361 research outputs found
Forced two layer beta-plane quasigeostrophic flow. Part I: Long-time existence and uniqueness of weak solutions
AbstractWe consider a model of quasigeostrophic turbulence that has proven useful in theoretical studies of large scale heat transport and coherent structure formation in planetary atmospheres and oceans. The model consists of a coupled pair of hyperbolic PDEs with a forcing which represents domain-scale thermal energy source. Although the use to which the model is typically put involves gathering information from very long numerical integrations, little of a rigorous nature is known about long-time properties of solutions to the equations. In this first paper we define a notion of weak solution, and show using Galerkin methods the long-time existence and uniqueness of such solutions
Long-time Behavior of a Two-layer Model of Baroclinic Quasi-geostrophic Turbulence
We study a viscous two-layer quasi-geostrophic beta-plane model that is
forced by imposition of a spatially uniform vertical shear in the eastward
(zonal) component of the layer flows, or equivalently a spatially uniform
north-south temperature gradient. We prove that the model is linearly unstable,
but that non-linear solutions are bounded in time by a bound which is
independent of the initial data and is determined only by the physical
parameters of the model. We further prove, using arguments first presented in
the study of the Kuramoto-Sivashinsky equation, the existence of an absorbing
ball in appropriate function spaces, and in fact the existence of a compact
finite-dimensional attractor, and provide upper bounds for the fractal and
Hausdorff dimensions of the attractor. Finally, we show the existence of an
inertial manifold for the dynamical system generated by the model's solution
operator. Our results provide rigorous justification for observations made by
Panetta based on long-time numerical integrations of the model equations
Fermi observations of high-energy gamma-ray emission from GRB 090217A
The Fermi observatory is advancing our knowledge of Gamma-Ray Bursts (GRBs)
through pioneering observations at high energies, covering more than 7 decades
in energy with the two on-board detectors, the Large Area Telescope (LAT) and
the Gamma-ray Burst Monitor (GBM). Here we report on the observation of the
long GRB 090217A which triggered the GBM and has been detected by the LAT with
a significance greater than 9 sigma. We present the GBM and LAT observations
and on-ground analyses, including the time-resolved spectra and the study of
the temporal profile from 8 keV up to 1 GeV. All spectra are well reproduced by
a Band model. We compare these observations to the first two LAT-detected, long
bursts GRB 080825C and GRB 080916C. These bursts were found to have
time-dependent spectra and exhibited a delayed onset of the high-energy
emission, which are not observed in the case of GRB 090217A. We discuss some
theoretical implications for the high-energy emission of GRBs.Comment: 17 pages, 4 figures. Contact Authors: Fred, Piron; Sara, Cutini;
Andreas, von Kienli
Fermi LAT Observations of the Supernova Remnant W28 (G6.4-0.1)
We present detailed analysis of the two gamma-ray sources,1FGL J1801.3-2322c
and 1FGL J1800.5-2359c,that have been found toward the supernova remnant(SNR)
W28 with the Large Area Telescope(LAT) on board the Fermi Gamma-ray Space
Telescope.1FGL J1801.3-2322c is found to be an extended source within the
boundary of SNR W28,and to extensively overlap with the TeV gamma-ray source
HESS J1801-233,which is associated with a dense molecular cloud interacting
with the supernova remnant.The gamma-ray spectrum measured with LAT from
0.2--100 GeV can be described by a broken power-law function with a break of
~1GeV,and photon indices of 2.090.08(stat)0.28(sys) below the break
and 2.740.06(stat)0.09(sys) above the break.Given the clear
association between HESS J1801-233 and the shocked molecular cloud and a
smoothly connected spectrum in the GeV--TeV band,we consider the origin of the
gamma-ray emission in both GeV and TeV ranges to be the interaction between
particles accelerated in the SNR and the molecular cloud.The decay of neutral
pions produced in interactions between accelerated hadrons and dense molecular
gas provide a reasonable explanation for the broadband gamma-ray spectrum. 1FGL
J1800.5-2359c, located outside the southern boundary of SNR W28, cannot be
resolved.An upper limit on the size of the gamma-ray emission was estimated to
be ~16 using events above ~2GeV under the assumption of a circular shape
with uniform surface brightness. It appears to coincide with the TeV source
HESS J1800-240B,which is considered to be associated with a dense molecular
cloud that contains the ultra compact HII region W28A2(G5.89-0.39).We found no
significant gamma-ray emission in the LAT energy band at the positions of TeV
sources HESS J1800-230A and HESS J1800-230C.The LAT data for HESS J1800-230A
combined with the TeV data points indicate a spectral break between 10GeV and
100GeV.Comment: 23 pages, 6 figures. Accepted for publication in the Astrophysical
Journal. Corresponding authors: H. Katagiri, H. Tajima, T. Tanaka, and Y.
