1,744 research outputs found
Numerical Study of the Ghost-Ghost-Gluon Vertex on the Lattice
It is well known that, in Landau gauge, the renormalization function of the
ghost-ghost-gluon vertex \widetilde{Z}_1(p^2) is finite and constant, at least
to all orders of perturbation theory. On the other hand, a direct
non-perturbative verification of this result using numerical simulations of
lattice QCD is still missing. Here we present a preliminary numerical study of
the ghost-ghost-gluon vertex and of its corresponding renormalization function
using Monte Carlo simulations in SU(2) lattice Landau gauge. Data were obtained
in 4 dimensions for lattice couplings beta = 2.2, 2.3, 2.4 and lattice sides N
= 4, 8, 16.Comment: 3 pages, 1 figure, presented by A. Mihara at the IX Hadron Physics
and VII Relativistic Aspects of Nuclear Physics Workshops, Angra dos Reis,
Rio de Janeiro, Brazil (March 28--April 3, 2004
Warm absorber, reflection and Fe K line in the X-ray spectrum of IC 4329A
Results from the X-ray spectral analysis of the ASCA PV phase observation of
the Seyfert 1 galaxy IC 4329A are presented. We find that the 0.4 - 10 keV
spectrum of IC 4329A is best described by the sum of a steep () power-law spectrum passing through a warm absorber plus a strong
reflection component and associated Fe K line, confirming recent results
(Madejski et al. 1995, Mushotsky et al. 1995). Further cold absorption in
excess of the Galactic value and covering the entire source is also required by
the data, consistent with the edge-on galactic disk and previous X-ray
measurements. The effect of the warm absorber at soft X-ray energies is best
parameterized by two absorption edges, one consistent with OVI, OVII or NVII,
the other consistent with OVIII. A description of the soft excess in terms of
blackbody emission, as observed in some other Seyfert 1 galaxies, is ruled out
by the data. A large amount of reflection is detected in both the GIS and SIS
detectors, at similar intensities. We find a strong correlation between the
amount of reflection and the photon index, but argue that the best solution
with the present data is that given by the best statistical fit. The model
dependence of the Fe K line parameters is also discussed. Our best fit gives a
slightly broad ( keV) and redshifted (E keV) Fe K line, with equivalent width 89 33 eV.
The presence of a weak Fe K line with a strong reflection can be reconciled if
one assumes iron underabundances or ionized reflection. We also have modeled
the line with a theoretical line profile produced by an accretion disk. This
yields results in better agreement with the constraints obtained from the
reflection component.Comment: Accepted for publication in The Astrophysical Journal, 10th February
1996 issue; 24 pages and 8 figures + 1 table tared, compressed and uuencoded
(with uufiles
Probing the stellar wind environment of Vela X-1 with MAXI
Vela X-1 is among the best studied and most luminous accreting X-ray pulsars.
The supergiant optical companion produces a strong radiatively-driven stellar
wind, which is accreted onto the neutron star producing highly variable X-ray
emission. A complex phenomenology, due to both gravitational and radiative
effects, needs to be taken into account in order to reproduce orbital spectral
variations. We have investigated the spectral and light curve properties of the
X-ray emission from Vela X-1 along the binary orbit. These studies allow to
constrain the stellar wind properties and its perturbations induced by the
compact object. We took advantage of the All Sky Monitor MAXI/GSC data to
analyze Vela X-1 spectra and light curves. By studying the orbital profiles in
the and keV energy bands, we extracted a sample of orbital light
curves (% of the total) showing a dip around the inferior
conjunction, i.e., a double-peaked shape. We analyzed orbital phase-averaged
and phase-resolved spectra of both the double-peaked and the standard sample.
The dip in the double-peaked sample needs cm to
be explained by absorption solely, which is not observed in our analysis. We
show how Thomson scattering from an extended and ionized accretion wake can
contribute to the observed dip. Fitted by a cutoff power-law model, the two
analyzed samples show orbital modulation of the photon index, hardening by
around the inferior conjunction, compared to earlier and later
phases, hinting a likely inadequacy of this model. On the contrary, including a
partial covering component at certain orbital phase bins allows a constant
photon index along the orbital phases, indicating a highly inhomogeneous
environment. We discuss our results in the framework of possible scenarios.Comment: 10 pages, 9 figures, accepted for publication in A&
Comparison among HB-inspired algorithms for continuous-spin systems and gauge fields
We propose a new local algorithm for the thermalization of n-vector spin
models, which can also be used in the numerical simulation of SU(N) lattice
gauge theories. The algorithm combines heat-bath (HB) and micro-canonical
updates in a single step -- as opposed to the hybrid overrelaxation method,
which alternates between the two kinds of update steps -- while preserving
ergodicity. We test our proposed algorithm in the case of the one-dimensional
4-vector spin model and compare its performance with the standard HB algorithm
and with other HB-inspired algorithms.Comment: 6 pages, 4 figures. Work presented at the IV Brazilian Meeting on
Simulational Physics -- Ouro Preto - MG/Brazil, August 200
Footprints in the wind of Vela X-1 traced with MAXI
The stellar wind around the compact object in luminous wind-accreting high
mass X-ray binaries is expected to be strongly ionized with the X-rays coming
from the compact object. The stellar wind of hot stars is mostly driven by
light absorption in lines of heavier elements, and X-ray photo-ionization
significantly reduces the radiative force within the so-called Stroemgren
region leading to wind stagnation around the compact object. In close binaries
like Vela X-1 this effect might alter the wind structure throughout the system.
