20,260 research outputs found
The C-metric as a colliding plane wave space-time
It is explicitly shown that part of the C-metric space-time inside the black
hole horizon may be interpreted as the interaction region of two colliding
plane waves with aligned linear polarization, provided the rotational
coordinate is replaced by a linear one. This is a one-parameter generalization
of the degenerate Ferrari-Ibanez solution in which the focussing singularity is
a Cauchy horizon rather than a curvature singularity.Comment: 6 pages. To appear in Classical and Quantum Gravit
Insurance for autonomous underwater vehicles
The background and practice of insurance for autonomous underwater vehicles (AUVs) are examined. Key topics include: relationships between clients, brokers and underwriters; contract wording to provide appropriate coverage; and actions to take when an incident occurs. Factors that affect cost of insurance are discussed, including level of autonomy, team experience and operating environment. Four case studies from industry and academia illustrate how AUV insurance has worked in practice. The paper concludes by stressing the importance of effective dialogue between client, broker and underwriter to review, assess and reduce risk to the benefit of all parties
Solving the characteristic initial value problem for colliding plane gravitational and electromagnetic waves
A method is presented for solving the characteristic initial value problem
for the collision and subsequent nonlinear interaction of plane gravitational
or gravitational and electromagnetic waves in a Minkowski background. This
method generalizes the monodromy transform approach to fields with nonanalytic
behaviour on the characteristics inherent to waves with distinct wave fronts.
The crux of the method is in a reformulation of the main nonlinear symmetry
reduced field equations as linear integral equations whose solutions are
determined by generalized (``dynamical'') monodromy data which evolve from data
specified on the initial characteristics (the wavefronts).Comment: 4 pages, RevTe
Collision of plane gravitational and electromagnetic waves in a Minkowski background: solution of the characteristic initial value problem
We consider the collisions of plane gravitational and electromagnetic waves
with distinct wavefronts and of arbitrary polarizations in a Minkowski
background. We first present a new, completely geometric formulation of the
characteristic initial value problem for solutions in the wave interaction
region for which initial data are those associated with the approaching waves.
We present also a general approach to the solution of this problem which
enables us in principle to construct solutions in terms of the specified
initial data. This is achieved by re-formulating the nonlinear dynamical
equations for waves in terms of an associated linear problem on the spectral
plane. A system of linear integral ``evolution'' equations which solve this
spectral problem for specified initial data is constructed. It is then
demonstrated explicitly how various colliding plane wave space-times can be
constructed from given characteristic initial data.Comment: 33 pages, 3 figures, LaTeX. Accepted for publication in Classical and
Quantum Gravit
Complex X-ray spectral variability in Mkn 421 observed with XMM-Newton
The bright blazar Mkn 421 has been observed four times for uninterrupted
durations of ~ 9 - 13 hr during the performance verification and calibration
phases of the XMM-Newton mission. The source was strongly variable in all
epochs, with variability amplitudes that generally increased to higher energy
bands. Although the detailed relationship between soft (0.1 - 0.75 keV) and
hard (2 - 10 keV) band differed from one epoch to the next, in no case was
there any evidence for a measurable interband lag, with robust upper limits of
hr in the best-correlated light curves. This is in conflict
with previous claims of both hard and soft lags of ~1 hr in this and other
blazars. However, previous observations suffered a repeated 1.6 hr feature
induced by the low-Earth orbital period, a feature that is not present in the
uninterrupted XMM-Newton data. The new upper limit on leads to a lower
limit on the magnetic field strength and Doppler factor of B \delta^{1/3} \gs
4.7 G, mildly out of line with the predictions from a variety of homogeneous
synchrotron self-Compton emission models in the literature of G. Time-dependent spectral fitting was performed on all epochs,
and no detectable spectral hysteresis was seen. We note however that the source
exhibited significantly different spectral evolutionary behavior from one epoch
to the next, with the strongest correlations in the first and last and an
actual divergance between soft and hard X-ray bands in the third. This
indicates that the range of spectral variability behavior in Mkn 421 is not
fully described in these short snippets; significantly longer uninterrupted
light curves are required, and can be obtained with XMM-Newton.Comment: 21 pages, 4 figures, accepted for ApJ, scheduled for August 1, 200
Economic evaluation of a nursing-led intermediate care unit
Objectives: The aim of this paper is to examine the costs of introducing a nursing-led ward program together with examining the impact this may have on patients' outcomes. Methods; The study had a sample size of 177 patients with a mean age of 77, and randomized to either a treatment group (care on a nursing-led ward, n = 97) or a control group (standard care usually on a consultant-led acute ward, n = 80). Resource use data including length of stay, tests and investigations performed, and multidisciplinary involvement in care were collected. Results: There were no significant differences in outcome between the two groups. The inpatient costs for the treatment group were significantly higher, due to the longer length of stay in this group. However, the postdischarge costs were significantly lower for the treatment group. Conclusions: The provision of nursing-led intermediate care units has been proposed as a solution to inappropriate use of acute medical wards by patients who require additional nursing rather than medical care. Whether the treatment group is ultimately cost-additive is dependent on how long reductions in postdischarge resource use are maintained
Correlation inequalities for noninteracting Bose gases
For a noninteracting Bose gas with a fixed one-body Hamiltonian H^0
independent of the number of particles we derive the inequalities _N <
_{N+1}, _N _N _N for i\neq j, \partial
_N/\partial \beta >0 and ^+_N _N. Here N_i is the occupation
number of the ith eigenstate of H^0, \beta is the inverse temperature and the
superscript + refers to adding an extra level to those of H^0. The results
follow from the convexity of the N-particle free energy as a function of N.Comment: a further inequality adde
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