5,325 research outputs found
Null Energy Condition Violation and Classical Stability in the Bianchi I Metric
The stability of isotropic cosmological solutions in the Bianchi I model is
considered. We prove that the stability of isotropic solutions in the Bianchi I
metric for a positive Hubble parameter follows from their stability in the
Friedmann-Robertson-Walker metric. This result is applied to models inspired by
string field theory, which violate the null energy condition. Examples of
stable isotropic solutions are presented. We also consider the k-essence model
and analyse the stability of solutions of the form .Comment: 27 pages, references added, accepted for publication in Phys. Rev.
Space Weather Monitoring for ISS Space Environments Engineering and Crew Auroral Observations
The awareness of potentially significant impacts of space weather on spaceand ground ]based technological systems has generated a strong desire in many sectors of government and industry to effectively transform knowledge and understanding of the variable space environment into useful tools and applications for use by those entities responsible for systems that may be vulnerable to space weather impacts. Essentially, effectively transitioning science knowledge to useful applications relevant to space weather has become important. This talk will present proven methodologies that have been demonstrated to be effective, and how in the current environment those can be applied to space weather transition efforts
Transmission of correlated electrons through sharp domain walls in magnetic nanowires: a renormalization group approach
The transmission of correlated electrons through a domain wall in a
ferromagnetic one dimensional system is studied theoretically in the limit of a
domain wall width smaller or comparable to the electron Fermi wavelength. The
domain wall gives rise to both potential and spin dependent scattering of the
charge carriers. Using a poor man's renormalization group approach for the
electron-electron interactions, we obtain the low temperature behavior of the
reflection and transmission coefficients. The results show that the
low-temperature conductance is governed by the electron correlations, which may
suppress charge transport without suppressing spin current. The results may
account for a huge magnetoresistance associated with a domain wall in ballistic
nanocontacs.Comment: 13 pages, 6 figure
Evolution of Global Relativistic Jets: Collimations and Expansion with kKHI and the Weibel Instability
One of the key open questions in the study of relativistic jets is their
interaction with the environment. Here, we study the initial evolution of both
electron-proton and electron-positron relativistic jets, focusing on their
lateral interaction with the ambient plasma. We trace the generation and
evolution of the toroidal magnetic field generated by both kinetic
Kelvin-Helmholtz (kKH) and Mushroom instabilities (MI). This magnetic field
collimates the jet. We show that in electron-proton jet, electrons are
perpendicularly accelerated with jet collimation. The magnetic polarity
switches from the clockwise to anti-clockwise in the middle of jet, as the
instabilities weaken. For the electron-positron jet, we find strong mixture of
electron-positron with the ambient plasma, that results in the creation of a
bow shock. Merger of magnetic field current filaments generate density bumps
which initiate a forward shock. The strong mixing between jet and ambient
particles prevents full development of the jet on the studied scale. Our
results therefore provide a direct evidence for both jet collimation and
particle acceleration in the created bow shock. Differences in the magnetic
field structures generated by electron-proton and electron-positron jets may
contribute to observable differences in the polarized properties of emission by
electrons.Comment: 25 pages, 12 figures, ApJ, accepte
The Hypervelocity Star SDSS J090745.0+024507 is a Short-Period Variable
We present high-precision photometry of the hypervelocity star SDSS
J090745.0+024507 (HVS), which has a Galactic rest-frame radial velocity of
v=709 km/s, and so has likely been ejected from the supermassive black hole in
the Galactic center. Our data were obtained on two nights using the MMT 6.5m
telescope, and is supplemented by lower precision photometry obtained on four
nights using the FLWO 1.2m telescope. The high-precision photometry indicates
that the HVS is a short-period, low-amplitude variable, with period P=0.2-2
days and amplitude A = 2-10%. Together with the known effective temperature of
T_eff ~ 10,500 K (spectral type B9), this variability implies that the HVS is a
member of the class of slowly pulsating B-type main sequence stars, thus
resolving the previously-reported two-fold degeneracy in the luminosity and
distance of the star. The HVS has a heliocentric distance of 71 kpc, and an age
of ~0.35 Gyr. The time of ejection from the center of the Galaxy is < 100 Myr,
and thus the existence of the OS constitutes observational evidence of a
population of young stars in the proximity of the central supermassive black
hole ~0.1 Gyr ago. It is possible that the HVS was a member of a binary that
was tidally disrupted by the central black hole; we discuss constraints on the
properties of the companion's orbit.Comment: ApJL, submitted, 4 pages, 4 figure
