591 research outputs found
Reducing the standard deviation in multiple-assay experiments where the variation matters but the absolute value does not
You measure the value of a quantity x for a number of systems (cells,
molecules, people, chunks of metal, DNA vectors, etc.). You repeat the whole
set of measures in different occasions or assays, which you try to design as
equal to one another as possible. Despite the effort, you find that the results
are too different from one assay to another. As a consequence, some systems'
averages present standard deviations that are too large to render the results
statistically significant. In this work, we present a novel correction method
of very low mathematical and numerical complexity that can reduce the standard
deviation in your results and increase their statistical significance as long
as two conditions are met: inter-system variations of x matter to you but its
absolute value does not, and the different assays display a similar tendency in
the values of x; in other words, the results corresponding to different assays
present high linear correlation. We demonstrate the improvement that this
method brings about on a real cell biology experiment, but the method can be
applied to any problem that conforms to the described structure and
requirements, in any quantitative scientific field that has to deal with data
subject to uncertainty.Comment: Supplementary material at http://bit.ly/14I718
Study of the use of vapor and vacuum techniques for the development of high strength filamentary materials Summary technical report, 22 May 1965 - 21 May 1966
Vapor deposition of pyrolytic graphite thin films on liquid metal substrates for producing high strength filament
Discovery of periodic dips in the light curve of GX 13+1: the X-ray orbital ephemeris of the source
The bright low-mass X-ray binary (LMXB) GX 13+1 is one of the most peculiar
Galactic binary systems. A periodicity of 24.27 d with a formal statistical
error of 0.03 d was observed in its power spectrum density obtained with RXTE
All Sky Monitor (ASM) data spanning 14 years. Starting from a recent study,
indicating GX 13+1 as a possible dipping source candidate, we systematically
searched for periodic dips in the X-ray light curves of GX 13+1 from 1996 up to
2013 using RXTE/ASM, and MAXI data to determine for the first time the X-ray
orbital ephemeris of GX 13+1. We searched for a periodic signal in the ASM and
MAXI light curves, finding a common periodicity of 24.53 d. We folded the 1.3-5
keV and 5-12.1 keV ASM light curves and the 2-4 and 4-10 keV MAXI light curves
at the period of 24.53 d finding a periodic dip. To refine the value of the
period we used the timing technique dividing the ASM light curve in eight
intervals and the MAXI light curve in two intervals, obtaining four and two dip
arrival times from the ASM and MAXI light curves, respectively. We improved the
X-ray position of GX 13+1 using a recent Chandra observation. The new X-ray
position is discrepant by \sim 7\arcsec from the previous one, while it is
compatible with the infrared and radio counterpart positions. We detected an
X-ray dip, that is totally covered by the Chandra observation, in the light
curve of GX 13+1 and showed, a-posteriori, that it is a periodic dip. We
obtained seven dip arrival times from ASM, MAXI, and Chandra light curves. We
calculated the delays of the detected dip arrival times with respect to the
expected times for a 24.52 d periodicity. Fitting the delays with a linear
function we find that the orbital period and the epoch of reference of GX 13+1
are 24.5274(2) days and 50,086.79(3) MJD, respectively.(Abridged)Comment: 12 pages, including 16 figures. Accepted for publication in A&
Chandra X-ray spectroscopy of a clear dip in GX 13+1
The source GX 13+1 is a persistent, bright Galactic X-ray binary hosting an
accreting neutron star. It shows highly ionized absorption features, with a
blueshift of 400 km s and an outflow-mass rate similar to the
accretion rate. Many other X-ray sources exhibit warm absorption features, and
they all show periodic dipping behavior at the same time. Recently, a dipping
periodicity has also been determined for GX 13+1 using long-term X-ray folded
light-curves, leading to a clear identification of one of such periodic dips in
an archival Chandra observation. We give the first spectral characterization of
the periodic dip of GX 13+1 found in this archival Chandra observation
performed in 2010. We used Chandra/HETGS data (1.0-10 keV band) and
contemporaneous RXTE/PCA data (3.5-25 keV) to analyze the broadband X-ray
spectrum. We adopted different spectral models to describe the continuum
emission and used the XSTAR-derived warm absorber component to constrain the
highly ionized absorption features. The 1.0-25 keV continuum emission is
consistent with a model of soft accretion-disk emission and an optically thick,
harder Comptonized component. The dip event, lasting 450 s, is
spectrally resolved with an increase in the column density of the neutral
absorber, while we do not find significant variations in the column density and
ionization parameter of the warm absorber with respect to the out-of-dip
spectrum. We argue that the very low dipping duty-cycle with respect to other
sources of the same class can be ascribed to its long orbital period and the
mostly neutral bulge, that is relatively small compared with the dimensions of
the outer disk radius.Comment: 13 pages, 15 figures, accepted for publication in Astronomy and
Astrophysic
Comment on "Correlated electron-nuclear dynamics: Exact factorization of the molecular wavefunction" [J. Chem. Phys. 137, 22A530 (2012)]
In spite of the relevance of the proposal introduced in the recent work A.
