1,019 research outputs found
Doppler temperatures from O(<sup>1</sup>D) airglow in the daytime thermosphere as observed by the Wind Imaging Interferometer (WINDII) on the UARS satellite
International audienceFrom 1992 to 1997, the WINDII interferometer on board the UARS satellite acquired a large set of thermospheric data from the O(1D) and O(1S) airglows. We report here for the first time on daytime O(1D) Doppler temperatures obtained with version 5.11 of the WINDII data processing software. Using a statistical analysis of the temperatures independently measured by the two WINDII fields of view, we estimate that the temperature variations larger than 40 K can be considered as geophysical. Comparisons of WINDII temperatures measured during magnetically quiet days with temperatures obtained by the MSIS-90 and DTM-94 thermospheric models show a 100 K bias. We demonstrate, however, that the modeled temperature variations represent very well the mean temperature variation observed by WINDII over 4 years. We also show that the observed latitudinal/local time variation is in very good agreement with the two empirical models. Finally, the temperature variations during a magnetically disturbed day are found to be qualitatively well represented in form by the models, but largely underestimated. The presence of non-thermal atoms and instrument related issues are discussed as possible explanations for the 100 K bias between the WINDII Doppler temperatures and the empirical models
Multiwavelength observations revealing the outbursts of the two soft X-ray transients XTE J1859+226 and XTE J1118+480
We report here on multiwavelength observations of the two new soft X-ray
transients (SXTs) XTE J1859+226 and XTE J1118+480, which we observed with
HST/RXTE/UKIRT. For XTE J1118+480 we also used EUVE since it is located at a
very high galactic latitude and suffers from very low extinction. The two
sources exhibited very different behaviour. XTE J1859+226 seems quite normal
and therefore a good object for testing the accretion mechanisms in place
during the outbursts, XTE J1118+480 is much more unusual because it exhibits i)
a low X-ray to optical ratio and ii) a strong non-thermal contribution in the
radio to optical domain, which is likely to be due to synchrotron emission. We
concentrate here on the near-infrared (NIR) and optical observations of these
two systems.Comment: 4 pages, 6 figures, conference, Proceedings of the 4th INTEGRAL
Workshop (Alicante 2000), to be published in ESA-SP (2001
A time varying speed of light as a solution to cosmological puzzles
We consider the cosmological implications of light travelling faster in the
early Universe. We propose a prescription for deriving corrections to the
cosmological evolution equations while the speed of light is changing. We
then show how the horizon, flatness, and cosmological constant problems may be
solved. We also study cosmological perturbations in this scenario and show how
one may solve the homogeneity and isotropy problems. As it stands, our scenario
appears to most easily produce extreme homogeneity, requiring structure to be
produced in the Standard Big Bang epoch. Producing significant perturbations
during the earlier epoch would require a rather careful design of the function
. The large entropy inside the horizon nowadays can also be accounted for
in this scenario.Comment: To be published in Physical Review D. Note added referring to John
Moffat's early work on VSL theorie
OSIRIS: A Decade of Scattered Light
Into year 11 of a 2-yr mission, OSIRIS is redefining how limb-scattered sunlight can be used to probe the atmosphere, even into the upper troposphere
A survey for pulsating subdwarf B stars with the Nordic Optical Telescope
A search programme for pulsating subdwarf B stars was conducted with the
Nordic Optical Telescope on La Palma over 59 nights between 1999 and 2009. The
purpose of the programme was to significantly extend the number of rapidly
pulsating sdB stars to better understand the properties of this new group of
variable compact stars. Candidates were selected initially from the HS and HE
surveys, but were supplemented with additional objects from other surveys.
