1,259 research outputs found
Scintillation Caustics in Planetary Occultation Light Curves
We revisit the GSC5249-01240 light curve obtained during its occultation by
Saturn's North polar region. In addition to refractive scintillations, the
power spectrum of intensity fluctuations shows an enhancement of power between
refractive and diffractive regimes. We identify this excess power as due to
high amplitude spikes in the light curve and suggest that these spikes are due
to caustics associated with ray crossing situations. The flux variation in
individual spikes follows the expected caustic behavior, including diffraction
fringes which we have observed for the first time in a planetary occultation
light curve. The presence of caustics in scintillation light curves require an
inner scale cut off to the power spectrum of underlying density fluctuations
associated with turbulence. Another possibility is the presence of gravity
waves in the atmosphere. While occultation light curves previously showed the
existence of refractive scintillations, a combination of small projected
stellar size and a low relative velocity during the event have allowed us to
identify caustics in this occultation. This has led us to re-examine previous
data sets, in which we have also found likely examples of caustics.Comment: 4 pages, 3 figures; ApJL submitte
On the morphology of the electron-positron annihilation emission as seen by SPI/INTEGRAL
The 511 keV positron annihilation emission remains a mysterious component of
the high energy emission of our Galaxy. Its study was one of the key scientific
objective of the SPI spectrometer on-board the INTEGRAL satellite. In fact, a
lot of observing time has been dedicated to the Galactic disk with a particular
emphasis on the central region. A crucial issue in such an analysis concerns
the reduction technique used to treat this huge quantity of data, and more
particularly the background modeling. Our method, after validation through a
variety of tests, is based on detector pattern determination per ~6 month
periods, together with a normalisation variable on a few hour timescale. The
Galactic bulge is detected at a level of ~70 sigma allowing more detailed
investigations. The main result is that the bulge morphology can be modelled
with two axisymmetric Gaussians of 3.2 deg. and 11.8 deg. FWHM and respective
fluxes of 2.5 and 5.4 x 10^-4 photons/(cm^2.s^1). We found a possible shift of
the bulge centre towards negative longitude at l=-0.6 +/- 0.2 degrees. In
addition to the bulge, a more extended structure is detected significantly with
flux ranging from 1.7 to 2.9 x10^-3 photons/(cm^2.s^1) depending on its assumed
geometry (pure disk or disk plus halo). The disk emission is also found to be
symmetric within the limits of the statistical errors.Comment: This paper has 12 pages and 14 figures. Accepted for publication by
the Astrophysical Journa
Kuiper Belt Object Occultations: Expected Rates, False Positives, and Survey Design
A novel method of generating artificial scintillation noise is developed and
used to evaluate occultation rates and false positive rates for surveys probing
the Kuiper Belt with the method of serendipitous stellar occultations. A
thorough examination of survey design shows that: (1) diffraction-dominated
occultations are critically (Nyquist) sampled at a rate of 2 Fsu^{-1},
corresponding to 40 s^{-1} for objects at 40 AU, (2) occultation detection
rates are maximized when targets are observed at solar opposition, (3) Main
Belt Asteroids will produce occultations lightcurves identical to those of
Kuiper Belt Objects if target stars are observed at solar elongations of: 116
deg < epsilon < 125 deg, or 131 deg < epsilon < 141 deg, and (4) genuine KBO
occultations are likely to be so rare that a detection threshold of >7-8 sigma
should be adopted to ensure that viable candidate events can be disentangled
from false positives.Comment: Accepted AJ, 12 pages, 12 figure
Mid-Infrared Ethane Emission on Neptune and Uranus
We report 8- to 13-micron spectral observations of Neptune and Uranus from
the NASA Infrared Telescope Facility spanning more than a decade. The
spectroscopic data indicate a steady increase in Neptune's mean atmospheric
12-micron ethane emission from 1985 to 2003, followed by a slight decrease in
2004. The simplest explanation for the intensity variation is an increase in
stratospheric effective temperature from 155 +/- 3 K in 1985 to 176 +/- 3 K in
2003 (an average rate of 1.2 K/year), and subsequent decrease to 165 +/- 3 K in
