604 research outputs found
The Distribution of Redshifts in New Samples of Quasi-stellar Objects
Two new samples of QSOs have been constructed from recent surveys to test the
hypothesis that the redshift distribution of bright QSOs is periodic in
. The first of these comprises 57 different redshifts among all
known close pairs or multiple QSOs, with image separations 10\arcsec,
and the second consists of 39 QSOs selected through their X-ray emission and
their proximity to bright comparatively nearby active galaxies. The redshift
distributions of the samples are found to exhibit distinct peaks with a
periodic separation of in identical to that claimed
in earlier samples but now extended out to higher redshift peaks and 4.47, predicted by the formula but never seen before. The periodicity
is also seen in a third sample, the 78 QSOs of the 3C and 3CR catalogues. It is
present in these three datasets at an overall significance level -
, and appears not to be explicable by spectroscopic or similar
selection effects. Possible interpretations are briefly discussed.Comment: submitted for publication in the Astronomical Journal, 15 figure
Weak Charge-Changing Flow in Expanding r-Process Environments
We assess the prospects for attaining steady nuclear flow equilibrium in
expanding r-process environments where beta decay and/or neutrino capture
determine the nuclear charge-changing rates. For very rapid expansions, we find
that weak steady flow equilibrium normally cannot be attained. However, even
when neutron capture processes freeze out in such nonequilibrium conditions,
abundance ratios of nuclear species in the r-process peaks might still mimic
those attained in weak steady flow. This result suggests that the r-process
yield in a regime of rapid expansion can be calculated reliably only when all
neutron capture, photodisintegration, and weak interaction processes are fully
coupled in a dynamical calculation. We discuss the implications of these
results for models of the r-process sited in rapidly expanding neutrino-heated
ejecta.Comment: 21 pages, AAS LaTex, 2 postscript figure
Possible interpretations of the magnitude-redshift relation for supernovae of type Ia
It has been shown by Riess et al. and Perlmutter et al. that the observed redshift-magnitude relation for supernovae of type Ia, which suggests that the deceleration parameter q0 is negative, can be explained in a Friedmann model with a positive cosmological constant. We show that a quasi-steady state cosmology (QSSC) model can also fit the supernova data. Since most of the emphasis and publicity have been concentrated on explanations involving the Friedmann model, we show how a good fit can be obtained to the observations in the framework of the QSSC. Using this model, we show that absorption due to intergalactic dust may play an important role. This may explain why a few of the supernovae observed show large deviations from the curve determined by the majority of the data
Cosmology and Cosmogony in a Cyclic Universe
In this paper we discuss the properties of the quasi-steady state
cosmological model (QSSC) developed in 1993 in its role as a cyclic model of
the universe driven by a negative energy scalar field. We discuss the origin of
such a scalar field in the primary creation process first described by F. Hoyle
and J. V. Narlikar forty years ago. It is shown that the creation processes
which takes place in the nuclei of galaxies are closely linked to the high
energy and explosive phenomena, which are commonly observed in galaxies at all
redshifts.
The cyclic nature of the universe provides a natural link between the places
of origin of the microwave background radiation (arising in hydrogen burning in
stars), and the origin of the lightest nuclei (H, D, He and He). It
also allows us to relate the large scale cyclic properties of the universe to
events taking place in the nuclei of galaxies. Observational evidence shows
that ejection of matter and energy from these centers in the form of compact
objects, gas and relativistic particles is responsible for the population of
quasi-stellar objects (QSOs) and gamma-ray burst sources in the universe.