Short sequences of time-series photometry were made on the candidates to
determine the presence of rapid pulsations. In total twenty new pulsators were
found in this survey, most of which have already been published and some
extensively studied. We present four new short period pulsators, bringing the
total of such pulsators up to 49. We also give limits on pulsation amplitudes
for 285 objects with no obvious periodic variations, summarise the results of
the survey, and provide improved physical parameters on the composite pulsators
for which only preliminary estimates were published earlier.Comment: 17 pages, accepted for publication in A&
Strength and uncertainty of phytoplankton metrics for assessing eutrophication impacts in lakes
Phytoplankton constitutes a diverse array of short-lived organisms which derive their nutrients from the water column of lakes. These features make this community the most direct and earliest indicator of the impacts of changing nutrient conditions on lake ecosystems. It also makes them particularly suitable for measuring the success of restoration measures following reductions in nutrient loads. This paper integrates a large volume of work on a number of measures, or metrics, developed for using phytoplankton to assess the ecological status of European lakes, as required for the Water Framework Directive. It assesses the indicator strength of these metrics, specifically in relation to representing the impacts of eutrophication. It also examines how these measures vary naturally at different locations within a lake, as well as between lakes, and how much variability is associated with different replicate samples, different months within a year and between years. On the basis of this analysis, three of the strongest metrics (chlorophyll-a, phytoplankton trophic index (PTI), and cyanobacterial biovolume) are recommended for use as robust measures for assessing the ecological quality of lakes in relation to nutrient-enrichment pressures and a minimum recommended sampling frequency is provided for these three metrics
Normal modes and discovery of high-order cross-frequencies in the DBV white dwarf GD 358
We present a detailed mode identification performed on the 1994 Whole Earth Telescope (WET) run on GD 358. The results are compared with that obtained for the same star from the 1990 WET data. The two temporal spectra show very few qualitative differences, although amplitude changes are seen in most modes, including the disappearance of the mode identified as k=14 in the 1990 data. The excellent coverage and signal-to-noise ratio obtained during the 1994 run lead to the secure identification of combination frequencies up to fourth order, i.e. peaks that are sums or differences of up to four parent frequencies, including a virtually complete set of second-order frequencies, as expected from harmonic distortion. We show how the third-order frequencies are expected to affect the triplet structure of the normal modes by back-interacting with them. Finally, a search for ℓ=2 modes was unsuccessful, not verifying the suspicion that such modes had been uncovered in the 1990 data set
Timing Analysis of the Light Curve of the Dipping-Bursting X-ray Binary X1916-053
We present the timing analysis results for our observations of the x-ray dip
source X1916-053 conducted with RXTE between February and October of 1996. Our
goal was to finally measure the binary period - as either the x-ray dip period
or ~1% longer optical modulation period, thereby establishing if the binary has
a precessing disk (SU UMa model) or a third star (triple model). Combined with
historical data (1979-96), the x-ray dip period is measured to be 3000.6508
0.0009 sec with a 2 upper limit . From our quasi-simultaneous optical observations (May 14-23, 1996)
and historical data (1987-96), we measure the optical modulation period to be
3027.5510 0.0052 sec with a 2 upper limit . The two periods are therefore each stable (over all recorded
data) and require a d beat period. This beat period, and
several of its harmonics is also observed as variations in the dip shape. Phase
modulation of x-ray dips, observed in a 10 consecutive day observation, is
highly correlated with the 3.9d dip shape modulation. The 1987-1996
optical observations show that the optical phase fluctuations are a factor of 3
larger than those in the x-ray. We discuss SU UMa vs. triple models to describe
the X1916-053 light curve behavior and conclude that the x-ray dip period, with
smaller phase jitter, is probably the binary period so that the required
precession is most likely similar to that observed in SU UMa and x-ray nova
systems. However the ``precession'' period stability and especially the fact
that the times of x-ray bursts may partially cluster to occur just after x-ray
dips, continue to suggest that this system may be a hierarchical triple.Comment: 33 pages, 13 figures, accepted by Ap
Observations of the pulsating subdwarf B star Feige 48: Constraints on evolution and companions
Since pulsating subdwarf B (sdBV or EC14026) stars were first discovered
(Kilkenny et al, 1997), observational efforts have tried to realize their
potential for constraining the interior physics of extreme horizontal branch
(EHB) stars. Difficulties encountered along the way include uncertain mode
identifications and a lack of stable pulsation mode properties. Here we report
on Feige 48, an sdBV star for which follow-up observations have been obtained
spanning more than four years, which shows some stable pulsation modes.
We resolve the temporal spectrum into five stable pulsation periods in the
range 340 to 380 seconds with amplitudes less than 1%, and two additional
periods that appear in one dataset each. The three largest amplitude
periodicities are nearly equally spaced, and we explore the consequences of
identifying them as a rotationally split l=1 triplet by consulting with a
representative stellar model.
The general stability of the pulsation amplitudes and phases allows us to use
the pulsation phases to constrain the timescale of evolution for this sdBV
star. Additionally, we are able to place interesting limits on any stellar or
planetary companion to Feige 48.Comment: accepted for publication in MNRA
The fundamental constants and their variation: observational status and theoretical motivations
This article describes the various experimental bounds on the variation of
the fundamental constants of nature. After a discussion on the role of
fundamental constants, of their definition and link with metrology, the various
constraints on the variation of the fine structure constant, the gravitational,
weak and strong interactions couplings and the electron to proton mass ratio
are reviewed. This review aims (1) to provide the basics of each measurement,
(2) to show as clearly as possible why it constrains a given constant and (3)
to point out the underlying hypotheses. Such an investigation is of importance
to compare the different results, particularly in view of understanding the
recent claims of the detections of a variation of the fine structure constant
and of the electron to proton mass ratio in quasar absorption spectra. The
theoretical models leading to the prediction of such variation are also
reviewed, including Kaluza-Klein theories, string theories and other
alternative theories and cosmological implications of these results are
discussed. The links with the tests of general relativity are emphasized.Comment: 56 pages, l7 figures, submitted to Rev. Mod. Phy
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