Context.
The full third
Gaia
data release will provide, for the first time, the calibrated spectra obtained with the blue and red
Gaia
slitless spectrophotometers (BP and RP, respectively).
Gaia
is a ...very complex mission and cannot be considered as a single instrument, but rather as many instruments. The two lines of sight with wide fields of view introduce strong variations of the observations across the large focal plane with more than one hundred different detectors. 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.
Aims.
This work gives a detailed description of the internal calibration model for the spectrophotometric data used to obtain the content of 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.
Methods.
In order to describe all observations on a common mean flux and pseudo-wavelength scale, we constructed a suitable representation of the internally calibrated mean spectra via basis functions, and we described the transformation between non-calibrated epoch spectra and calibrated mean spectra via a discrete convolution, parametrising the convolution kernel to recover the relevant coefficients.
Results.
The model proposed here for the internal calibration of the
Gaia
spectrophotometric observations 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. We derived a calibration model that can handle the self-calibrating nature of the problem. 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.
Gaia Data Release 2 Evans, D. W.; Riello, M.; De Angeli, F. ...
Astronomy and astrophysics (Berlin),
08/2018, Letnik:
616
Journal Article
Recenzirano
Odprti dostop
Aims.
We describe the photometric content of the second data release of the
Gaia
project (
Gaia
DR2) and its validation along with the quality of the data.
Methods.
The validation was mainly carried ...out using an internal analysis of the photometry. External comparisons were also made, but were limited by the precision and systematics that may be present in the external catalogues used.
Results.
In addition to the photometric quality assessment, we present the best estimates of the three photometric passbands. Various colour-colour transformations are also derived to enable the users to convert between the
Gaia
and commonly used passbands.
Conclusions.
The internal analysis of the data shows that the photometric calibrations can reach a precision as low as 2 mmag on individual CCD measurements. Other tests show that systematic effects are present in the data at the 10 mmag level.
Context.
Photometric variability is an essential feature that sheds light on the intrinsic properties of celestial variable sources, the more so when photometry is available in various bands. In this ...respect, the all-sky
Gaia
mission is particularly attractive as it collects, among other quantities, epoch photometry measured quasi-simultaneously in three optical bands for sources ranging from a few magnitudes to fainter than magnitude 20.
Aims.
The second data release (DR2) of the mission provides mean
G
,
G
BP
, and
G
RP
photometry for ∼1.4 billion sources, but light curves and variability properties are available for only ∼0.5 million of them. Here, we provide a census of large-amplitude variables (LAVs) with amplitudes larger than ∼0.2 mag in the
G
band for objects with mean brightnesses between 5.5 and 19 mag.
Methods.
To achieve this, we rely on variability amplitude proxies in
G
,
G
BP
, and
G
RP
computed from the uncertainties on the magnitudes published in DR2. We then apply successive filters to identify two subsets containing sources with reliable mean
G
BP
and
G
RP
(for studies using colours) and sources having compatible amplitude proxies in
G
,
G
BP
, and
G
RP
(for multi-band variability studies).
Results.
The full catalogue gathers 23 315 874 LAV candidates, and the two subsets with increased levels of purity contain, respectively, 1 148 861 and 618 966 sources. A multi-band variability analysis of the catalogue shows that different types of variable stars can be categorized according to their colours and blue-to-red amplitude ratios as determined from the
G
,
G
BP
, and
G
RP
amplitude proxies. More specifically, four groups are globally identified. They include: long-period variables in a first group with amplitudes more than twice larger in the blue than in the red; hot compact variables in a second group with amplitudes smaller in the blue than in the red; classical instability strip pulsators in a third group with amplitudes larger in the blue than in the red by 50% to 80%; and other non-pulsating variables in a fourth group, mainly achromatic, but 10% of them still having 20% to 50% larger amplitudes in the blue than in the red.
Conclusions.
The catalogue constitutes the first census of
Gaia
LAV candidates extracted from the public DR2 archive. The overview presented here illustrates the added value of the mission for multi-band variability studies, even at this stage when epoch photometry is not yet available for all sources.
Gaia Data Release 1 Carrasco, J M; Evans, D W; Montegriffo, P ...
Astronomy and astrophysics (Berlin),
11/2016, Letnik:
595
Journal Article
Recenzirano
Odprti dostop
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 GaiaG 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.