Using the spectral data from Monitor of All-sky X-ray Image (MAXI), we study
the observed dependence of the photoelectric absorption as function of orbital
phase in Vela X-1, and find that it is inconsistent with expectations for a
spherically-symmetric smooth wind. Taking into account previous investigations
we develop a simple model for wind structure with a stream-like photoionization
wake region of slower and denser wind trailing the neutron star responsible for
the observed absorption curve.Comment: 5 pages, 3 figures, accepted in A&
ASCA Observations of the Composite Warm Absorber in NGC 3516
We obtained X-ray spectra of the Seyfert 1 galaxy NGC~3516 in March 1995
using ASCA. Simultaneous far-UV observations were obtained with HUT on the
Astro-2 shuttle mission. The ASCA spectrum shows a lightly absorbed power law
of energy index 0.78. The low energy absorbing column is significantly less
than previously seen. Prominent O~vii and O~viii absorption edges are visible,
but, consistent with the much lower total absorbing column, no Fe K absorption
edge is detectable. A weak, narrow Fe~K emission line from cold
material is present as well as a broad Fe~K line. These features are
similar to those reported in other Seyfert 1 galaxies. A single warm absorber
model provides only an imperfect description of the low energy absorption. In
addition to a highly ionized absorber with ionization parameter and
a total column density of , adding a lower
ionization absorber with and a total column of significantly improves the fit. The contribution of
resonant line scattering to our warm absorber models limits the Doppler
parameter to at 90\% confidence. Turbulence at the sound
speed of the photoionized gas provides the best fit. None of the warm absorber
models fit to the X-ray spectrum can match the observed equivalent widths of
all the UV absorption lines. Accounting for the X-ray and UV absorption
simultaneously requires an absorbing region with a broad range of ionization
parameters and column densities.Comment: 14 pages, 4 Postscript figures, uses aaspp4.sty To appear in the
August 20, 1996, issue of The Astrophysical Journa
Test of the Conserved Vector Current Hypothesis by beta-ray Angular Distribution Measurement in the Mass-8 System
The beta-ray angular correlations for the spin alignments of 8Li and 8B have
been observed in order to test the conserved vector current (CVC) hypothesis.
The alignment correlation terms were combined with the known beta-alpha-angular
correlation terms to determine all the matrix elements contributing to the
correlation terms. The weak magnetism term, 7.5\pm0.2, deduced from the
beta-ray correlation terms was consistent with the CVC prediction 7.3\pm0.2,
deduced from the analog-gamma-decay measurement based on the CVC hypothesis.
However, there was no consistent CVC prediction for the second-forbidden term
associated with the weak vector current. The experimental value for the
second-forbidden term was 1.0 \pm 0.3, while the CVC prediction was 0.1 \pm 0.4
or 2.1 \pm 0.5.Comment: 31 pages, 12 figures, Accepted for publication in Phys. Rev.
Photoemission from buried interfaces in SrTiO3/LaTiO3 superlattices
We have measured photoemission spectra of SrTiO3/LaTiO3 superlattices with a
topmost SrTiO3 layer of variable thickness. Finite coherent spectral weight
with a clear Fermi cut-off was observed at chemically abrupt SrTiO3/LaTiO3
interfaces, indicating that an ``electronic reconstruction'' occurs at the
interface between the Mott insulator LaTiO3 and the band insulator SrTiO3. For
SrTiO3/LaTiO3 interfaces annealed at high temperatures (~ 1000 C), which leads
to Sr/La atomic interdiffusion and hence to the formation of La1-xSrxTiO3-like
material, the intensity of the incoherent part was found to be dramatically
reduced whereas the coherent part with a sharp Fermi cut-off is enhanced due to
the spread of charge. These important experimental features are well reproduced
by layer dynamical-mean-field-theory calculation
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