Abedi, N. T. Maitra and E. K. U. Gross, J. Chem. Phys. 137, 22A530, 2012, there
is an important ingredient which is missing. Namely, the proof that the norms
of the electronic and nuclear wavefunctions which are the solutions to the
nonlinear equations of motion are preserved by the evolution. To prove the
conservation of these norms is precisely the objective of this Comment.Comment: 2 pages, published versio
X-ray spectroscopy of the ADC source X1822-371 with Chandra and XMM-Newton
The eclipsing low-mass X-ray binary X1822-371 is the prototype of the
accretion disc corona (ADC) sources. We analyse two Chandra observations and
one XMM-Newton observation to study the discrete features and their variation
as a function of the orbital phase, deriving constraints on the temperature,
density, and location of the plasma responsible for emission lines. The HETGS
and XMM/Epic-pn observed X1822-371 for 140 and 50 ks, respectively. We
extracted an averaged spectrum and five spectra from five selected
orbital-phase intervals that are 0.04-0.25, 0.25-0.50, 0.50-0.75, 0.75-0.95,
and, finally, 0.95-1.04; the orbital phase zero corresponds to the eclipse
time. All spectra cover the energy band between 0.35 and 12 keV. We confirm the
presence of local neutral matter that partially covers the X-ray emitting
region; the equivalent hydrogen column is cm and the
covered fraction is about 60-65%. We identify emission lines from highly
ionised elements, and a prominent fluorescence iron line associated with a
blending of FeI-FeXV resonant transitions. The transitions of He-like ions show
that the intercombination dominates over the forbidden and resonance lines. The
line fluxes are the highest during the orbital phases between 0.04 and 0.75. We
discuss the presence of an extended, optically thin corona with optical depth
of about 0.01 that scatters the X-ray photons from the innermost region into
the line of sight. The photoionised plasma producing most of the observed lines
is placed in the bulge at the outer radius of the disc distant from the central
source of cm. The OVII and the fluorescence iron line are
probably produced in the photoionised surface of the disc at inner radii.
(Abridged)Comment: 18 pages including 12 figures. Accepted for publication in A&
Broad-band Spectral Evolution of Scorpius X-1 along its Color-Color Diagram
We analyze a large collection of RXTE archive data from April 1997 to August
2003 of the bright X-ray source Scorpius X-1 in order to study the broadband
spectral evolution of the source for different values of the inferred mass
accretion rate by studying energy spectra from selected regions in the Z-track
of its Color-Color Diagram. A two-component model, consisting of a soft thermal
component interpreted as thermal emission from an accretion disk and a thermal
Comptonization component, is unable to fit the whole 3--200 keV energy spectrum
at low accretion rates. Strong residuals in the highest energy band of the
spectrum require the addition of a third component that can be fitted with a
power-law component, that could represent a second thermal Comptonization from
a much hotter plasma, or a hybrid thermal/non-thermal Comptonization. We
discuss the physical implications derived from the results of our analysis,
with a particular emphasis on the hardest part of the X-ray emission and its
possible origins.Comment: 18 pages. Accepted for publication in Ap
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