372 research outputs found
How to detect an anti-spacetime
Is it possible, in principle, to measure the sign of the Lapse? We show that
fermion dynamics distinguishes spacetimes having the same metric but different
tetrads, for instance a Lapse with opposite sign. This sign might be a physical
quantity not captured by the metric. We discuss its possible role in quantum
gravity.Comment: Article awarded with an "Honorable Mention" from the 2012 Gravity
Foundation Award. 6 pages, 8 (pretty) figure
HST color-magnitude diagrams of 74 galactic globular clusters in the HST F439W and F555W bands
We present the complete photometric database and the color-magnitude diagrams
for 74 Galactic globular clusters observed with the HST/WFPC2 camera in the
F439W and F555W bands. A detailed discussion of the various reduction steps is
also presented, and of the procedures to transform instrumental magnitudes into
both the HST F439W and F555W flight system and the standard Johnson B and V
systems. We also describe the artificial star experiments which have been
performed to derive the star count completeness in all the relevant branches of
the color magnitude diagram. The entire photometric database and the
completeness function will be made available on the Web immediately after the
publication of the present paper.Comment: 21 pages, 77 figures. High resolution version of this paper can be
retrived at http://dipastro.pd.astro.it/globular
Ag-sensitized Tb3+/Yb3+ codoped silica-zirconia glasses and glass-ceramics: Systematic and detailed investigation of the broadband energy-transfer and downconversion processes
Abstract Various studies report that Tb3+/Yb3+ co-doped materials can split one UV or 488 nm (visible) photon in two near infrared (NIR) photons at 980 nm by an energy-transfer process involving one Tb3+ and two Yb3+ ions. Additionally, it was demonstrated that Ag multimers can provide an efficient optical sensitizing effect for rare earth ions (RE3+ ions), resulting in a broadband enhanced excitation, which could have a significant technological impact, overcoming their limited spectral absorptions and small excitation cross sections. However, a systematic and detailed investigation of the down-conversion process enhanced by Ag nanoaggregates is still lacking, which is the focus of this paper. Specifically, a step by step analysis of the energy-transfer quantum-cutting chain in Ag-exchanged Tb3+/Yb3+ co-doped glasses and glass-ceramics is presented. Moreover, the direct Ag-Yb3+ energy-transfer is also considered. Results of structural, compositional, and optical characterizations are given, providing quantitative data for the efficient broadband Ag-sensitization of Tb3+/Yb3+ quantum cutting. A deeper understanding of the physical processes beneath the optical properties of the developed materials will allow a wiser realization of more efficient energy-related devices, such as spectral converters for silicon solar cells and light-emitting devices (LEDs) in the visible and NIR spectral regions
Internal calibration of Gaia BP/RP low-resolution spectra
The full third Gaia data release will provide the calibrated spectra obtained
with the blue and red Gaia slit-less spectrophotometers. The main challenge
when facing Gaia spectral calibration is that no lamp spectra or flat fields
are available during the mission. Also, the significant size of the line spread
function with respect to the dispersion of the prisms produces alien photons
contaminating neighbouring positions of the spectra. This makes the calibration
special and different from standard approaches.
This work gives a detailed description of the internal calibration model to
obtain the spectrophotometric data in the Gaia catalogue. The main purpose of
the internal calibration is to bring all the epoch spectra onto a common flux
and pixel (pseudo-wavelength) scale, taking into account variations over the
focal plane and with time, producing a mean spectrum from all the observations
of the same source.
In order to describe all observations in a common mean flux and
pseudo-wavelength scale, we construct a suitable representation of the
internally calibrated mean spectra via basis functions and we describe the
transformation between non calibrated epoch spectra and calibrated mean spectra
via a discrete convolution, parametrising the convolution kernel to recover the
relevant coefficients.
The model proposed here is able to combine all observations into a mean
instrument to allow the comparison of different sources and observations
obtained with different instrumental conditions along the mission and the
generation of mean spectra from a number of observations of the same source.
The output of this model provides the internal mean spectra, not as a sampled
function (flux and wavelength), but as a linear combination of basis functions,
although sampled spectra can easily be derived from them.Comment: 20 pages, 22 figure
Gaia data release 1: Principles of the photometric calibration of the G band
Context. Gaia is an ESA cornerstone mission launched on 19 December 2013
aiming to obtain the most complete and precise 3D map of our Galaxy by
observing more than one billion sources. This paper is part of a series of
documents explaining the data processing and its results for Gaia Data Release
1, focussing on the G band photometry. Aims. This paper describes the
calibration model of the Gaia photometric passband for Gaia Data Release 1.
