4,637 research outputs found
A Slower Superluminal Velocity for the Quasar 1156+295
As part of an ongoing effort to observe high energy gamma-ray blazars with
VLBI, we have produced 8 and 2 GHz VLBI images, at ten epochs spanning the
years 1988 to 1996, of the quasar 1156+295. The VLBI data have been taken from
the Washington VLBI correlator's geodetic database. We have detected detected
four components and have measured their apparent speeds to be 8.8 +/- 2.3, 5.3
+/- 1.1, 5.5 +/- 0.9, and 3.5 +/- 1.2 h^{-1}c from the outermost component
inwards. (H_{0}=100h km/(s Mpc), q_{0}=0.5 throughout paper). These velocities
contradict a previously published very high superluminal velocity of 26 h^{-1}c
for this source.Comment: 8 pages, 2 figures, accepted to ApJ Letter
Generalized Affine Coherent States: A Natural Framework for Quantization of Metric-like Variables
Affine variables, which have the virtue of preserving the positive-definite
character of matrix-like objects, have been suggested as replacements for the
canonical variables of standard quantization schemes, especially in the context
of quantum gravity. We develop the kinematics of such variables, discussing
suitable coherent states, their associated resolution of unity, polarizations,
and finally the realization of the coherent-state overlap function in terms of
suitable path-integral formulations.Comment: 17 pages, LaTeX, no figure
Interplay of spin-orbit coupling and Zeeman splitting in the absorption lineshape of 2D fermions
We suggest that electron spin resonance (ESR) experiment can be used as a
probe of spinon excitations of hypothetical spin-liquid state of frustrated
antiferromagnet in the presence of asymmetric Dzyaloshinskii-Moriya (DM)
interaction. We describe assumptions under which the ESR response is reduced to
the response of 2D electron gas with Rashba spin-orbit coupling. Unlike
previous treatments, the spin-orbit coupling, \Delta_{SO}, is not assumed small
compared to the Zeeman splitting, \Delta_Z. We demonstrate that ESR response
diverges at the edges of the absorption spectrum for ac magnetic field
perpendicular to the static field. At the compensation point,
\Delta_{SO}\approx \Delta_Z, the broad absorption spectrum exhibits features
that evolve with temperature, T, even when T is comparable to the Fermi energy.Comment: 11 pages, 6 figure
It's Hard to Learn How Gravity and Electromagnetism Couple
We construct the most general effective Lagrangian coupling gravity and
electromagnetism up to mass dimension 6 by enumerating all possible non-minimal
coupling terms respecting both diffeomorphism and gauge invariance. In all,
there are only two unique terms after field re-definitions; one is known to
arise from loop effects in QED while the other is a parity violating term which
may be generated by weak interactions within the standard model of particle
physics. We show that neither the cosmological propagation of light nor,
contrary to earlier claims, solar system tests of General Relativity are useful
probes of these terms. These non-minimal couplings of gravity and
electromagnetism may remain a mystery for the foreseeable future.Comment: 9 pages. Minor corrections made. To appear in Phys. Rev.
In the Shadow of the Accretion Disk: Higher Resolution Imaging of the Central Parsec in NGC 4261
The physical conditions in the inner parsec of accretion disks believed to
orbit the central black holes in active galactic nuclei can be probed by
imaging the absorption (by ionized gas in the disk) of background emission from
a radio counterjet. We report high angular resolution VLBI observations of the
nearby (about 40 Mpc) radio galaxy NGC 4261 that confirm free-free absorption
of radio emission from a counterjet by a geometrically thin, nearly edge-on
disk at 1.6, 4.8, and 8.4 GHz. The angular width and depth of the absorption
appears to increase with decreasing frequency, as expected. We derive an
average electron density of ~10E4 per cc at a disk radius of about 0.2 pc,
assuming that the inner disk inclination and opening angles are the same as at
larger radii. Pressure balance between the thermal gas and the magnetic field
in the disk implies an average field strength of 0.1 milligauss at a radius of
0.2 pc. These are the closest-in free-free absorption measurements to date of
the conditions in an extragalactic accretion disk orbiting a black hole with a
well-determined mass. If a standard advection-dominated accretion flow exists
in the disk center, then the transition between thin and thick disk regions
must occur at a radius less than 0.2 pc (4000 Schwarzschild radii).Comment: 20 pages including 12 figures. Accepted for publication in Ap
Constraining the initial temperature and shear viscosity in a hybrid hydrodynamic model of =200 GeV Au+Au collisions using pion spectra, elliptic flow, and femtoscopic radii
A new framework for evaluating hydrodynamic models of relativistic heavy ion
collisions has been developed. This framework, a Comprehesive Heavy Ion Model
Evaluation and Reporting Algorithm (CHIMERA) has been implemented by augmenting
UVH 2+1D viscous hydrodynamic model with eccentricity fluctuations,
pre-equilibrium flow, and the Ultra-relativistic Quantum Molecular Dynamic
(UrQMD) hadronic cascade. A range of initial temperatures and shear viscosity
to entropy ratios were evaluated for four initial profiles, and
scaling with and without pre-equilibrium flow. The model results
were compared to pion spectra, elliptic flow, and femtoscopic radii from 200
GeV Au+Au collisions for the 0--20% centrality range.Two sets of initial
density profiles, scaling with pre-equilibrium flow and
scaling without were shown to provide a consistent description of all three
measurements.Comment: 21 pages, 32 figures, version 3 includes additional text for
clarification, division of figures into more manageable units, and placement
of chi-squared values in tables for ease of viewin
Chapter: “Health Law and Ethics”
Law and ethics are both essential attributes of a high-functioning health care system and powerful explainers of why the existing system is so difficult to improve. U.S. health law is not seamless; rather, it derives from multiple sources and is based on various theories that may be in tension with one another. There are state laws and federal laws, laws setting standards and laws providing funding, laws reinforcing professional prerogatives, laws furthering social goals, and laws promoting market competition. Complying with law is important, but health professionals also should understand that the legal and ethical constraints under which health systems operate must themselves adapt if health systems science is to advance
Global Stationary Phase and the Sign Problem
We present a computational strategy for reducing the sign problem in the
evaluation of high dimensional integrals with non-positive definite weights.
