24,920 research outputs found
Application of a multi-level grid method to transonic flow calculations
A multi-level grid method was studied as a possible means of accelerating convergence in relaxation calculations for transonic flows. The method employs a hierarchy of grids, ranging from very coarse to fine. The coarser grids are used to diminish the magnitude of the smooth part of the residuals. The method was applied to the solution of the transonic small disturbance equation for the velocity potential in conservation form. Nonlifting transonic flow past a parabolic arc airfoil is studied with meshes of both constant and variable step size
The multigrid method: Fast relaxation
A multi-level grid method was studied as a possible means of accelerating convergence in relaxation calculations for transonic flows. The method employs a hierarchy of grids, ranging from very coarse (e.g. 4 x 2 mesh cells) to fine (e.g. 64 x 32); the coarser grids are used to diminish the magnitude of the smooth part of the residuals, hopefully with far less total work than would be required with optimal iterations on the finest grid. To date the method was applied quite successfully to the solution of the transonic small-disturbance equation for the velocity potential in conservation form. Nonlifting transonic flow past a parabolic arc airfoil is the example studied, with meshes of both constant and variable step size
X-ray absorption in the strong FeII narrow-line Seyfert 1 galaxy Mrk 507
We present results from spectral analysis of ASCA data on the NLS1 Mrk 507.
This galaxy was found to have an exceptionally flat ROSAT spectrum among the
NLS1s. The ASCA spectrum however shows a clear absorption feature in the energy
band below 2 keV, which partly accounts for the flat spectrum observed with the
ROSAT PSPC. The absorption is mainly due to cold gas with a column density of
(2-3)E21 cm-2. A reanalysis of the PSPC data indicates that the absorber is
slightly ionized, covers only part of the central source, or there is extra
soft thermal emission from an extended region. There is also evidence that the
X-ray absorption is complex; an edge feature marginally detected at 0.84 keV
suggests the presence of an additional high ionization absorber which imposes a
strong OVIII edge on the spectrum. After correction for the absorption, the
photon index of the intrinsic continuum, 1.8, obtained from the ASCA data is
quite similar to that of ordinary Seyfert 1 galaxies. Mrk 507 still has one of
the flattest continuum slopes among NLS1, but is no longer exceptional. The
strong optical FeII emission remains unusual in the light of the correlation
between FeII strengths and steepness of soft X-ray slope.Comment: 6 pages, 3 Postscript figures, to be published in MNRA
A longer XMM-Newton look at I Zwicky 1: Distinct modes of X-ray spectral variability
The short-term spectral variability of the narrow-line Seyfert 1 galaxy I
Zwicky 1 (I Zw 1) as observed in an 85 ks XMM-Newton observation is discussed
in detail. I Zw 1 shows distinct modes of variability prior to and after a flux
dip in the broad-band light curve. Before the dip the variability can be
described as arising from changes in shape and normalisation of the spectral
components. Only changes in normalisation are manifested after the dip. The
change in the mode of behaviour occurs on dynamically short timescales in I Zw
1. The data suggest that the accretion-disc corona in I Zw 1 could have two
components that are co-existing. The first, a uniform, physically diffuse
plasma responsible for the "typical'' long-term (e.g. years) behaviour; and a
second compact, centrally located component causing the rapid flux and spectral
changes. This compact component could be the base of a short or aborted jet as
sometimes proposed for radio-quiet active galaxies. Modelling of the average
and time-resolved rms spectra demonstrate that a blurred Compton-reflection
model can describe the spectral variability if we allow for pivoting of the
continuum component prior to the dip.Comment: 8 pages, 9 figures. Accepted by MNRA
On the X-ray Properties of OH Megamaser Sources: Chandra Snapshot Observations
We present Chandra snapshot observations for a sample of 7 sources selected
from the Arecibo OH megamaser (OHM) survey at z~0.13-0.22 and with far-infrared
luminosities in excess of 10^{11} L_sun. In contrast with the known H2O
megamasers, which are mostly associated with powerful Active Galactic Nuclei
(AGN), the situation is far less clear for OHMs, which have been poorly studied
in the X-ray band thus far. All of the observed sources are X-ray weak, with
only one OHM, IRAS FSC 03521+0028 (z=0.15), being detected by Chandra (with 5
counts). The results from this pilot program indicate that the X-ray emission,
with luminosities of less than ~10^{42} erg/s, is consistent with that from
star formation (as also suggested in some cases by the optical spectra) and
low-luminosity AGN emission. If an AGN is present, its contribution to the
broad-band emission of OHM galaxies is likely modest. Under reasonable
assumptions about the intrinsic X-ray spectral shape, the observed count
distribution from stacking analysis suggests absorption of ~10^{22} cm^{-2}.Comment: 8 pages, 3 figures, accepted for publication in MNRA
The Effective Potential in Non-Conformal Gauge Theories
By using the renormalization group (RG) equation it has proved possible to
sum logarithmic corrections to quantities that arise due to quantum effects in
field theories. In particular, the effective potential V in the Standard Model
in the limit that there are no massive parameters in the classical action (the
"conformal limit") has been subject to this analysis, as has the effective
potential in a scalar theory with a quartic self coupling and in massless
scalar electrodynamics. Having multiple coupling constants and/or mass
parameters in the initial action complicates this analysis, as then several
mass scales arise. We show how to address this problem by considering the
effective potential in scalar electrodynamics when the scalar field has a tree
level mass term. In addition to summing logarithmic corrections by using the RG
equation, we also consider the consequences of the condition V'(v)=0 where v is
the vacuum expectation value of the scalar. If V is expanded in powers of the
logarithms that arise, then it proves possible to show that either v is zero or
that V is independent of the scalar. (That is, either there is no spontaneous
symmetry breaking or the vacuum expectation value is not determined by
minimizing V as V is "flat".
Quantum statistical correlations in thermal field theories: boundary effective theory
We show that the one-loop effective action at finite temperature for a scalar
field with quartic interaction has the same renormalized expression as at zero
temperature if written in terms of a certain classical field , and if
we trade free propagators at zero temperature for their finite-temperature
counterparts. The result follows if we write the partition function as an
integral over field eigenstates (boundary fields) of the density matrix element
in the functional Schr\"{o}dinger field-representation, and perform a
semiclassical expansion in two steps: first, we integrate around the
saddle-point for fixed boundary fields, which is the classical field ,
a functional of the boundary fields; then, we perform a saddle-point
integration over the boundary fields, whose correlations characterize the
thermal properties of the system. This procedure provides a
dimensionally-reduced effective theory for the thermal system. We calculate the
two-point correlation as an example.Comment: 13 pages, 1 figur
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