6,649 research outputs found
Alternating groups and moduli space lifting Invariants
Main Theorem: Spaces of r-branch point 3-cycle covers, degree n or Galois of
degree n!/2 have one (resp. two) component(s) if r=n-1 (resp. r\ge n). Improves
Fried-Serre on deciding when sphere covers with odd-order branching lift to
unramified Spin covers. We produce Hurwitz-Torelli automorphic functions on
Hurwitz spaces, and draw Inverse Galois conclusions. Example: Absolute spaces
of 3-cycle covers with +1 (resp. -1) lift invariant carry canonical even (resp.
odd) theta functions when r is even (resp. odd). For inner spaces the result is
independent of r. Another use appears in,
http://www.math.uci.edu/~mfried/paplist-mt/twoorbit.html, "Connectedness of
families of sphere covers of A_n-Type." This shows the M(odular) T(ower)s for
the prime p=2 lying over Hurwitz spaces first studied by,
http://www.math.uci.edu/~mfried/othlist-cov/hurwitzLiu-Oss.pdf, Liu and
Osserman have 2-cusps. That is sufficient to establish the Main Conjecture: (*)
High tower levels are general-type varieties and have no rational points.For
infinitely many of those MTs, the tree of cusps contains a subtree -- a spire
-- isomorphic to the tree of cusps on a modular curve tower. This makes
plausible a version of Serre's O(pen) I(mage) T(heorem) on such MTs.
Establishing these modular curve-like properties opens, to MTs, modular
curve-like thinking where modular curves have never gone before. A fuller html
description of this paper is at
http://www.math.uci.edu/~mfried/paplist-cov/hf-can0611591.html .Comment: To appear in the Israel Journal as of 1/5/09; v4 is corrected from
proof sheets, but does include some proof simplification in \S
Resummations in the Bloch-Nordsieck model
We studied different levels of resummations of the exactly solvable
Bloch-Nordsieck model in order to be able to compare the approximations with an
exact result. We studied one-loop perturbation theory, 2PI resummation and
Schwinger-Dyson equations truncated in a way to maintain Ward-identities. At
all levels we carefully performed renormalization. We found that although the
2PI resummation does not exhibit infrared sensitivity at the mass shell (the
one-loop perturbation theory does), but it is still far from the exact
solution. The method of truncated Schwinger-Dyson equations, however, is exact
in this model, so it provides a new way of solving the Bloch-Nordsieck model.
This method can also be generalized to other, more complicated theories.Comment: 12 pages, 3 figures, revtex
Atmospheric image blur with finite outer scale or partial adaptive correction
Seeing-limited resolution in large telescopes working over wide wavelength
range depends substantially on the turbulence outer scale and cannot be
adequately described by one "seeing" value. We attempt to clarify frequent
confusions on this matter. We study the effects of finite turbulence outer
scale and partial adaptive corrections by means of analytical calculations and
numerical simulations. If a von Karman turbulence model is adopted, a simple
approximate formula captures the dependence of atmospheric long-exposure
resolution on the outer scale over the entire practically interesting range of
telescope diameters and wavelengths. In the infrared (IR), the difference with
the standard Kolmogorov seeing formula can exceed a factor of two. We find that
low-order adaptive turbulence correction produces residual wave-fronts with
effectively small outer scale, so even very low compensation order leads to a
substantial improvement in resolution over seeing, compared to the standard
theory. Seeing-limited resolution of large telescopes, especially in the IR, is
currently under-estimated by not accounting for the outer scale. On the other
hand, adaptive-optics systems designed for diffraction-limited imaging in the
IR can improve the resolution in the visible by as much as two times.Comment: A&A accepte
The Double Quasar Q2138-431: Lensing by a Dark Galaxy?
We report the discovery of a new gravitational lens candidate Q2138-431AB,
comprising two quasar images at a redshift of 1.641 separated by 4.5 arcsecs.
The spectra of the two images are very similar, and the redshifts agree to
better than 115 km.sec. The two images have magnitudes and
, and in spite of a deep search and image subtraction procedure, no
lensing galaxy has been found with . Modelling of the system
configuration implies that the mass-to-light ratio of any lensing galaxy is
likely to be around , with an absolute lower limit of
for an Einstein-de Sitter universe. We conclude that
the most likely explanation of the observations is gravitational lensing by a
dark galaxy, although it is possible we are seeing a binary quasar.Comment: 17 pages (Latex), 8 postscript figures included, accepted by MNRA
Obliquely propagating electromagnetic waves in magnetized kappa plasmas
Velocity distribution functions (VDFs) that exhibit a power-law dependence on
the high-energy tail have been the subject of intense research by the plasma
physics community. Such functions, known as kappa or superthermal
distributions, have been found to provide a better fitting to the VDFs measured
by spacecraft in the solar wind. One of the problems that is being addressed on
this new light is the temperature anisotropy of solar wind protons and
electrons. In the literature, the general treatment for waves excited by
(bi-)Maxwellian plasmas is well-established. However, for kappa distributions,
the wave characteristics have been studied mostly for the limiting cases of
purely parallel or perpendicular propagation, relative to the ambient magnetic
field. Contributions to the general case of obliquely-propagating
electromagnetic waves have been scarcely reported so far. The absence of a
general treatment prevents a complete analysis of the wave-particle interaction
in kappa plasmas, since some instabilities can operate simultaneously both in
the parallel and oblique directions. In a recent work, Gaelzer and Ziebell [J.