Experimental and theoretical lifetimes and transition probabilities in Sb I
We present experimental atomic lifetimes for 12 levels in Sb I, out of which
seven are reported for the first time. The levels belong to the 5p(P)6s
P, P and 5p(P)5d P, F and F terms. The
lifetimes were measured using time-resolved laser-induced fluorescence. In
addition, we report new calculations of transition probabilities in Sb I using
a Multiconfigurational Dirac-Hartree-Fock method. The physical model being
tested through comparisons between theoretical and experimental lifetimes for
5d and 6s levels. The lifetimes of the 5d F levels (19.5,
7.8 and 54 ns, respectively) depend strongly on the -value. This is
explained by different degrees of level mixing for the different levels in the
F term.Comment: 10 page
A Note on Scalar Field Theory in AdS_3/CFT_2
We consider a scalar field theory in AdS_{d+1}, and introduce a formalism on
surfaces at equal values of the radial coordinate. In particular, we define the
corresponding conjugate momentum. We compute the Noether currents for
isometries in the bulk, and perform the asymptotic limit on the corresponding
charges. We then introduce Poisson brackets at the border, and show that the
asymptotic values of the bulk scalar field and the conjugate momentum transform
as conformal fields of scaling dimensions \Delta_{-} and \Delta_{+},
respectively, where \Delta_{\pm} are the standard parameters giving the
asymptotic behavior of the scalar field in AdS. Then we consider the case d=2,
where we obtain two copies of the Virasoro algebra, with vanishing central
charge at the classical level. An AdS_3/CFT_2 prescription, giving the
commutators of the boundary CFT in terms of the Poisson brackets at the border,
arises in a natural way. We find that the boundary CFT is similar to a
generalized ghost system. We introduce two different ground states, and then
compute the normal ordering constants and quantum central charges, which depend
on the mass of the scalar field and the AdS radius. We discuss certain
implications of the results.Comment: 24 pages. v2: added minor clarification. v3: added several comments
and discussions, abstract sligthly changed. Version to be publishe
Stochastic Flux-Freezing and Magnetic Dynamo
We argue that magnetic flux-conservation in turbulent plasmas at high
magnetic Reynolds numbers neither holds in the conventional sense nor is
entirely broken, but instead is valid in a novel statistical sense associated
to the "spontaneous stochasticity" of Lagrangian particle tra jectories. The
latter phenomenon is due to the explosive separation of particles undergoing
turbulent Richardson diffusion, which leads to a breakdown of Laplacian
determinism for classical dynamics. We discuss empirical evidence for
spontaneous stochasticity, including our own new numerical results. We then use
a Lagrangian path-integral approach to establish stochastic flux-freezing for
resistive hydromagnetic equations and to argue, based on the properties of
Richardson diffusion, that flux-conservation must remain stochastic at infinite
magnetic Reynolds number. As an important application of these results we
consider the kinematic, fluctuation dynamo in non-helical, incompressible
turbulence at unit magnetic Prandtl number. We present results on the
Lagrangian dynamo mechanisms by a stochastic particle method which demonstrate
a strong similarity between the Pr = 1 and Pr = 0 dynamos. Stochasticity of
field-line motion is an essential ingredient of both. We finally consider
briefly some consequences for nonlinear MHD turbulence, dynamo and reconnectionComment: 29 pages, 10 figure
XMM-Newton observations of the first unidentified TeV gamma-ray source TeV J2032+4130
(abridged) The first unidentified very high energy gamma ray source (TeV
J2032+4130) in the Cygnus region has been the subject of intensive search for a
counterpart source at other wavelengths. A deep ( ksec) exposure of
TeV J2032+4130 with \textit{XMM-Newton} has been obtained. The contribution of
point sources to the observed X-ray emission from TeV J2032+4130 is subtracted
from the data. The point-source subtracted X-ray data are analyzed using blank
sky exposures and regions adjacent to the position of TeV J2032+4130 in the
field of view covered by the XMM-Newton telescopes to search for diffuse X-ray
emission. An extended X-ray emission region with a full width half maximum
(FWHM) size of arc min is found. The centroid of the emission is
co-located with the position of TeV J2032+4130.The energy spectrum of the
emission coinciding with the position and extension of TeV J2032+4130 can be
modeled by a power-law model with a photon index
and an energy flux
integrated between 2 and 10 keV of ergs/(cm s) which is lower than the very high energy gamma-ray
flux observed from TeV J2032+4130. We conclude that the faint extended X-ray
emission discovered in this observation is the X-ray counterpart of TeV
J2032+4130. Formally, it can not be excluded that the extended emission is due
to an unrelated population of faint, hot ( keV) unresolved
point-sources which by chance coincides with the position and extension of TeV
J2032+4130. We discuss our findings in the frame of both hadronic and leptonic
gamma-ray production scenarios.Comment: 5 Pages, 3 Figures, accepted for publication in A&
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