23,773 research outputs found
A Tidal Flare Candidate in Abell 1795
As part of our ongoing archival X-ray survey of galaxy clusters for tidal
flares, we present evidence of an X-ray transient source within 1 arcmin of the
core of Abell 1795. The extreme variability (a factor of nearly 50), luminosity
(> 2 x 10^42 erg s^{-1}), long duration (> 5 years) and supersoft X-ray
spectrum (< 0.1 keV) are characteristic signatures of a stellar tidal
disruption event according to theoretical predictions and to existing X-ray
observations, implying a massive >~10^5 M_sun black hole at the centre of that
galaxy. The large number of X-ray source counts (~700) and long temporal
baseline (~12 years with Chandra and XMM-Newton) make this one of the
best-sampled examples of any tidal flare candidate to date. The transient may
be the same EUV source originally found contaminating the diffuse ICM
observations of Bowyer et al. (1999), which would make it the only tidal flare
candidate with reported EUV observations and implies an early source luminosity
1-2 orders of magnitude greater. If the host galaxy is a cluster member then it
must be a dwarf galaxy, an order of magnitude less massive than the quiescent
galaxy Henize 2-10 which hosts a massive black hole that is difficult to
reconcile with its low mass. The unusual faintness of the host galaxy may be
explained by tidal stripping in the cluster core.Comment: Accepted by MNRAS 2013 July 23. 27 pages, 10 figure
Connections of activated hopping processes with the breakdown of the Stokes-Einstein relation and with aspects of dynamical heterogeneities
We develop a new extended version of the mode-coupling theory (MCT) for glass
transition, which incorporates activated hopping processes via the dynamical
theory originally formulated to describe diffusion-jump processes in crystals.
The dynamical-theory approach adapted here to glass-forming liquids treats
hopping as arising from vibrational fluctuations in quasi-arrested state where
particles are trapped inside their cages, and the hopping rate is formulated in
terms of the Debye-Waller factors characterizing the structure of the
quasi-arrested state. The resulting expression for the hopping rate takes an
activated form, and the barrier height for the hopping is ``self-generated'' in
the sense that it is present only in those states where the dynamics exhibits a
well defined plateau. It is discussed how such a hopping rate can be
incorporated into MCT so that the sharp nonergodic transition predicted by the
idealized version of the theory is replaced by a rapid but smooth crossover. We
then show that the developed theory accounts for the breakdown of the
Stokes-Einstein relation observed in a variety of fragile glass formers. It is
also demonstrated that characteristic features of dynamical heterogeneities
revealed by recent computer simulations are reproduced by the theory. More
specifically, a substantial increase of the non-Gaussian parameter, double-peak
structure in the probability distribution of particle displacements, and the
presence of a growing dynamic length scale are predicted by the extended MCT
developed here, which the idealized version of the theory failed to reproduce.
These results of the theory are demonstrated for a model of the Lennard-Jones
system, and are compared with related computer-simulation results and
experimental data.Comment: 13 pages, 5 figure
Multilevel semantic analysis and problem-solving in the flight domain
A computer based cockpit system which is capable of assisting the pilot in such important tasks as monitoring, diagnosis, and trend analysis was developed. The system is properly organized and is endowed with a knowledge base so that it enhances the pilot's control over the aircraft while simultaneously reducing his workload
Branes from a non-Abelian (2,0) tensor multiplet with 3-algebra
In this paper, we study the equations of motion for non-Abelian N=(2,0)
tensor multiplets in six dimensions, which were recently proposed by Lambert
and Papageorgakis. Some equations are regarded as constraint equations. We
employ a loop extension of the Lorentzian three-algebra (3-algebra) and examine
the equations of motion around various solutions of the constraint equations.
The resultant equations take forms that allow Lagrangian descriptions. We find
various (5+d)-dimensional Lagrangians and investigate the relation between them
from the viewpoint of M-theory duality.Comment: 44+1 pages, reference added, typos corrected, and several discussions
added; v3, reference added, many typos corrected, the language improved; v4,
some typos and references corrected, final version to appear in J. Phys.