Geophys. Res. 119, 9334 (2014)] obtained expressions for the dielectric tensor
and dispersion relations for the low-frequency, quasi-perpendicular dispersive
Alfv\'en waves resulting from a kappa VDF. In the present work, the formalism
introduced by Ref. 1 is generalized for the general case of electrostatic
and/or electromagnetic waves propagating in a kappa plasma in any frequency
range and for arbitrary angles. An isotropic distribution is considered, but
the methods used here can be easily applied to more general anisotropic
distributions, such as the bi-kappa or product-bi-kappa.Comment: Accepted for publication in Physics of Plasmas; added references for
section
On the thermodynamics of the Swift–Hohenberg theory
We present the microbalance including the microforces, the first- and second-order microstresses for the Swift–Hohenberg equation concomitantly with their constitutive equations, which are consistent with the free-energy imbalance. We provide an explicit form for the microstress structure for a free-energy functional endowed with second-order spatial derivatives. Additionally, we generalize the Swift–Hohenberg theory via a proper constitutive process. Finally, we present one highly resolved three-dimensional numerical simulation to demonstrate the particular form of the resulting microstresses and their interactions in the evolution of the Swift–Hohenberg equation
Speckle Statistics in Adaptively Corrected Images
(abridged) Imaging observations are generally affected by a fluctuating
background of speckles, a particular problem when detecting faint stellar
companions at small angular separations. Knowing the distribution of the
speckle intensities at a given location in the image plane is important for
understanding the noise limits of companion detection. The speckle noise limit
in a long-exposure image is characterized by the intensity variance and the
speckle lifetime. In this paper we address the former quantity through the
distribution function of speckle intensity. Previous theoretical work has
predicted a form for this distribution function at a single location in the
image plane. We developed a fast readout mode to take short exposures of
stellar images corrected by adaptive optics at the ground-based UCO/Lick
Observatory, with integration times of 5 ms and a time between successive
frames of 14.5 ms ( m). These observations temporally
oversample and spatially Nyquist sample the observed speckle patterns. We show,
for various locations in the image plane, the observed distribution of speckle
intensities is consistent with the predicted form. Additionally, we demonstrate
a method by which and can be mapped over the image plane. As the
quantity is proportional to the PSF of the telescope free of random
atmospheric aberrations, this method can be used for PSF calibration and
reconstruction.Comment: 7 pages, 4 figures, ApJ accepte
Stimulated Raman spin coherence and spin-flip induced hole burning in charged GaAs quantum dots
High-resolution spectral hole burning (SHB) in coherent nondegenerate
differential transmission spectroscopy discloses spin-trion dynamics in an
ensemble of negatively charged quantum dots. In the Voigt geometry, stimulated
Raman spin coherence gives rise to Stokes and anti-Stokes sidebands on top of
the trion spectral hole. The prominent feature of an extremely narrow spike at
zero detuning arises from spin population pulsation dynamics. These SHB
features confirm coherent electron spin dynamics in charged dots, and the
linewidths reveal spin spectral diffusion processes.Comment: 5 pages, 5 figure
MODIFICATION BY FRAILTY STATUS OF AMBIENT AIR POLLUTION EFFECTS ON LUNG FUNCTION IN OLDER ADULTS IN THE CARDIOVASCULAR HEALTH STUDY
Older adult susceptibility to air pollution health effects is well-recognized. Advanced age may act as a partial surrogate for conditions associated with aging. The authors investigated whether gerontologic frailty (a clinical health status metric) modified the effects of ambient ozone or particulate matter (PM10) air pollution on lung function in 3382 older adults using 7 years of followup data from the Cardiovascular Health Study (CHS) and the CHS Environmental Factors Ancillary Study. Monthly average pollution and annual frailty assessments were related to up to 3 repeated measurements of lung function using novel cumulative summaries of pollution and frailty histories that account for duration as well as concentration. Frailty history was found to modify long-term pollution effects on Forced Vital Capacity (FVC). For example, the decrease in FVC associated with a 70 ppb-month increase in the cumulative sum of monthly average O3 exposure was 8.8 mL (95% confidence interval (CI): 7.4, 10.1) for a woman who had spent the prior 7 years prefrail or frail compared to 3.3 mL (95% CI: 2.7, 4.0) for a similar not frail woman (interaction P\u3c0.001)
The optically-powerful quasar E1821+643 is associated with a 300-kpc scale FRI radio structure
We present a deep image of the optically-powerful quasar E1821+643 at 18cm
made with the Very Large Array (VLA). This image reveals radio emission, over
280 kpc in extent, elongated way beyond the quasar's host galaxy. Its radio
structure has decreasing surface brightness with increasing distance from the
bright core, characteristic of FRI sources (Fanaroff & Riley 1974). Its radio
luminosity at 5GHz falls in the classification for `radio-quiet' quasars (it is
only 10^23.9 W/Hz/sr; see e.g. Kellermann et al 1994). Its radio luminosity at
151MHz (which is 10^25.3 W/Hz/sr) is at the transition luminosity observed to
separate FRIs and FRIIs. Hitherto, no optically-powerful quasar had been found
to have a conventional FRI radio structure. For searches at low-frequency this
is unsurprising given current sensitivity and plausible radio spectral indices
for radio-quiet quasars. We demonstrate the inevitability of the extent of any
FRqI radio structures being seriously under-estimated by existing targetted
follow-up observations of other optically-selected quasars, which are typically
short exposures of z > 0.3 objects, and discuss the implications for the
purported radio bimodality in quasars.
The nature of the inner arcsec-scale jet in E1821+643, together with its
large-scale radio structure, suggest that the jet-axis in this quasar is
precessing (cf. Galactic jet sources such as SS433). A possible explanation for
this is that its central engine is a binary whose black holes have yet to
coalesce. The ubiquity of precession in `radio-quiet' quasars, perhaps as a
means of reducing the observable radio luminosity expected in highly-accreting
systems, remains to be established.Comment: Accepted by ApJ Letters; higher quality versions of figures available
at http://www-astro.physics.ox.ac.uk/~km
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