2004. We also detected variation of the overall spectral structure of the
ethane band, specifically an apparent absorption structure in the central
portion of the band; this structure arises from coarse spectral sampling
coupled with a non-uniform response function within the detector elements. We
also report a probable direct detection of ethane emission on Uranus. The
deduced peak mole fraction is approximately an order of magnitude higher than
previous upper limits for Uranus. The model fit suggests an effective
temperature of 114 +/- 3 K for the globally-averaged stratosphere of Uranus,
which is consistent with recent measurements indicative of seasonal variation.Comment: Accepted for publication in ApJ. 16 pages, 10 figures, 2 table
Adaptation in a heterogeneous environment II: To be three or not to be
We propose a model to describe the adaptation of a phenotypically structured
population in a -patch environment connected by migration, with each patch
associated with a different phenotypic optimum, and we perform a rigorous
mathematical analysis of this model. We show that the large-time behaviour of
the solution (persistence or extinction) depends on the sign of a principal
eigenvalue, , and we study the dependency of with
respect to . This analysis sheds new light on the effect of increasing the
number of patches on the persistence of a population, which has implications in
agroecology and for understanding zoonoses; in such cases we consider a
pathogenic population and the patches correspond to different host species. The
occurrence of a springboard effect, where the addition of a patch contributes
to persistence, or on the contrary the emergence of a detrimental effect by
increasing the number of patches on the persistence, depends in a rather
complex way on the respective positions in the phenotypic space of the optimal
phenotypes associated with each patch. From a mathematical point of view, an
important part of the difficulty in dealing with , compared to or
, comes from the lack of symmetry. Our results, which are based on a fixed
point theorem, comparison principles, integral estimates, variational
arguments, rearrangement techniques, and numerical simulations, provide a
better understanding of these dependencies. In particular, we propose a precise
characterisation of the situations where the addition of a third patch
increases or decreases the chances of persistence, compared to a situation with
only two patches
Changes in Grape Maturity Induced by Spraying Ethanol
Three different ethanol solutions were sprayed onto Cabernet Sauvignon (Vitis vinifera L.) clusters during the ripening period: 2.5, 5 and 10% by volume in water. Controls were sprayed with water alone. Three different times of spraying were also tested: 8, 10 and 13 weeks post-flowering. One of the observed changes was a lower titratable acidity in grape samples at harvest, when the clusters were sprayed with ethanol at 10 weeks, in comparison with controls. The wines made with grapes treated with ethanol after mid-veraison, had higher ODs at 520 nm than did the controls. This may due to a combined effect of red pigment levels and acidity. In addition, following malolactic fermentation, the acidity levels of wines made with ethanoltreated grapes were slightly higher than those made with the control grapes. Spraying ethanol at 13 weeks post-flowering increased the berry weight by 10% at harvest without decreasing the °Brix value. The corresponding wines had similar degrees of alcohol. This observation was made for the first time in 2001
Detection and period measurements of GX1+4 at hard x ray energies with the SIGMA telescope
The galactic Low Mass X ray Binary GX1+4 was detected by the coded aperture hard X ray gamma ray SIGMA telescope during the Feb. to April 1991 observations of the galactic center regions. The source, whose emission varied during the survey of a factor greater than 40 pct., reached a maximum luminosity in the 40 to 140 energy range of 1.03 x 10(exp 37) erg/s (D = 8.5 kpc), thus approaching the emission level of the 1970 to 1980 high state. Two minute flux pulsations were detected on Mar. 22 and on Mar. 31 and Apr. 1. Comparison with the last period measurements shows that the current spin-down phase of GX1+4 is ending. Concerning the proposed association of this source with the galactic center 511 keV annihilation emission, upper limits were derived
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