Methods. The overall principle of splitting the process into internal and
external calibrations is outlined. In the internal calibration, a
self-consistent photometric system is generated. Then, the external calibration
provides the link to the absolute photometric flux scales. Results. The Gaia
photometric calibration pipeline explained here was applied to the first data
release with good results. Details are given of the various calibration
elements including the mathematical formulation of the models used and of the
extraction and preparation of the required input parameters (e.g. colour
terms). The external calibration in this first release provides the absolute
zero point and photometric transformations from the Gaia G passband to other
common photometric systems. Conclusions. This paper describes the photometric
calibration implemented for the first Gaia data release and the instrumental
effects taken into account. For this first release no aperture losses,
radiation damage, and other second-order effects have not yet been implemented
in the calibration.Comment: 15 pages, 16 figures, Gaia data release 1 documentation special
volum
Gaia Data Release 2: processing of the photometric data
CONTEXT. The second Gaia data release is based on 22 months of mission data with an average of 0.9 billion individual CCD observations per day. A data volume of this size and granularity requires a robust and reliable but still flexible system to achieve the demanding accuracy and precision constraints that Gaia is capable of delivering. AIMS. We aim to describe the input data, the treatment of blue photometer/red photometer (BP/RP) low–resolution spectra required to produce the integrated GBP and GRP fluxes, the process used to establish the internal Gaia photometric system, and finally, the generation of the mean source photometry from the calibrated epoch data for Gaia DR2. METHODS. The internal Gaia photometric system was initialised using an iterative process that is solely based on Gaia data. A set of calibrations was derived for the entire Gaia DR2 baseline and then used to produce the final mean source photometry. The photometric catalogue contains 2.5 billion sources comprised of three different grades depending on the availability of colour information and the procedure used to calibrate them: 1.5 billion gold, 144 million silver, and 0.9 billion bronze. These figures reflect the results of the photometric processing; the content of the data release will be different due to the validation and data quality filters applied during the catalogue preparation. The photometric processing pipeline, PhotPipe, implements all the processing and calibration workflows in terms of Map/Reduce jobs based on the Hadoop platform. This is the first example of a processing system for a large astrophysical survey project to make use of these technologies. RESULTS. The improvements in the generation of the integrated G–band fluxes, in the attitude modelling, in the cross–matching, and and in the identification of spurious detections led to a much cleaner input stream for the photometric processing. This, combined with the improvements in the definition of the internal photometric system and calibration flow, produced high-quality photometry. Hadoop proved to be an excellent platform choice for the implementation of PhotPipe in terms of overall performance, scalability, downtime, and manpower required for operations and maintenance
Gaia Early Data Release 3:Modelling and calibration of Gaia's point and line spread functions
Context: The unprecedented astrometric precision of the Gaia mission relies
on accurate estimates of the locations of sources in the Gaia data stream. This
is ultimately performed by point spread function (PSF) fitting, which in turn
requires an accurate reconstruction of the PSF. Gaia Early Data Release 3
(EDR3) will, for the first time, use a PSF calibration that models several of
the strongest dependences, leading to signficantly reduced systematic errors.
Aims: We describe the PSF model and calibration pipeline implemented for Gaia
EDR3, including an analysis of the calibration results over the 34 months of
data. We include a discussion of the limitations of the current pipeline and
directions for future releases. This will be of use both to users of Gaia data
and as a reference for other precision astrometry missions. Methods: We develop
models of the 1D line spread function (LSF) and 2D PSF profiles based on a
linear combination of basis components. We fit the models to selected primary
sources in independent time ranges, using simple parameterisations for the
colour and other dependences. Variation in time is smoothed by merging the
independent calibrations in a square root information filter, with resets at
certain mission events that induce a discontinuous change in the PSF. Results:
The PSF calibration shows strong time and colour dependences that accurately
reproduce the varying state of the Gaia astrometric instrument. Analysis of the
residuals reveals both the performance and the limitations of the current
models and calibration pipeline, and indicates the directions for future
development. Conclusions: The PSF modelling and calibration carried out for
Gaia EDR3 represents a major step forwards in the data processing and will lead
to reduced systematic errors in the core mission data products. Further
significant improvements are expected in the future data releases.Comment: Accepted by A&A for inclusion in Gaia EDR3 special issu
Top-down synthesis of multifunctional iron oxide nanoparticles formacrophage labelling and manipulation
Multifunctional iron oxide (FeOx) magnetic nanoparticles (MNPs) are promising items for biomedical
applications. They are studied as theranostic agents for cancer treatment, selective probes for
bioanalytical assays, controllable carriers for drug delivery and biocompatible tools for cell sorting or
tissue repair. Here we report a new method for the synthesis in water of FeOx–MNPs via a top-down
physical technique consisting in Laser Ablation Synthesis in Solution (LASiS). LASiS is a green
method that does not require chemicals or stabilizers, because nanoparticles are directly obtained in
water as a stable colloidal system. A gamut of characterization techniques was used for investigating
the structure of FeOx–MNPs that have a polycrystalline structure prevalently composed of magnetite
(ca. 75%) and hematite (ca. 22%). The FeOx–MNPs exhibit very good magnetic properties if compared
to what is usually reported for iron oxide nanoparticles, with saturation magnetization close to the bulk
value (ca. 80 emu g1) and typical signatures of the coexistence of ferrimagnetic and antiferromagnetic
phases in the same particle. The functionalization of FeOx–MNPs after the synthesis was possible with
a variety of ligands. In particular, we succeeded in the functionalization of FeOx–MNPs with
carboxylated phosphonates, fluorescent alkylamines, fluorescent isothiocyanates and bovine serum
albumin. Our FeOx–MNPs showed excellent biocompatibility. Multifunctional FeOx–MNPs were
exploited for macrophage cell labelling with fluorescent probes as well as for cell sorting and
manipulation by external magnetic fields
Quantum simplicial geometry in the group field theory formalism: reconsidering the Barrett-Crane model
A dual formulation of group field theories, obtained by a Fourier transform
mapping functions on a group to functions on its Lie algebra, has been proposed
recently. In the case of the Ooguri model for SO(4) BF theory, the variables of
the dual field variables are thus so(4) bivectors, which have a direct
interpretation as the discrete B variables. Here we study a modification of the
model by means of a constraint operator implementing the simplicity of the
bivectors, in such a way that projected fields describe metric tetrahedra. This
involves a extension of the usual GFT framework, where boundary operators are
labelled by projected spin network states. By construction, the Feynman
amplitudes are simplicial path integrals for constrained BF theory. We show
that the spin foam formulation of these amplitudes corresponds to a variant of
the Barrett-Crane model for quantum gravity. We then re-examin the arguments
against the Barrett-Crane model(s), in light of our construction.Comment: revtex, 24 page
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