26,116 research outputs found
Singular order parameter interaction at nematic quantum critical point in two dimensional electron systems
We analyze the infrared behavior of effective N-point interactions between
order parameter fluctuations for nematic and other quantum critical electron
systems with a scalar order parameter in two dimensions. The interactions
exhibit a singular momentum and energy dependence and thus cannot be
represented by local vertices. They diverge for all N greater or equal 4 in a
collinear static limit, where energy variables scale to zero faster than
momenta, and momenta become increasingly collinear. The degree of divergence is
not reduced by any cancellations and renders all N-point interactions marginal.
A truncation of the order parameter action at quartic or any other finite order
is therefore not justified. The same conclusion can be drawn for the effective
action describing fermions coupled to a U(1) gauge field in two dimensions.Comment: 18 pages, 1 figur
Error bounds on block Gauss Seidel solutions of coupled\ud multiphysics problems
Mathematical models in many fields often consist of coupled sub–models, each of which describe a different physical process. For many applications, the quantity of interest from these models may be written as a linear functional of the solution to the governing equations. Mature numerical solution techniques for the individual sub–models often exist. Rather than derive a numerical solution technique for the full coupled model, it is therefore natural to investigate whether these techniques may be used by coupling in a block Gauss–Seidel fashion. In this study, we derive two a posteriori bounds for such linear functionals. These bounds may be used on each Gauss–Seidel iteration to estimate the error in the linear functional computed using the single physics solvers, without actually solving the full, coupled problem. We demonstrate the use of the bound first by using a model problem from linear algebra, and then a linear ordinary differential equation example. We then investigate the effectiveness of the bound using a non–linear coupled fluid–temperature problem. One of the bounds derived is very sharp for most linear functionals considered, allowing us to predict very accurately when to terminate our block Gauss–Seidel iteration.\ud
\ud
Copyright c 2000 John Wiley & Sons, Ltd
The spectroscopic evolution of the -ray emitting classical nova Nova Mon 2012. I. Implications for the ONe subclass of classical novae
Nova Mon 2012 was the first classical nova to be detected as a high energy
-ray transient, by Fermi-LAT, before its optical discovery. We study a
time sequence of high resolution optical echelle spectra (Nordic Optical
Telescope) and contemporaneous NOT, STIS UV, and CHIRON echelle spectra (Nov
20/21/22). We use [O III] and H line fluxs to constrain the properties
of the ejecta. We derive the structure from the optical and UV line profiles
and compare our measured line fluxes for with predictions using Cloudy with
abundances from other ONe novae. Mon 2012 is confirmed as an ONe nova. We find
E(B-V)=0.850.05 and hydrogen column density
cm. The corrected continuum luminosity is nearly the same in the entire
observed energy range as V1974 Cyg, V382 Mon, and Nova LMC 2000 at the same
epoch after outburst. The distance, about 3.6 kpc, is quite similar to V1974
Cyg. The line profiles can be modeled using an axisymmetric bipolar geometry
for the ejecta with various inclinations of the axis to the line of sight, 60
\le i \le 80 degrees, an opening angle of \approx\Delta
R/R(t)\approx 0.4f\approx 0.1-0.3\leq 6\times
10^{-5}_\odot\gamma$-ray emission may be a generic phenomenon, common to all ONe novae,
possibly to all classical novae, and connected with acceleration and emission
processes within the ejecta (abstract severely truncated).Comment: Submitted to A&A 9/1/2013; Accepted 27/2/2013 (in press
Dynamics of Warm-Absorbing Gas in Seyfert Galaxies: NGC 5548
A hydromagnetic (MHD) wind from a clumpy molecular accretion disk is invoked
to explain observations of warm absorbing (WA) gas in UVX from Sy galaxies.
