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Search Results: 1 - 10 of 190450 matches for " G. Petit "
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Precise Point Positioning for TAI Computation
Gérard Petit,Zhiheng Jiang
International Journal of Navigation and Observation , 2008, DOI: 10.1155/2008/562878
Abstract: We discuss the use of some new time transfer techniques for computing TAI time links. Precise point positioning (PPP) uses GPS dual frequency carrier phase and code measurements to compute the link between a local clock and a reference time scale with the precision of the carrier phase and the accuracy of the code. The time link between any two stations can then be computed by a simple difference. We show that this technique is well adapted and has better short-term stability than other techniques used in TAI. We present a method of combining PPP and two-way time transfer that takes advantage of the qualities of each technique, and shows that it would bring significant improvement to TAI links.
Stellar magnetic field parameters from a Bayesian analysis of high-resolution spectropolarimetric observations
V. Petit,G. A. Wade
Physics , 2011, DOI: 10.1111/j.1365-2966.2011.20091.x
Abstract: In this paper we describe a Bayesian statistical method designed to infer the magnetic properties of stars observed using high-resolution circular spectropolarimetry in the context of large surveys. This approach is well suited for analysing stars for which the stellar rotation period is not known, and therefore the rotational phases of the observations are ambiguous. The model assumes that the magnetic observations correspond to a dipole oblique rotator, a situation commonly encountered in intermediate and high-mass stars. Using reasonable assumptions regarding the model parameter prior probability density distributions, the Bayesian algorithm determines the posterior probability densities corresponding to the surface magnetic field geometry and strength by performing a comparison between the observed and computed Stokes V profiles. Based on the results of numerical simulations, we conclude that this method yields a useful estimate of the surface dipole field strength based on a small number (i.e. 1 or 2) of observations. On the other hand, the method provides only weak constraints on the dipole geometry. The odds ratio, a parameter computed by the algorithm that quantifies the relative appropriateness of the magnetic dipole model versus the non-magnetic model, provides a more sensitive diagnostic of the presence of weak magnetic signals embedded in noise than traditional techniques. To illustrate the application of the technique to real data, we analyse seven ESPaDOnS and Narval observations of the early B-type magnetic star LP Ori. Insufficient information is available to determine the rotational period of the star and therefore the phase of the data; hence traditional modelling techniques fail to infer the dipole strength. In contrast, the Bayesian method allows a robust determination of the dipole polar strength, $B_d=911^{+138}_{-244}$ G.
One blind and three targeted searches for (sub)millisecond pulsars
E. Davoust,G. Petit,T. Fayard
Physics , 2011, DOI: 10.1051/0004-6361/201117384
Abstract: We conducted one blind and three targeted searches for millisecond and submillisecond pulsars. The blind search was conducted within 3deg of the Galactic plane and at longitudes between 20 and 110deg. It takes 22073 pointings to cover this region, and 5487 different positions in the sky. The first targeted search was aimed at Galactic globular clusters, the second one at 24 bright polarized and pointlike radiosources with steep spectra, and the third at 65 faint polarized and pointlike radiosources. The observations were conducted at the large radiotelescope of Nancay Observatory, at a frequency near 1400 MHz. Two successive backends were used, first a VLBI S2 system, second a digital acquisition board and a PC with large storage capacity sampling the signal at 50 Mb/s on one bit, over a 24-MHz band and in one polarization. The bandwidth of acquisition of the second backend was later increased to 48 MHz and the sampling rate to 100 Mb/s. The survey used the three successive setups, with respective sensitivities of 3.5, 2.2, and 1.7 mJy. The targeted-search data were obtained with the third setup and reduced with a method based on the Hough transform, yielding a sensitivity of 0.9 mJy. The processing of the data was done in slightly differed time by soft-correlation in all cases. No new short-period millisecond pulsars were discovered in the different searches. To better understand the null result of the blind survey, we estimate the probability of detecting one or more short-period pulsars among a given Galactic population of synthetic pulsars with our setup: 25% for the actual incomplete survey and 79% if we had completed the whole survey with a uniform nominal sensitivity of 1.7 mJy. The alternative of surveying a smaller, presumably more densely populated, region with a higher sensitivity would have a low return and would be impractical at a transit instrument. (abridged)
Likelihood for generally coarsened observations from multi-state or counting process models
Daniel Commenges,Anne Gégout-Petit
Mathematics , 2008, DOI: 10.1111/j.1467-9469.2006.00518.x
Abstract: We consider first the mixed discrete-continuous scheme of observation in multistate models; this is a classical pattern in epidemiology because very often clinical status is assessed at discrete visit times while times of death or other events are observed exactly. A heuristic likelihood can be written for such models, at least for Markov models; however, a formal proof is not easy and has not been given yet. We present a general class of possibly non-Markov multistate models which can be represented naturally as multivariate counting processes. We give a rigorous derivation of the likelihood based on applying Jacod's formula for the full likelihood and taking conditional expectation for the observed likelihood. A local description of the likelihood allows us to extend the result to a more general coarsening observation scheme proposed by Commenges & G\'egout-Petit. The approach is illustrated by considering models for dementia, institutionalization and death.
