12,515 research outputs found
Superconductivity in Heavy Alkaline-Earths Intercalated Graphites
We report the discovery of superconductivity below 1.65(6) K in
Sr-intercalated graphite SrC6, by susceptibility and specific heat (Cp)
measurements. In comparison with CaC6, we found that the anisotropy of the
upper critical fields for SrC6 is much reduced. The Cp anomaly at Tc is smaller
than the BCS prediction indicating an anisotropic superconducting gap for SrC6
similar to CaC6. The significantly lower Tc of SrC6 as compared to CaC6 can be
understood in terms of "negative" pressure effects, which decreases the
electron-phonon coupling for both in-plane intercalant and the out-of-plane C
phonon modes. We observed no superconductivity for BaC6 down to 0.3 K.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Let
A universal GRB photon energy-peak luminosity relation
The energetics and emission mechanism of GRBs are not well understood. Here
we demonstrate that the instantaneous peak flux or equivalent isotropic peak
luminosity, L_iso ergs s^-1, rather than the integrated fluence or equivalent
isotropic energy, E_iso ergs, underpins the known high-energy correlations.
Using new spectral/temporal parameters calculated for 101 bursts with redshifts
from BATSE, BeppoSAX, HETE-II and Swift we describe a parameter space which
characterises the apparently diverse properties of the prompt emission. We show
that a source frame characteristic-photon-energy/peak luminosity ratio, K_z,
can be constructed which is constant within a factor of 2 for all bursts
whatever their duration, spectrum, luminosity and the instrumentation used to
detect them. The new parameterization embodies the Amati relation but indicates
that some correlation between E_peak and E_iso follows as a direct mathematical
inference from the Band function and that a simple transformation of E_iso to
L_iso yields a universal high energy correlation for GRBs. The existence of K_z
indicates that the mechanism responsible for the prompt emission from all GRBs
is probably predominantly thermal.Comment: Submitted to Ap
Locally Optimally Emitting Clouds and the Origin of Quasar Emission Lines
The similarity of quasar line spectra has been taken as an indication that
the emission line clouds have preferred parameters, suggesting that the
environment is subject to a fine tuning process. We show here that the observed
spectrum is a natural consequence of powerful selection effects. We computed a
large grid of photoionization models covering the widest possible range of
cloud gas density and distance from the central continuum source. For each line
only a narrow range of density and distance from the continuum source results
in maximum reprocessing efficiency, corresponding to ``locally
optimally-emitting clouds'' (LOC). These parameters depend on the ionization
and excitation potentials of the line, and its thermalization density. The mean
QSO line spectrum can be reproduced by simply adding together the full family
of clouds, with an appropriate covering fraction distribution. The observed
quasar spectrum is a natural consequence of the ability of various clouds to
reprocess the underlying continuum, and can arise in a chaotic environment with
no preferred pressure, gas density, or ionization parameter.Comment: 9 pages including 1 ps figure. LaTeX format using aaspp4.st
Time-reversal and super-resolving phase measurements
We demonstrate phase super-resolution in the absence of entangled states. The
key insight is to use the inherent time-reversal symmetry of quantum mechanics:
our theory shows that it is possible to \emph{measure}, as opposed to prepare,
entangled states. Our approach is robust, requiring only photons that exhibit
classical interference: we experimentally demonstrate high-visibility phase
super-resolution with three, four, and six photons using a standard laser and
photon counters. Our six-photon experiment demonstrates the best phase
super-resolution yet reported with high visibility and resolution.Comment: 4 pages, 3 figure
Echo-Mapping of Swift J1753.5-0127
We present two epochs of coordinated X-ray-optical timing observations of the
black hole candidate Swift J1753.5-0127 during its 2005 outburst. The first
epoch in July occurred at outburst peak. Two consecutive nights of observations
using the McDonald Observatory Argos camera with the Rossi X-ray Timing
Explorer show a consistent correlation with an immediate response and an
extended tail lasting ~5s. The properties of the variability and the
correlation are consistent with thermal reprocessing in an accretion disk. The
shortness of the lag suggests a short orbital period consistent with that
recently claimed. The second epoch in August used the VLT FORS2 HIT mode again
in conjunction with RXTE. Again a repeatable correlation is seen between two
independent subsets of the data. In this case, though, the cross-correlation
function has an unusual structure comprising a dip followed by a double-peak.
