255 research outputs found
Time and frequency transfer with a microwave link in the ACES/PHARAO mission
The Atomic Clocks Ensemble in Space (ACES/PHARAO mission), which will be
installed on board the International Space Station (ISS), uses a dedicated
two-way Micro-Wave Link (MWL) in order to compare the timescale generated on
board with those provided by many ground stations disseminated on the Earth.
Phase accuracy and stability of this long range link will have a key role in
the success of the ACES/PHARAO experiment. SYRTE laboratory is heavily involved
in the design and development of the data processing software : from
theoretical modelling and numerical simulations to the development of a
software prototype. Our team is working on a wide range of problems that need
to be solved in order to achieve high accuracy in (almost) real time. In this
article we present some key aspects of the measurement, as well as current
status of the software's development.Comment: Proceedings of the European Frequency and Time Forum (EFTF) 2012 held
in Gothenburg, Sweden, April 201
The ruminal level of trans-10 fatty acids of dairy cows is linked to the composition of bacterial community
The ruminal level of trans-10 fatty acids of dairy cows is linked to the composition of bacterial communit
Dynamique sub-picoseconde de l'interaction laser de puissance – agrégats de gaz rare : émission intense de rayons X et production d'ions multichargés.
National audienceLors de campagnes d'expériences réalisées sur le Laser Ultra Court Accordable du CEA/Saclay, nous avons étudié le rayonnement X, tant qualitativement (spectroscopie et énergie moyenne des photons) que quantitativement (taux absolus et lois d'évolution), émis lors de l'interaction d'un jet effusif d'agrégats de gaz rare (Ar, Kr, Xe comprenant entre 10^4 et 10^6 atomes/agrégat) avec un laser femtoseconde de puissance (éclairement jusqu'à quelques 10^17 W/cm2). Les résultats présentés dans ce manuscrit sont uniquement dédiés aux agrégats d'Ar pour lesquels nous avons observé un rayonnement X issu d'ions fortement multichargés (jusqu'à l'Ar16+) présentant des lacunes en couches K. La technique de spectroscopie X utilisée a permis de déterminer pour la première fois des taux absolus ainsi que les lois d'évolution de l'émission X en fonction de l'ensemble des paramètres gouvernant l'interaction (intensité, polarisation, longueur d'onde et durée du pulse laser aussi bien que taille, densité et numéro atomique des agrégats)
Very High Energy Gamma-ray spectral properties of Mrk 501 from CAT Cerenkov telescope observations in 1997
The BL Lac object Mrk 501 went into a very high state of activity during
1997, both in VHE gamma-rays and X-rays. We present here results from
observations at energies above 250 GeV carried out between March and October
1997 with the CAT Cerenkov imaging Telescope. The average differential spectrum
between 30 GeV and 13 TeV shows significant curvature and is well represented
by phi_0 * E_TeV^{-(alpha + beta*log10(E_TeV))}, with: phi_0 = 5.19 +/- 0.13
{stat} +/- 0.12 {sys-MC} +1.66/-1.04 {sys-atm} * 10^-11 /cm^2/s/TeV alpha =
2.24 +/- 0.04 {stat} +/- 0.05 {sys} beta = 0.50 +/- 0.07 {stat} (negligible
systematics). The TeV spectral energy distribution of Mrk 501 clearly peaks in
the range 500 GeV-1 TeV. Investigation of spectral variations shows a
significant hardness-intensity correlation with no measurable effect on the
curvature. This can be described as an increase of the peak TeV emission energy
with intensity. Simultaneous and quasi-simultaneous CAT VHE gamma-ray and
BeppoSAX hard X-ray detections for the highest recorded flare on 16th April and
for lower-activity states of the same period show correlated variability with a
higher luminosity in X-rays than in gamma-rays. The observed spectral energy
distribution and the correlated variability between X-rays and gamma-rays, both
in amplitude and in hardening of spectra, favour a two-component emission
scheme where the low and high energy components are attributed to synchrotron
and inverse Compton (IC) radiation, respectively.Comment: Submitted to Astronomy and Astrophysics, 8 pages including 6 figures.
Published with minor change
Spectrum and Variability of Mrk501 as observed by the CAT Imaging Telescope
The CAT Imaging Telescope has observed the BL Lac object Markarian 501
between March and August 1997. We report here on the variability over this time
including several large flares. We present also preliminary spectra for all
these data, for the low emission state, and for the largest flare.Comment: 4 pages, 4 figures, Late
A clock network for geodesy and fundamental science
Leveraging the unrivaled performance of optical clocks in applications in
fundamental physics beyond the standard model, in geo-sciences, and in
astronomy requires comparing the frequency of distant optical clocks
truthfully. Meeting this requirement, we report on the first comparison and
agreement of fully independent optical clocks separated by 700 km being only
limited by the uncertainties of the clocks themselves. This is achieved by a
phase-coherent optical frequency transfer via a 1415 km long telecom fiber link
that enables substantially better precision than classical means of frequency
transfer. The fractional precision in comparing the optical clocks of three
parts in was reached after only 1000 s averaging time, which is
already 10 times better and more than four orders of magnitude faster than with
any other existing frequency transfer method. The capability of performing high
resolution international clock comparisons paves the way for a redefinition of
the unit of time and an all-optical dissemination of the SI-second.Comment: 14 pages, 3 figures, 1 tabl
Observation of the Crab Nebula Gamma-Ray Emission Above 220 Gev by the Cat Cherenkov Imaging Telescope
The CAT imaging telescope, recently built on the site of the former solar
plant Themis (French Pyrenees), observed gamma-rays from the Crab nebula from
October 1996 to March 1997. This steady source, often considered as the
standard candle of very-high-energy gamma-ray astronomy, is used as a test-beam
to probe the performances of the new telescope, particularly its energy
threshold (220 GeV at 20 degrees zenith angle) and the stability of its
response. Due to the fine-grain camera, an accurate analysis of the
longitudinal profiles of shower images is performed, yielding the source
position in two dimensions for each individual shower.Comment: 5 pages, 3 figures, Tex, contribution to 25th ICRC Durba
Detection of Vhe Gamma-Rays from MRK 501 with the Cat Imaging Telescope
The CAT imaging telescope on the site on the former solar plant Themis has
been observing gamma-rays from Mrk501 above 220 GeV in March and April 1997.
This source is shown to be highly variable and the light curve is presented.
The detected gamma-ray rate for the most intense flare is in excess of 10 per
minute.Comment: 5 pages, 4 figures, Tex, contribution to 25th ICRC Durba
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