Celestial mechanics in Kerr spacetime
The dynamical parameters conventionally used to specify the orbit of a test
particle in Kerr spacetime are the energy , the axial component of the
angular momentum, , and Carter's constant . These parameters are
obtained by solving the Hamilton-Jacobi equation for the dynamical problem of
geodesic motion. Employing the action-angle variable formalism, on the other
hand, yields a different set of constants of motion, namely, the fundamental
frequencies , and associated with
the radial, polar and azimuthal components of orbital motion. These
frequencies, naturally, determine the time scales of orbital motion and,
furthermore, the instantaneous gravitational wave spectrum in the adiabatic
approximation. In this article, it is shown that the fundamental frequencies
are geometric invariants and explicit formulas in terms of quadratures are
derived. The numerical evaluation of these formulas in the case of a rapidly
rotating black hole illustrates the behaviour of the fundamental frequencies as
orbital parameters such as the semi-latus rectum , the eccentricity or
the inclination parameter are varied. The limiting cases of
circular, equatorial and Keplerian motion are investigated as well and it is
shown that known results are recovered from the general formulas.Comment: 25 pages (LaTeX), 5 figures, submitted to Class. Quantum Gra
Superconductivity, magnetic order, and quadrupolar order in the filled skutterudite system PrNdOsSb
Superconductivity, magnetic order, and quadrupolar order have been
investigated in the filled skutterudite system
PrNdOsSb as a function of composition in magnetic
fields up to 9 tesla and at temperatures between 50 mK and 10 K. Electrical
resistivity measurements indicate that the high field ordered phase (HFOP),
which has been identified with antiferroquadruoplar order, persists to
0.5. The superconducting critical temperature of PrOsSb
is depressed linearly with Nd concentration to 0.55, whereas the
Curie temperature of NdOsSb is depressed linearly with Pr
composition to () 0.45. In the superconducting region, the upper
critical field is depressed quadratically with in the range 0
0.3, exhibits a kink at 0.3, and then
decreases linearly with in the range 0.3 0.6. The
behavior of appears to be due to pair breaking caused by the
applied magnetic field and the exhange field associated with the polarization
of the Nd magnetic moments, in the superconducting state. From magnetic
susceptibility measurements, the correlations between the Nd moments in the
superconducting state appear to change from ferromagnetic in the range 0.3
0.6 to antiferromagnetic in the range 0
0.3. Specific heat measurements on a sample with 0.45
indicate that magnetic order occurs in the superconducting state, as is also
inferred from the depression of with .Comment: 7 pages, 7 figures, currently submitted to Phys. Rev.
Magnetic Hydrogen Atmosphere Models and the Neutron Star RX J1856.5-3754
RX J1856.5-3754 is one of the brightest nearby isolated neutron stars, and
considerable observational resources have been devoted to it. However, current
models are unable to satisfactorily explain the data. We show that our latest
models of a thin, magnetic, partially ionized hydrogen atmosphere on top of a
condensed surface can fit the entire spectrum, from X-rays to optical, of RX
J1856.5-3754, within the uncertainties. In our simplest model, the best-fit
parameters are an interstellar column density N_H \approx 1x10^20 cm^-2 and an
emitting area with R^infty \approx 17 km (assuming a distance to RX
J1856.5-3754 of 140 pc), temperature T^infty \approx 4.3x10^5 K, gravitational
redshift z_g \sim 0.22, atmospheric hydrogen column y_H \approx 1 g cm^-2, and
magnetic field B \approx (3-4)x10^12 G; the values for the temperature and
magnetic field indicate an effective average over the surface. We also
calculate a more realistic model, which accounts for magnetic field and
temperature variations over the neutron star surface as well as general
relativistic effects, to determine pulsations; we find there exist viewing
geometries that produce pulsations near the currently observed limits. The
origin of the thin atmospheres required to fit the data is an important
question, and we briefly discuss mechanisms for producing these atmospheres.
Our model thus represents the most self-consistent picture to date for
explaining all the observations of RX J1856.5-3754.Comment: 11 pages, 8 figures; MNRAS, accepte
Automatic spacecraft docking using computer vision-based guidance and control techniques
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76754/1/AIAA-21001-304.pd
On the quantisation of the angular momentum
When a hydrogen-like atom is treated as a two dimensional system whose
configuration space is multiply connected, then in order to obtain the same
energy spectrum as in the Bohr model the angular momentum must be
half-integral.Comment: Latex, 5 page
Particle abundance in a thermal plasma: quantum kinetics vs. Boltzmann equation
We study the abundance of a particle species in a thermalized plasma by
introducing a quantum kinetic description based on the non-equilibrium
effective action. A stochastic interpretation of quantum kinetics in terms of a
Langevin equation emerges naturally. We consider a particle species that is
stable in the vacuum and interacts with \emph{heavier} particles that
constitute a thermal bath in equilibrium and define of a fully renormalized
single particle distribution function. The distribution function thermalizes on
a time scale determined by the \emph{quasiparticle} relaxation rate. The
equilibrium distribution function depends on the full spectral density and
features off-shell contributions to the particle abundance. A model of a
bosonic field in interaction with two \emph{heavier} bosonic fields is
studied. We find substantial departures from the Bose-Einstein result both in
the high temperature and the low temperature but high momentum region. In the
latter the abundance is exponentially suppressed but larger than the
Bose-Einstein result. We obtain the Boltzmann equation in renormalized
perturbation theory and highlight the origin of the differences. We argue that
the corrections to the abundance of cold dark matter candidates are
observationally negligible and that recombination erases any possible spectral
distortions of the CMB. However we expect that the enhancement at high
temperature may be important for baryogenesis.Comment: 39 pages, 11 figures. Clarifying remarks. To appear in Physical
Review
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