We suggest that this may be equivalent to the dip plus single peak structure
seen by Kanbach et al. (2001) in XTE J1118+480 and attributed there to
synchrotron emission; a similar structure was seen during later activity of
Swift J1753.5-0127 by Durant et al. (2008).Comment: 7 pages, accepted for publication in Monthly Notices of the Royal
Astronomical Societ
The Impact of Differential Cost Sharing of Non-Steroidal Anti-Inflammatory Agents on the Use and Costs of Analgesic Drugs
OBJECTIVE: To estimate the effect of differential cost sharing (DCS) schemes for non-steroidal anti-inflammatory drugs (NSAIDs) on drug subsidy program and beneficiary expenditures. DATA SOURCES/STUDY SETTING: Monthly aggregate claims data from Pharmacare, the public drug subsidy program for seniors in British Columbia, Canada over the period 1989-11 to 2001-06. STUDY DESIGN: DCS limits insurance reimbursement of a group of therapeutically similar drugs to the cost of the lowest priced drugs, with beneficiaries responsible for costs above the reimbursement limit. Pharmacare introduced two different forms of DCS, generic substitution (GS) and reference pricing (RP), in April 1994 and November 1995, respectively, to the NSAIDs. Under GS, generic and brand versions of the same NSAID are considered interchangeable, whereas under RP different NSAIDs are. We extrapolated average reimbursement per day of NSAID therapy over the months before GS and RP to estimate what expenditures would have been without the policies. These counterfactual predictions were compared to actual values to estimate the impact of the policies; the estimated impacts on reimbursement rates were multiplied by the post-policy volume of NSAIDS dispensed, which appeared unaffected by the policies, to estimate expenditure changes. DATA COLLECTION: The cleaned NSAID claims data, obtained from Pharmacare’s databases, were aggregated by month and by their reimbursement status under the GS and RP policies. PRINCIPAL FINDINGS: After RP, program expenditures declined by 4 million annually, cutting expenditure by half. Most savings accrued from the substitution of low cost NSAIDs for more costly alternatives. About 20% of savings represented expenditures by seniors who elected to pay for partially-reimbursed drugs. GS produced one quarter the savings of RP. CONCLUSIONS: RP of NSAIDs achieved its goal of reducing drug expenditures and was more effective than GS. The effects of RP on patient health and associated health care costs remain to be investigated.Reference pricing; generic substitution; prescription drugs; drug cost containment; NSAIDs.
An Atlas of Computed Equivalent Widths of Quasar Broad Emission Lines
We present graphically the results of several thousand photoionization
calculations of broad emission line clouds in quasars, spanning seven orders of
magnitude in hydrogen ionizing flux and particle density. The equivalent widths
of 42 quasar emission lines are presented as contours in the particle density -
ionizing flux plane for a typical incident continuum shape, solar chemical
abundances, and cloud column density of . Results are
similarly given for a small subset of emission lines for two other column
densities ( and ), five other incident
continuum shapes, and a gas metallicity of 5 \Zsun. These graphs should prove
useful in the analysis of quasar emission line data and in the detailed
modeling of quasar broad emission line regions. The digital results of these
emission line grids and many more are available over the Internet.Comment: 16 pages, LaTeX (AASTeX aaspp4.sty); to appear in the 1997 ApJS: full
contents of the 9 photoionization grids presented in this paper may be found
at http://www.pa.uky.edu/~korista/grids/grids.htm
Swift observations of the 2006 outburst of the recurrent nova RS Ophiuchi: II. 1D hydrodynamical models of wind driven shocks
Following the early Swift X-ray observations of the latest outburst of the
recurrent nova RS Ophiuchi in February 2006 (Paper I), we present new 1D
hydrodynamical models of the system which take into account all three phases of
the remnant evolution. The models suggest a novel way of modelling the system
by treating the outburst as a sudden increase then decrease in wind mass-loss
rate and velocity. The differences between this wind model and previous
Primakoff-type simulations are described. A more complex structure, even in 1D,
is revealed through the presence of both forward and reverse shocks, with a
separating contact discontinuity. The effects of radiative cooling are
investigated and key outburst parameters such as mass-loss rate, ejecta
velocity and mass are varied. The shock velocities as a function of time are
compared to the ones derived in Paper I. We show how the manner in which the
matter is ejected controls the evolution of the shock and that for a
well-cooled remnant, the shock deceleration rate depends on the amount of
energy that is radiated away.Comment: 9 pages, 5 figure
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