A general definition of influence between stochastic processes
Anne Gégout-Petit,Daniel Commenges
Mathematics , 2009,
Abstract: We extend the study of weak local conditional independence (WCLI) based on a measurability condition made by Commenges and G\'egout-Petit (2009) to a larger class of processes that we call D'. We also give a definition related to the same concept based on certain likelihood processes, using the Girsanov theorem. Under certain conditions, the two definitions coincide on D'. These results may be used in causal models in that we define what may be the largest class of processes in which influences of one component of a stochastic process on another can be described without ambiguity. From WCLI we can contruct a concept of strong local conditional independence (SCLI). When WCLI does not hold, there is a direct influence while when SCLI does not hold there is direct or indirect influence. We investigate whether WCLI and SCLI can be defined via conventional independence conditions and find that this is the case for the latter but not for the former. Finally we recall that causal interpretation does not follow from mere mathematical definitions, but requires working with a good system and with the true probability.
A search for weak or complex magnetic fields in the the B3V star iota Herculis
G. A. Wade,C. P. Folsom,P. Petit,V. Petit,F. Lignieres,M. Auriere,T. Bohm
Physics , 2014, DOI: 10.1093/mnras/stu1541
Abstract: We obtained 128 high signal-to-noise ratio Stokes $V$ spectra of the B3V star $\iota$~Her on 5 consecutive nights in 2012 with the ESPaDOnS spectropolarimeter at the Canada-France-Hawaii Telescope, with the aim of searching for the presence of weak and/or complex magnetic fields. Least-Squares Deconvolution mean profiles were computed from individual spectra, averaged over individual nights and over the entire run. No Zeeman signatures are detected in any of the profiles. The longitudinal magnetic field in the grand average profile was measured to be $-0.24 \pm 0.32 $ G, as compared to $-0.22 \pm 0.32$ G in the null profile. Our observations therefore provide no evidence for the presence of Zeeman signatures analogous to those observed in the A0V star Vega by Lignieres et al. (2009). We interpret these observations in three ways. First, we compare the LSD profiles with synthetic Stokes $V$ profiles corresponding to organised (dipolar) magnetic fields, for which we find an upper limit of about 8~G on the polar strength of any surface dipole present. Secondly, we compare the grand average profile with calculations corresponding to the random magnetic spot topologies of Kochukhov & Sudnik (2013), inferring that spots, if present, of 2 degrees radius with strengths of 2-4 G and a filling factor of 50% should have been detected in our data. Finally, we compare the observations with synthetic $V$ profiles corresponding to the surface magnetic maps of Vega (Petit et al. 2010) computed for the spectral characteristics of $\iota$ Her. We conclude that while it is unlikely we would have detected a magnetic field identical to Vega's, we would have likely detected one with a peak strength of about 30 G, i.e. approximately four times as strong as that of Vega.
