton method for calculating Schwinger
pair production rates in (i) one-loop QED (ii) multiloop QED and (iii) one-loop
open string theory.Comment: 8 pages, 3 figures, talk given at XIV Mexican School of Particles and
Fields, November 8 - 12, 2010, Morelia, Mexico, to appear in the conference
proceeding
Improved Approximations for Fermion Pair Production in Inhomogeneous Electric Fields
Reformulating the instantons in a complex plane for tunneling or transmitting
states, we calculate the pair-production rate of charged fermions in a
spatially localized electric field, illustrated by the Sauter electric field
E_0 sech^2 (z/L), and in a temporally localized electric field such as E_0
sech^2 (t/T). The integration of the quadratic part of WKB instanton actions
over the frequency and transverse momentum leads to the pair-production rate
obtained by the worldline instanton method, including the prefactor, of Phys.
Rev. D72, 105004 (2005) and D73, 065028 (2006). It is further shown that the
WKB instanton action plus the next-to-leading order contribution in spinor QED
equals the WKB instanton action in scalar QED, thus justifying why the WKB
instanton in scalar QED can work for the pair production of fermions. Finally
we obtain the pair-production rate in a spatially localized electric field
together with a constant magnetic field in the same direction.Comment: RevTex, 12 pages, two figures; replaced by the version accepted in
Phys. Rev.
A young contracting white dwarf in the peculiar binary HD 49798/RX J0648.0--4418?
HD 49798/RX J0648.0--4418 is a peculiar X-ray binary with a hot subdwarf
(sdO) mass donor. The nature of the accreting compact object is not known, but
its spin period ~s and s~s, prove
that it can be only either a white dwarf or a neutron star. The spin-up has
been very stable for more than 20 years. We demonstrate that the continuous
stable spin-up of the compact companion of HD 49798 can be best explained by
contraction of a young white dwarf with an age ~Myrs. This allows us to
interpret all the basic parameters of the system in the framework of an
accreting white dwarf. We present examples of binary evolution which result in
such systems. If correct, this is the first direct evidence for a white dwarf
contraction on early evolutionary stages.Comment: 9 pages, accepted to MNRA
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