Aims.In this paper we derive the structure of the Galactic stellar warp and flare. Methods.We use 2MASS red clump and red giant stars, selected at mean and fixed heliocentric distances of ...$R_{\odot}\simeq3$, 7 and 17 kpc. Results.Our results can be summarized as follows: (i) a clear stellar warp signature is derived for the 3 selected rings, proving that the warp starts already within the solar circle; (ii) the derived stellar warp is consistent (both in amplitude and phase-angle) with that for the Galactic interstellar dust and neutral atomic hydrogen; (iii) the consistency and regularity of the stellar-gaseous warp is traced out to about $R_{\rm GC}\sim20$ kpc; (iv) the Sun seems not to fall on the line of nodes. The stellar warp phase-angle orientation ($\phi\sim15^{\circ}$) is close to the orientation angle of the Galactic bar and this, most importantly, produces an asymmetric warp for the inner $R_{\odot}\simeq3$ and 7 kpc rings; (v) a Northern/Southern warp symmetry is observed only for the ring at $R_{\odot}\simeq17$ kpc, at which the dependency on ϕ is weakened; (vi) treating a mixture of thin and thick disk stellar populations, we trace the variation with RGC of the disk thickness (flaring) and derive an almost constant scale-height (~0.65 kpc) within $R_{\rm GC}\sim15$ kpc. Further out, the disk flaring increase gradually reaching a mean scale-height of ~1.5 kpc at $R_{\rm GC}\sim23$ kpc; (vii) the derived outer disk warping and flaring provide further robust evidence that there is no disk radial truncation at $R_{\rm GC}\sim14$ kpc. Conclusions.In the particular case of the Canis Major (CMa) over-density we confirm its coincidence with the Southern stellar maximum warp occurring near $l\sim240^{\circ}$ (for $R_{\odot}\simeq7$ kpc) which brings down the Milky Way mid-plane by ~$3^{\circ}$ in this direction. The regularity and consistency of the stellar, gaseous and dust warp argues strongly against a recent merger scenario for Canis Major. We present evidence to conclude that all observed parameters (e.g. number density, radial velocities, proper motion etc) of CMa are consistent with it being a normal Milky Way outer-disk population, thereby leaving no justification for more complex interpretations of its origin. The present analysis or outer disk structure does not provide a conclusive test of the structure or origin of the Monoceros Ring. Nevertheless, we show that a warped flared Milky Way contributes significantly at the locations of the Monoceros Ring. Comparison of outer Milky Way $\ion{H}{i}$ and CO properties with those of other galaxies favors the suggestion that complex structures close to planar in outer disks are common, and are a natural aspect of warped and flaring disks.
ABSTRACT
We present the flux tables of the spectrophotometric standard stars (SPSS) used to calibrate in flux the Gaia DR2 and (E)DR3 data releases. The latest SPSS grid version contains 112 stars, ...whose flux tables agree to better than 1 per cent with the CALSPEC spectra of 11 flux standards for the calibration of the Hubble Space Telescope. The synthetic magnitudes computed on the SPSS spectra also agree to better than 1 per cent with the Landolt magnitudes of 37 stars in common. The typical spreads in both comparisons are of the order of 1 per cent. These uncertainties already meet the initial requirements for the Gaia SPSS project, but further improvements are expected in the next SPSS versions, that will be used to calibrate future Gaia releases. We complement the SPSS flux tables with literature spectra of 60 additional stars that did not pass all the criteria to be SPSS, the Passband Validation Library (PVL). The PVL contains stars of extreme spectral types, such as bright O and B stars and late M stars and brown dwarfs, and was useful to investigate systematic effects in the previous Gaia DR2 release and to minimize them in the EDR3 one. The PVL literature spectra are recalibrated as accurately as possible on to the SPSS reference scale, so that the two sets together can be used in a variety of validation and comparison studies.
Studies have previously shown that anthracene and naphthalene derivatives serve as compounds for trapping and chemically generating singlet molecular oxygen O2(1Δg), respectively. Simple and ...efficient synthetic routes to anthracene and naphthalene derivatives are needed, for improved capture and release of O2(1Δg) in cellular environments. Because of this need, we have synthesized a dihydroxypropyl amide naphthlene endoperoxide as a O2(1Δg) donor, as well as five anthracene derivatives as O2(1Δg) acceptor. The anthracene derivatives bear dihydroxypropyl amide, ester, and sulfonate ion end groups connected to 9,10-positions by way of unsaturated (vinyl) and saturated (ethyl) bridging groups. Heck reactions were found to yield these six compounds in easy-to-carry out 3-step reactions in yields of 50-76%. Preliminary results point to the potential of the anthracene compounds to serve as O2(1Δg) acceptors and would be amenable for future use in biological systems to expand the understanding of O2(1Δg) in biochemistry.