Microalbuminuria como indicador precoz de Hipertrofia Ventricular Izquierda en pacientes diabéticos tipo 1 y 2, no hipertensos
Y Petit,F Contreras,A Lara,G Díaz
Archivos Venezolanos de Farmacología y Terapéutica , 2004,
Abstract: Se dise a trabajo de tipo descriptivo, transversal y correlacional cuyo objetivo es relacionar la Microalbuminuria (MA) en pacientes diabéticos tipo 1 y 2 No hipertensos, como marcador bioquímico precoz de Hipertrofia Ventricular Izquierda (HVI). A lo efectos se incluyeron 197 pacientes de la consulta de diabetes del Hospital Francisco Antonio Risques de Caracas; luego de aplicar criterios de inclusión se seleccionaron 16 Pacientes con DM tipo 1 y 2. En el grupo control se incluyeron 8 sujetos sanos. A los dos grupos se les realizó ecocardiograma MB y determinó por turbidimetría los niveles de albúmina en muestra parcial de orina de 2 horas. Los datos fueron analizados aplicando pruebas estadísticas descriptivas y correlacionales a través del programa SPSS 10.0. La MA resultó positiva en 75% de los pacientes diabéticos seleccionados; 94% de los pacientes diabéticos presento HVI, de estos 80% presentó simultáneamente microalbuminuria. Estos datos permitan inferir una tendencia entre MA e HVI, en razón de p=0.07. A descriptive, transversal and correlational study was designed to assess the relationship between non hypertensive diabetic type 1 and 2, microalbuminurea (MA) as a indicator for left ventricular hypertrophy (LVI). We include 197 patients from Francisco Antonio Risques Caracas Hospital′s diabetes consult; exclusion criteria were applied, and 16 diabetic (both type 1 and 2) patients were selected. Control group was formed by 8 healthy subjects. Ekg MB and 2 hour urine sample albumine was collected from each group. Descriptive and correlational analysis were performed to the data obtained by the SPSS 10.0 software. The MA resulted positive in 75% of diabetic patients, and 95% presented LVI; 80% resulted both positive for MA and LVI. This data allows to establish a positive correlation between MA and LVI with p=0,07.
Optimized XUV source at 100 kHz repetition rate
Cabasse A.,Machinet G.,Hazera C.,Petit S.
EPJ Web of Conferences , 2013, DOI: 10.1051/epjconf/20134101015
Abstract: Fiber based laser systems at high repetition rate are becoming very attractive for high-order harmonics generation as they allow to reach high intensities. Their characteristics (central wavelength, energy and pulse duration) require specific optimizing conditions. We analyse how the harmonic efficiency with such a system is influenced by several parameters (atomic gas media, pressure, interaction geometry (a jet or a cell)). This study allows us to define optimal conditions for HHG appropriate to this new type of laser and experimentally, a photon flux higher than 1012 photons/s is measured.
Neutron-induced fission cross sections of short-lived actinides with the surrogate reaction method
Jurado B.,Kessedjian G.,Petit M.,A?che M.
EPJ Web of Conferences , 2010, DOI: 10.1051/epjconf/20100206004
Abstract: We present a review of the fission cross section measurements made by the CENBG collaboration over the last years using the surrogate reaction method. For example the neutron-induced fission cross sections of 233Pa(T1/2 = 27 d), 242Cm(T1/2 d) and 243Cm(T1/2 y) have been obtained by our group with this technique. The advantages and the difficulties of the surrogate method are discussed. Special attention is paid to the comparison between cross sections measured with the surrogate method and those obtained directly with neutrons at low energies. This comparison provides information on possible differences between the spin-parity distributions achieved in the two methods.
Reduction of the COSMOS Southern Sky galaxy survey data to the RC3 standard system
J. Rousseau,H. Di Nella,G. Paturel,C. Petit
Physics , 1996, DOI: 10.1093/mnras/282.1.144
Abstract: After having cross-identified a subsample of LEDA galaxies in the COSMOS database, we defined the best relations to convert COSMOS parameters (coordinates, position angle, diameter, axis ratio and apparent magnitude) into RC3 system used in the LEDA database. Tiny secondary effects can be tested: distance to plate cenetrs effect and air-mass effect. The converted COSMOS parameters are used to add missing parameters on LEDA galaxies. Key words: galaxies - catalogue - photometry
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