48,668 research outputs found
X-ray Variability and Period Determinations in the Eclipsing Polar DP Leo
An analysis of ROSAT observations for the eclipsing magnetic cataclysmic
binary DP Leo provides constraints on the origin, size, temperature,
variability and structure of the soft X-ray emission region on the surface of
the white dwarf. These data, when combined with prior observations, show a
progression of approximately 2 degrees per year in the impact position of the
accretion stream onto the white dwarf. One explanation for the observed drift
in stream position is that a magnetic activity cycle on the secondary produces
orbital period oscillations. These oscillations result in an orbital period
which cycles above and below the rotational period of the nearly synchronous
white dwarf. The accretion stream and X-ray emission regions are modeled to fit
the observational data. A distance to the system is also calculated. [An
erroneous value for the cyclotron luminosity, included in an earlier paper
version of the preprint, is corrected here.]Comment: uuencoded PostScript file (25 pages) + 8 figures available by
anonymous ftp to ftp.astro.psu.edu (in the directory /pub/robinson), to
appear in ApJ, PSU preprint 1994-1
Positivity and strong ellipticity
We consider second-order partial differential operators in divergence
form on \Ri^d with a positive-semidefinite, symmetric, matrix of real
-coefficients and establish that is strongly elliptic if and only
if the associated semigroup kernel satisfies local lower bounds, or, if and
only if the kernel satisfies Gaussian upper and lower bounds.Comment: 9 page
A procedure for assessing aircraft turbulence- penetration performance
Subsonic transport aircraft performance assessment during atmospheric turbulenc
Comparative analysis of techniques for evaluating the effectiveness of aircraft computing systems
Performability analysis is a technique developed for evaluating the effectiveness of fault-tolerant computing systems in multiphase missions. Performability was evaluated for its accuracy, practical usefulness, and relative cost. The evaluation was performed by applying performability and the fault tree method to a set of sample problems ranging from simple to moderately complex. The problems involved as many as five outcomes, two to five mission phases, permanent faults, and some functional dependencies. Transient faults and software errors were not considered. A different analyst was responsible for each technique. Significantly more time and effort were required to learn performability analysis than the fault tree method. Performability is inherently as accurate as fault tree analysis. For the sample problems, fault trees were more practical and less time consuming to apply, while performability required less ingenuity and was more checkable. Performability offers some advantages for evaluating very complex problems
Second-order operators with degenerate coefficients
We consider properties of second-order operators on \Ri^d with bounded real symmetric
measurable coefficients. We assume that almost
everywhere, but allow for the possibility that is singular. We associate
with a canonical self-adjoint viscosity operator and examine
properties of the viscosity semigroup generated by . The
semigroup extends to a positive contraction semigroup on the -spaces with
. We establish that it conserves probability, satisfies
~off-diagonal bounds and that the wave equation associated with has
finite speed of propagation. Nevertheless is not always strictly
positive because separation of the system can occur even for subelliptic
operators. This demonstrates that subelliptic semigroups are not ergodic in
general and their kernels are neither strictly positive nor H\"older
continuous. In particular one can construct examples for which both upper and
lower Gaussian bounds fail even with coefficients in C^{2-\varepsilon}(\Ri^d)
with .Comment: 44 page
Some exact solutions with torsion in 5-D Einstein-Gauss-Bonnet gravity
Exact solutions with torsion in Einstein-Gauss-Bonnet gravity are derived.
These solutions have a cross product structure of two constant curvature
manifolds. The equations of motion give a relation for the coupling constants
of the theory in order to have solutions with nontrivial torsion. This relation
is not the Chern-Simons combination. One of the solutions has a structure and is so the purely gravitational analogue of the
Bertotti-Robinson space-time where the torsion can be seen as the dual of the
covariantly constant electromagnetic field.Comment: 19 pages, LaTex, no figures. References added, notation clarified.
Accepted for publication on Physical Review
Critical current of a Josephson junction containing a conical magnet
We calculate the critical current of a
superconductor/ferromagnetic/superconductor (S/FM/S) Josephson junction in
which the FM layer has a conical magnetic structure composed of an in-plane
rotating antiferromagnetic phase and an out-of-plane ferromagnetic component.
In view of the realistic electronic properties and magnetic structures that can
be formed when conical magnets such as Ho are grown with a polycrystalline
structure in thin-film form by methods such as direct current sputtering and
evaporation, we have modeled this situation in the dirty limit with a large
magnetic coherence length (). This means that the electron mean free
path is much smaller than the normalized spiral length which in
turn is much smaller than (with as the length a complete
spiral makes along the growth direction of the FM). In this physically
reasonable limit we have employed the linearized Usadel equations: we find that
the triplet correlations are short ranged and manifested in the critical
current as a rapid oscillation on the scale of . These rapid
oscillations in the critical current are superimposed on a slower oscillation
which is related to the singlet correlations. Both oscillations decay on the
scale of . We derive an analytical solution and also describe a
computational method for obtaining the critical current as a function of the
conical magnetic layer thickness.Comment: Extended version of the published paper. Additional information about
the computational method is included in the appendi
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