Leptospirosis is a global zoonotic disease caused by pathogenic bacteria of the genus Leptospira. We sought to determine if rodents in U.S. Virgin Islands (USVI) are carriers of Leptospira. In total, ...140 rodents were sampled, including 112 Mus musculus and 28 Rattus rattus. A positive carrier status was identified for 64/140 (45.7%); 49 (35.0%) were positive by dark-field microscopy, 60 (42.9%) by culture, 63 (45.0%) by fluorescent antibody testing, and 61 (43.6%) by real-time polymerase chain reaction (rtPCR). Molecular typing indicated that 48 isolates were L. borgpetersenii and 3 were L. kirschneri; the remaining nine comprised mixed species. In the single culture-negative sample that was rtPCR positive, genotyping directly from the kidney identified L. interrogans. Serotyping of L. borgpetersenii isolates identified serogroup Ballum and L. kirschneri isolates as serogroup Icterohaemorrhagiae. These results demonstrate that rodents are significant Leptospira carriers and adds to understanding the ecoepidemiology of leptospirosis in USVI.
Gaia Data Release 1 Evans, D W; Riello, M; De Angeli, F ...
Astronomy and astrophysics (Berlin),
4/2017, Letnik:
600
Journal Article
Recenzirano
Aims. The photometric validation of the Gaia DR1 release of the ESA Gaia mission is described and the quality of the data shown. Methods. This is carried out via an internal analysis of the ...photometry using the most constant sources. Comparisons with external photometric catalogues are also made, but are limited by the accuracies and systematics present in these catalogues. An analysis of the quoted errors is also described. Investigations of the calibration coefficients reveal some of the systematic effects that affect the fluxes. Results. The analysis of the constant sources shows that the early-stage photometric calibrations can reach an accuracy as low as 3 mmag.
Arterial Spin Labelling (ASL) imaging derives a perfusion image by tracing the accumulation of magnetically labeled blood water in the brain. As the image generated has an intrinsically low signal to ...noise ratio (SNR), multiple measurements are routinely acquired and averaged, at a penalty of increased scan duration and opportunity for motion artefact. However, this strategy alone might be ineffective in clinical settings where the time available for acquisition is limited and patient motion are increased. This study investigates the use of an Independent Component Analysis (ICA) approach for denoising ASL data, and its potential for automation.
72 ASL datasets (pseudo-continuous ASL; 5 different post-labeling delays: 400, 800, 1200, 1600, 2000 m s; total volumes = 60) were collected from thirty consecutive acute stroke patients. The effects of ICA-based denoising (manual and automated) where compared to two different denoising approaches, aCompCor, a Principal Component-based method, and Enhancement of Automated Blood Flow Estimates (ENABLE), an algorithm based on the removal of corrupted volumes. Multiple metrics were used to assess the changes in the quality of the data following denoising, including changes in cerebral blood flow (CBF) and arterial transit time (ATT), SNR, and repeatability. Additionally, the relationship between SNR and number of repetitions acquired was estimated before and after denoising the data.
The use of an ICA-based denoising approach resulted in significantly higher mean CBF and ATT values (p < 0.001), lower CBF and ATT variance (p < 0.001), increased SNR (p < 0.001), and improved repeatability (p < 0.05) when compared to the raw data. The performance of manual and automated ICA-based denoising was comparable. These results went beyond the effects of aCompCor or ENABLE. Following ICA-based denoising, the SNR was higher using only 50% of the ASL-dataset collected than when using the whole raw data.
The results show that ICA can be used to separate signal from noise in ASL data, improving the quality of the data collected. In fact, this study suggests that the acquisition time could be reduced by 50% without penalty to data quality, something that merits further study. Independent component classification and regression can be carried out either manually, following simple criteria, or automatically.
•ICA can be used to separate signal from noise in ASL data, improving data quality.•Automated denoising reproduces the improvement seen by a manual approach.•ICA based denoising is superior to PCA or volume censoring approaches.•ASL acquisition time could be reduced by 50% without penalty to data quality.
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