18,095 research outputs found
A System of Subroutines For Iteratively Reweighted Least Squares Computations
A description of a system of subroutines to compute solutions to the iteratively reweighted least squares problem is presented. The weights are determined from the data and linear fit and are computed as functions of the scaled residuals. Iteratively reweighted least squares is a part of robust statistics where "robustness" means relative insensitivity to moderate departures from assumptions. The software for iteratively reweighted least squares is cast as semi-portable Fortran code whose performance is unaffected (in the sense that performance will not be degraded) by the computer or operating-system environment in which it is used. An [ell sub1] start and an [ell sub2] start are provided. Eight weight functions, a numerical rank determination, convergence criterion, and a stem-and-leaf display are included.
Ultraviolet HST Observations of the Jet in M87
We present new ultraviolet photometry of the jet in M87 obtained from HST
WFPC2 imaging. We combine these ultraviolet data with previously published
photometry for the knots of the jet in radio, optical, and X-ray, and fit three
theoretical synchrotron models to the full data set. The synchrotron models
consistently overpredict the flux in the ultraviolet when fit over the entire
dataset. We show that if the fit is restricted to the radio through ultraviolet
data, the synchrotron models can provide a good match to the data. The break
frequencies of these fits are much lower than previous estimates. The implied
synchrotron lifetimes for the bulk of the emitting population are longer than
earlier work, but still much shorter than the estimated kinematic lifetimes of
the knots. The observed X-ray flux cannot be successfully explained by the
simple synchrotron models that fit the ultraviolet and optical fluxes. We
discuss the possible implications of these results for the physical properties
of the M87 jet. We also observe increased flux for the HST-1 knot that is
consistent with previous results for flaring. This observation fills in a
significant gap in the time coverage early in the history of the flare, and
therefore sets constraints on the initial brightening of the flare.Comment: 14 pages, 2 figures, Accepted for publication in ApJ, changed
lightcurve and caption in Figure
Linear independence of Gamma values in positive characteristic
We investigate the arithmetic nature of special values of Thakur's function
field Gamma function at rational points. Our main result is that all linear
independence relations over the field of algebraic functions are consequences
of the known relations of Anderson and Thakur arising from the functional
equations of the Gamma function.Comment: 51 page
Rotating Boson Stars and Q-Balls
We consider axially symmetric, rotating boson stars. Their flat space limits
represent spinning Q-balls. We discuss their properties and determine their
domain of existence. Q-balls and boson stars are stationary solutions and exist
only in a limited frequency range. The coupling to gravity gives rise to a
spiral-like frequency dependence of the boson stars. We address the flat space
limit and the limit of strong gravitational coupling. For comparison we also
determine the properties of spherically symmetric Q-balls and boson stars.Comment: 22 pages, 18 figure
Thermodynamics of non-local materials: extra fluxes and internal powers
The most usual formulation of the Laws of Thermodynamics turns out to be
suitable for local or simple materials, while for non-local systems there are
two different ways: either modify this usual formulation by introducing
suitable extra fluxes or express the Laws of Thermodynamics in terms of
internal powers directly, as we propose in this paper. The first choice is
subject to the criticism that the vector fluxes must be introduced a posteriori
in order to obtain the compatibility with the Laws of Thermodynamics. On the
contrary, the formulation in terms of internal powers is more general, because
it is a priori defined on the basis of the constitutive equations. Besides it
allows to highlight, without ambiguity, the contribution of the internal powers
in the variation of the thermodynamic potentials. Finally, in this paper, we
consider some examples of non-local materials and derive the proper expressions
of their internal powers from the power balance laws.Comment: 16 pages, in press on Continuum Mechanics and Thermodynamic
The oscillation effects on thermalization of the neutrinos in the universe with low reheating temperature
We study how the oscillations of the neutrinos affect their thermalization
process during the reheating period with temperature O(1) MeV in the early
universe. We follow the evolution of the neutrino density matrices and
investigate how the predictions of big bang nucleosynthesis vary with the
reheating temperature. For the reheating temperature of several MeV, we find
that including the oscillations makes different predictions, especially for
He abundance. Also, the effects on the lower bound of the reheating
temperature from cosmological observations are discussed.Comment: 24 pages, 8 figures; references and explanatory comments added,
conclusion unchange
Dark Energy in an Axion Model with Explicit Z(N) Symmetry Breaking
We point out that a well known axion model with an explicit Z(N) symmetry
breaking term predicts both dark energy and cold dark matter. We estimate the
parameters of this model which fit the observed densities of the dark
components of the universe. We find that the parameters do not conflict with
any observations.Comment: 5 pages, minor change
Magnetic field-induced quantum critical point in YbPtIn and YbPtIn single crystals
Detailed anisotropic (Hab and Hc) resistivity and
specific heat measurements were performed on online-grown YbPtIn and
solution-grown YbPtIn single crystals for temperatures down to 0.4 K,
and fields up to 140 kG; Hab Hall resistivity was also measured on
the YbPtIn system for the same temperature and field ranges. All these
measurements indicate that the small change in stoichiometry between the two
compounds drastically affects their ordering temperatures (T
K in YbPtIn, and K in YbPtIn). Furthermore, a field-induced
quantum critical point is apparent in each of these heavy fermion systems, with
the corresponding critical field values of YbPtIn (H around
35-45 kG and H kG) also reduced compared to the analogous
values for YbPtIn (H kG and H kG
Minisuperspace Model for Revised Canonical Quantum Gravity
We present a reformulation of the canonical quantization of gravity, as
referred to the minisuperspace; the new approach is based on fixing a Gaussian
(or synchronous) reference frame and then quantizing the system via the
reconstruction of a suitable constraint; then the quantum dynamics is re-stated
in a generic coordinates system and it becomes dependent on the lapse function.
The analysis follows a parallelism with the case of the non-relativistic
particle and leads to the minisuperspace implementation of the so-called {\em
kinematical action} as proposed in \cite{M02} (here almost coinciding also with
the approach presented in \cite{KT91}). The new constraint leads to a
Schr\"odinger equation for the system. i.e. to non-vanishing eigenvalues for
the super-Hamiltonian operator; the physical interpretation of this feature
relies on the appearance of a ``dust fluid'' (non-positive definite) energy
density, i.e. a kind of ``materialization'' of the reference frame. As an
example of minisuperspace model, we consider a Bianchi type IX Universe, for
which some dynamical implications of the revised canonical quantum gravity are
discussed. We also show how, on the classical limit, the presence of the dust
fluid can have relevant cosmological issues. Finally we upgrade our analysis by
its extension to the generic cosmological solution, which is performed in the
so-called long-wavelength approximation. In fact, near the Big-Bang, we can
neglect the spatial gradients of the dynamical variables and arrive to
implement, in each space point, the same minisuperspace paradigm valid for the
Bianchi IX model.Comment: 16 pages, no figures, to appear on International Journal of Modern
Physics
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