The method involves stochastic sampling with a positive semidefinite weight
that is adaptively and optimally determined during the course of a simulation.
The optimal criterion, which follows from a variational principle for analytic
actions S(z), is a global stationary phase condition that the average gradient
of the phase Im(S) along the sampling path vanishes. Numerical results are
presented from simulations of a model adapted from statistical field theories
of classical fluids.Comment: 9 pages, 3 figures, submitted for publicatio
Surprising phenomena in a rich new class of inflationary models
We report on a new class of fast-roll inflationary models. In a huge part of
its parameter space, inflationary perturbations exhibit quite unusual phenomena
such as scalar and tensor modes freezing out at widely different times, as well
as scalar modes reentering the horizon during inflation. In another, narrower
range of parameters, this class of models agrees with observations. One
specific point in parameter space is characterized by extraordinary behavior of
the scalar perturbations. Freeze-out of scalar perturbations as well as
particle production at horizon crossing are absent. Also the behavior of the
perturbations around this quasi-de Sitter background is dual to a quantum field
theory in flat space-time. Finally, the form of the primordial power spectrum
is determined by the interaction between different modes of scalar
perturbations.Comment: 12 pages, 5 figures, 1 table, references + comments added, errors
corrected, conclusions unchanged, version published in JCA
A Search for the Damped Ly-alpha Absorber at z=1.86 toward QSO 1244+3443 with NICMOS
We have carried out a high-resolution imaging search for the galaxy
associated with the damped Lyman-alpha (DLA) absorber at z=1.859 toward the
z_{em}=2.48 quasar QSO 1244+3443, using the HST and the NICMOS. Images were
obtained in the broad filter F160W and the narrow filter F187N with camera 2 on
NICMOS with the goal of detecting the rest-frame optical continuum and the
H-alpha line emission from the DLA. After PSF subtraction, two weak features
are seen at projected separations of 0.16-0.24" from the quasar. Parts of these
features may be associated with the DLA absorber, although we cannot completely
rule out that they could be artifacts of the point spread function (PSF). If
associated with the DLA, the objects would be ~1-2 h_{70}^{-1} kpc in size with
integrated flux densities of 2.5 and 3.3 mu Jy in the F160W filter, implying
luminosities at lambda_{central}=5600 A in the DLA rest frame of 4.4-5.9 x
10^{9} h_{70}^{-2} L_{solar} at z=1.86, for q0=0.5. However, no significant
H-alpha line emission is seen from these objects, suggesting low star formation
rates (SFRs). Our 3 sigma upper limit on the SFR in the DLA is 1.3 h_{70}^{-2}
M_{solar}/yr for q0 = 0.5 (2.4 h_{70}^{-2} M_{solar} yr^{-1} for q0 = 0.1).
This together with our earlier result for LBQS 1210+1731 mark a significant
improvement over previous constraints on the star formation rates of DLAs. A
combination of low SFR and some dust extinction is likely to be responsible for
the lack of H-alpha emission. Alternatively, the objects, may be associated
with the quasar host galaxy. In any case, our observations suggest that the DLA
is not a large bright proto-disk, but a compact object or a low-surface
brightness galaxy. If the two features are PSF artifacts then the constraints
on DLA properties are even more severe.Comment: 28 pages, 9 figures. Accepted for publication in The Astrophysical
Journal. Figures are given at a slightly lower resolution here, to decrease
file sizes. The higher resolution versions can be found in the Ap
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