Uchiyam
GeV Gamma-ray Flux Upper Limits from Clusters of Galaxies
The detection of diffuse radio emission associated with clusters of galaxies
indicates populations of relativistic leptons infusing the intracluster medium.
Those electrons and positrons are either injected into and accelerated directly
in the intracluster medium, or produced as secondary pairs by cosmic-ray ions
scattering on ambient protons. Radiation mechanisms involving the energetic
leptons together with decay of neutral pions produced by hadronic interactions
have the potential to produce abundant GeV photons. Here, we report on the
search for GeV emission from clusters of galaxies using data collected by the
Large Area Telescope (LAT) on the Fermi Gamma-ray Space Telescope (Fermi) from
August 2008 to February 2010. Thirty-three galaxy clusters have been selected
according to their proximity and high mass, X-ray flux and temperature, and
indications of non-thermal activity for this study. We report upper limits on
the photon flux in the range 0.2-100 GeV towards a sample of observed clusters
(typical values 1-5 x 10^-9 ph cm^-2 s^-1) considering both point-like and
spatially resolved models for the high-energy emission, and discuss how these
results constrain the characteristics of energetic leptons and hadrons, and
magnetic fields in the intracluster medium. The volume-averaged
relativistic-hadron-to-thermal energy density ratio is found to be < 5-10% in
several clusters.Comment: 9 pages, 3 tables, 1 figure, accepted for publication in ApJ Letter
Development of a Surface Plasmon Resonance Biosensor for Real-Time Detection of Osteogenic Differentiation in Live Mesenchymal Stem Cells
Surface plasmon resonance (SPR) biosensors have been recognized as a useful tool and widely used for real-time dynamic analysis of molecular binding affinity because of its high sensitivity to the change of the refractive index of tested objects. The conventional methods in molecular biology to evaluate cell differentiation require cell lysis or fixation, which make investigation in live cells difficult. In addition, a certain amount of cells are needed in order to obtain adequate protein or messenger ribonucleic acid for various assays. To overcome this limitation, we developed a unique SPR-based biosensing apparatus for real-time detection of cell differentiation in live cells according to the differences of optical properties of the cell surface caused by specific antigen-antibody binding. In this study, we reported the application of this SPR-based system to evaluate the osteogenic differentiation of mesenchymal stem cells (MSCs). OB-cadherin expression, which is up-regulated during osteogenic differentiation, was targeted under our SPR system by conjugating antibodies against OB-cadherin on the surface of the object. A linear relationship between the duration of osteogenic induction and the difference in refractive angle shift with very high correlation coefficient was observed. To sum up, the SPR system and the protocol reported in this study can rapidly and accurately define osteogenic maturation of MSCs in a live cell and label-free manner with no need of cell breakage. This SPR biosensor will facilitate future advances in a vast array of fields in biomedical research and medical diagnosis
Measurement of the Branching Fraction for B- --> D0 K*-
We present a measurement of the branching fraction for the decay B- --> D0
K*- using a sample of approximately 86 million BBbar pairs collected by the
BaBar detector from e+e- collisions near the Y(4S) resonance. The D0 is
detected through its decays to K- pi+, K- pi+ pi0 and K- pi+ pi- pi+, and the
K*- through its decay to K0S pi-. We measure the branching fraction to be
B.F.(B- --> D0 K*-)= (6.3 +/- 0.7(stat.) +/- 0.5(syst.)) x 10^{-4}.Comment: 7 pages, 1 postscript figure, submitted to Phys. Rev. D (Rapid