This paper focuses on two issues: (1) compatibility of kinematics and dynamics
of MHD wind with the observed properties of WAs; and (2) relationship between
the UVX absorptions. We provide an in-depth comparison between the MHD model
and the Sy 1 galaxy NGC 5548, which at high spectral resolution exhibits a
number of discrete UV absorption components. We find that: (1) the total column
densities of Ovii, Oviii and H, are reproduced by constraining the UV ion
column densities of Civ and Nv in each component to lie within a factor of 2 of
their observed values and optimizing over the possible sets of component
ionization states and Civ column densities; (2) the WA exists in the outer part
of the wind and is not a continuation of the flow in the BLR; and (3) the WA
extends in radial and polar directions and is ionization-stratified. X-ray
absorption is found to be heavily biased towards smaller r, and UV absorption
originates at larger distances from the central continuum source. We show that
the discrete absorption components along the line-of-sight are intrinsically
clumpy. Density differences between kinematic components result in a range of
ionization and recombination timescales. We further test the applicability of
the MHD wind to WAs in general, by constructing a quasi-continuous flow model,
and extending it to arbitrary aspect angles. We estimate the fraction of Sy 1s
having detectable WAs with larger Ovii column density than Oviii, and the range
of total H column densities. We also find that the ratio of Ovii to Oviii
optical depths can serve as a new diagnostic of AGN aspect angle.Comment: Latex, 8 postscript figures. Astrophysical Journal, 536, June 10, in
pres
Stringent neutron-star limits on large extra dimensions
Supernovae (SNe) are copious sources for Kaluza-Klein gravitons which are
generic for theories with large extra dimensions. These massive particles are
produced with average velocities ~0.5 c so that many of them are
gravitationally retained by the SN core. Every neutron star thus has a halo of
KK gravitons which decay into nu bar-nu, e^+e^- and gamma gamma on time scales
\~10^9 years. The EGRET gamma-flux limits (E_gamma ~ 100 MeV) for nearby
neutron stars constrain the fundamental scale for n=2 extra dimensions to M
>500 TeV, and M>30 TeV for n=3. The upcoming GLAST satellite is a factor ~30
more sensitive and thus may detect KK decays, for example at the nearby neutron
star RX J185635--3754. The requirement that neutron stars are not excessively
heated by KK decays implies M>1700 TeV for n=2, and M>60 TeV for n=3.Comment: Minor changes, matches version to appear in PR
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Profoundly reduced neovascularization capacity of bone marrow mononuclear cells derived from patients with chronic ischemic heart disease
Background— Cell therapy with bone marrow–derived stem/progenitor cells is a novel option for improving neovascularization and cardiac function in ischemic heart disease. Circulating endothelial progenitor cells in patients with coronary heart disease are impaired with respect to number and functional activity. However, whether this impairment also extends to bone marrow–derived mononuclear cells (BM-MNCs) in patients with chronic ischemic cardiomyopathy (ICMP) is unclea
Transport coefficients from the Boson Uehling-Uhlenbeck Equation
We derive microscopic expressions for the bulk viscosity, shear viscosity and
thermal conductivity of a quantum degenerate Bose gas above , the critical
temperature for Bose-Einstein condensation. The gas interacts via a contact
potential and is described by the Uehling-Uhlenbeck equation. To derive the
transport coefficients, we use Rayleigh-Schrodinger perturbation theory rather
than the Chapman-Enskog approach. This approach illuminates the link between
transport coefficients and eigenvalues of the collision operator. We find that
a method of summing the second order contributions using the fact that the
relaxation rates have a known limit improves the accuracy of the computations.
We numerically compute the shear viscosity and thermal conductivity for any
boson gas that interacts via a contact potential. We find that the bulk
viscosity remains identically zero as it is for the classical case.Comment: 10 pages, 2 figures, submitted to Phys. Rev.
Complex X-ray Absorption and the Fe Kalpha Profile in NGC 3516
We present data from simultaneous Chandra, XMM-Newton and BeppoSAX
observations of the Seyfert 1 galaxy NGC 3516, taken during 2001 April and Nov.
We have investigated the nature of the very flat observed X-ray spectrum.