Communications
Gamma-Ray Emission Concurrent with the Nova in the Symbiotic Binary V407 Cygni
Novae are thermonuclear explosions on a white dwarf surface fueled by mass
accreted from a companion star. Current physical models posit that shocked
expanding gas from the nova shell can produce X-ray emission but emission at
higher energies has not been widely expected. Here, we report the Fermi Large
Area Telescope detection of variable gamma-ray (0.1-10 GeV) emission from the
recently-detected optical nova of the symbiotic star V407 Cygni. We propose
that the material of the nova shell interacts with the dense ambient medium of
the red giant primary, and that particles can be accelerated effectively to
produce pi0 decay gamma-rays from proton-proton interactions. Emission
involving inverse Compton scattering of the red giant radiation is also
considered and is not ruled out.Comment: 38 pages, includes Supplementary Online Material; corresponding
authors: C.C. Cheung, A.B. Hill, P. Jean, S. Razzaque, K.S. Woo
Searches for Cosmic-Ray Electron Anisotropies with the Fermi Large Area Telescope
The Large Area Telescope on board the \textit{Fermi} satellite
(\textit{Fermi}-LAT) detected more than 1.6 million cosmic-ray
electrons/positrons with energies above 60 GeV during its first year of
operation. The arrival directions of these events were searched for
anisotropies of angular scale extending from 10 up to
90, and of minimum energy extending from 60 GeV up to 480 GeV. Two
independent techniques were used to search for anisotropies, both resulting in
null results. Upper limits on the degree of the anisotropy were set that
depended on the analyzed energy range and on the anisotropy's angular scale.
The upper limits for a dipole anisotropy ranged from to .Comment: 16 pages, 10 figures, accepted for publication in Physical Review D -
contact authors: M.N. Mazziotta and V. Vasileio
Early Fermi Gamma-ray Space Telescope Observations of the Quasar 3C 454.3
This is the first report of Fermi Gamma-ray Space Telescope observations of
the quasar 3C 454.3, which has been undergoing pronounced long-term outbursts
since 2000. The data from the Large Area Telescope (LAT), covering 2008 July 7
- October 6, indicate strong, highly variable gamma-ray emission with an
average flux of ~3 x 10^{-6} photons cm^{-2} s^{-1}, for energies above 100
MeV. The gamma-ray flux is variable, with strong, distinct,
symmetrically-shaped flares for which the flux increases by a factor of several
on a time scale of about three days. This variability indicates a compact
emission region, and the requirement that the source is optically thin to
pair-production implies relativistic beaming with Doppler factor delta > 8,
consistent with the values inferred from VLBI observations of superluminal
expansion (delta ~ 25). The observed gamma-ray spectrum is not consistent with
a simple power-law, but instead steepens strongly above ~2 GeV, and is well
described by a broken power-law with photon indices of ~2.3 and ~3.5 below and
above the break, respectively. This is the first direct observation of a break
in the spectrum of a high luminosity blazar above 100 MeV, and it is likely
direct evidence for an intrinsic break in the energy distribution of the
radiating particles. Alternatively, the spectral softening above 2 GeV could be
due to gamma-ray absorption via photon-photon pair production on the soft X-ray
photon field of the host AGN, but such an interpretation would require the
dissipation region to be located very close (less than 100 gravitational radii)
to the black hole, which would be inconsistent with the X-ray spectrum of the
source.Comment: Accepted by the Astrophysical Journal; corresponding authors: Greg
Madejski ([email protected]) and Benoit Lott ([email protected]
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