Chandra grating data show the presence of X-ray absorption lines, revealing two
distinct components of the absorbing gas, one which is consistent with our
previous model of the UV/X-ray absorber while the other, which is outflowing at
a velocity of ~1100 km/s has a larger column density and is much more highly
ionized. The broad-band spectral characteristics of the X-ray continuum
observed with XMM during 2001 April, reveal the presence of a third layer of
absorption consisting of a very large column (~2.5 x 10E23 cm^-2) of highly
ionized gas with a covering fraction ~50%. This low covering fraction suggests
that the absorber lies within a few lt-days of the X-ray source and/or is
filamentary in structure. Interestingly, these absorbers are not in thermal
equilibrium with one another. The two new components are too highly ionized to
be radiatively accelerated, which we suggest is evidence for a hydromagnetic
origin for the outflow. Applying our model to the Nov dataset, we can account
for the spectral variability primarily by a drop in the ionization states of
the absorbers, as expected by the change in the continuum flux. When this
complex absorption is accounted for we find the underlying continuum to be
typical of Seyfert 1 galaxies. The spectral curvature attributed to the high
column absorber, in turn, reduces estimates of the flux and extent of any broad
Fe emission line from the accretion disk.Comment: 33 pages, 9 figures, accepted for publication in Ap
Variable UV Absorption in the Seyfert 1.5 Galaxy NGC 3516: The Case for Associated UV and X-ray Absorption
We present observations of the UV absorption lines in the Seyfert 1 galaxy
NGC 3516, obtained at a resolution of /
40,000 with the Space Telescope Imaging Spectrograph (STIS) on 2000 October 1.
The UV continuum was 4 times lower than that observed during 1995 with
the Goddard High Resolution Spectrograph (GHRS), and the X-ray flux from a
contemporaneous {\it Chandra X-ray Observatory (CXO)} observation was a factor
of 8 below that observed with {\it ASCA}. The STIS spectra show kinematic
components of absorption in Ly, C IV, and N V at radial velocities of
-376, -183, and -36 km s (components 1, 2, and 3+4, respectively), which
were detected in the earlier GHRS spectra; the last of these is a blend of two
GHRS components that have increased greatly in column density. Four additional
absorption components have appeared in the STIS spectra at radial velocities of
-692, -837, -994, and -1372 km s (components 5 through 8); these may
also have been present in earlier low-flux states observed by the {\it
International Ultraviolet Explorer (IUE)}. Based on photoionization models, we
suggest that the components are arranged in increasing radial distance in the
order, 3+4, 2, 1, followed by components 5 -- 8. We have achieved an acceptable
fit to the X-ray data using the combined X-ray opacity of the UV components 1,
2 and 3+4. By increasing the UV and X-ray fluxes of these models to match the
previous high states, we are able to match the GHRS C IV column densities,
absence of detectable C IV absorption in components 5 through 8, and the 1994
{\it ASCA} spectrum. We conclude that variability of the UV and X-ray
absorption in NGC 3516 is primarily due to changes in the ionizing flux.Comment: 7 figures (note that Fig6 is not referenced in the .Tex file and must
be printed separately). There are 6 tables in the .tex file and an additional
8 tables included as separate .ps files. Accepted for Publication in the
Astrophysical Journa
"Dark Matter" in Accretion Disks
Using Spitzer Space Telescope photometric observations of the eclipsing,
interacting binary WZ Sge, we have discovered that the accretion disk is far
more complex than previously believed. Our 4.5 and 8 micron time series
observations reveal that the well known gaseous accretion disk is surrounded by
an asymmetric disk of dusty material with a radius approximately 15 times
larger than the gaseous disk. This dust ring contains only a small amount of
mass and is completely invisible at optical and near-IR wavelengths, hence
consisting of "dark matter". We have produced a model dust ring using 1 micron
spherical particles with a density of 3 g/cm and with a temperature profile
ranging from 700-1500K. Our discovery about the accretion disk structure and
the presence of a larger, outer dust ring have great relevance for accretion
disks in general, including those in other interacting binary systems, pre-main
sequence stars, and active galaxies.Comment: 34 pages, 8 figures (3 in color). Accepted to Ap
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