In the later parts of the paper we briefly discuss the major unsolved
problems of this integrated cosmological and cosmogonical scheme. These are the
understanding of the origin of the intrinsic redshifts, and the periodicities
in the redshift distribution of the QSOs.Comment: 51 pages including 1 figur
Particle-Number Reprojection in the Shell Model Monte Carlo Method: Application to Nuclear Level Densities
We introduce a particle-number reprojection method in the shell model Monte
Carlo that enables the calculation of observables for a series of nuclei using
a Monte Carlo sampling for a single nucleus. The method is used to calculate
nuclear level densities in the complete -shell using a good-sign
Hamiltonian. Level densities of odd-A and odd-odd nuclei are reliably extracted
despite an additional sign problem. Both the mass and the dependence of
the experimental level densities are well described without any adjustable
parameters. The single-particle level density parameter is found to vary
smoothly with mass. The odd-even staggering observed in the calculated
backshift parameter follows the experimental data more closely than do
empirical formulae.Comment: 14 pages, 4 eps figures included, RevTe
A deep X-ray observation of NGC 4258 and its surrounding field
We present a deep X-ray observation of the low-luminosity active galactic
nucleus in NGC4258 (M106) using ASCA. The soft X-ray spectrum <2keV is
dominated by thermal emission from optically-thin plasma with kT~0.5keV. The
hard X-ray emission is clearly due to a power-law component with photon index
Gamma=1.8 absorbed by a column density of N_H=8x10^22/cm^2. The power-law is
readily identified with primary X-ray emission from the AGN central engine. We
also clearly detect a narrow iron K-alpha emission line at 6.4keV. No broad
component is detected. We suggest that the bulk of this narrow line comes from
the accretion disk and, furthermore, that the power-law X-ray source which
excites this line emission (which is typically identified with a disk corona)
must be at least 100GM/c^2 in extent. This is in stark contrast to many
higher-luminosity Seyfert galaxies which display a broad iron line indicating a
small 10 GM/c^2 X-ray emitting region. It must be stressed that this study
constrains the size of the X-ray emitting corona rather than the
presence/absence of a radiatively efficient accretion disk in the innermost
regions. If, instead, a substantial fraction of the observed narrow line
originates from material not associated with the accretion disk, limits can be
placed on the parameter space of possible allowed relativistically broad iron
lines. By comparing our data with previous ASCA observations, we find marginal
evidence for a change in absorbing column density through to the central
engine, and good evidence for a change in the AGN flux.Comment: 11 pages, 9 postscript figures. Accepted for publication in Ap
AcDc - A new code for the NLTE spectral analysis of accretion discs: application to the helium CV AM CVn
We present a recently developed code for detailed NLTE calculations of
accretion disc spectra of cataclysmic variables and compact X-ray binaries.
Assuming a radial structure of a standard alpha-disc, the disc is divided into
concentric rings. For each disc ring the solution of the radiation transfer
equation and the structure equations, comprising the hydrostatic and radiative
equilibrium, the population of the atomic levels as well as charge and particle
conservation, is done self-consistently. Metal-line blanketing and irradiation
by the central object are taken into account. As a first application, we show
the influence of different disc parameters on the disc spectrum for the helium
cataclysmic variable AM CVn.Comment: 7 pages, 11 figures to be published in A&
Hybrid nature of 0846+51W1: a BL Lac object with a narrow line Seyfert 1 nucleus
We have found a NLS1 nucleus in the extensively studied eruptive BL Lac,
0846+51W1, out of a large sample of NLS1 compiled from the spectroscopic
dataset of SDSS DR1. Its optical spectrum can be well decomposed into three
components, a power law component from the relativistic jet, a stellar
component from the host galaxy, and a component from a typical NLS1 nucleus.
The emission line properties of 0846+51W1, FWHM(Hbeta) ~ 1710 km s^-1 and
[OIII]5007/Hbeta ~ 0.32 when it was in faint state, fulfil the conventional
definition of NLS1. Strong FeII emission is detected in the SDSS spectrum,
which is also typical of NLS1s. We try to estimate its central black hole mass
using various techniques and find that 0846+51W1 is very likely emitting at a
few times 10% L_Edd. We speculate that Seyfert-like nuclei, including NLS1s,
might be concealed in a significant fraction of BL Lacs but have not been
sufficiently explored due to the fact that, by definition, the optical-UV
continuum of such kind of objects are often overwhelmed by the synchrotron
emission.Comment: ChJAA accepte
Molecular Gas, Dust and Star Formation in the Barred Spiral NGC 5383
We present multi-wavelength (interferometer and single-dish CO J=1-0, Halpha,
broadband optical and near-infrared) observations of the classic barred spiral
NGC 5383. We compare the observed central gas and dust morphology to the
predictions of recent hydrodynamic simulations. In the nuclear region, our
observations reveal three peaks lying along a S-shaped gas and dust
distribution. In contrast, the model predicts a circumnuclear ring, not the
observed S-shaped distribution; moreover, the predicted surface density
contrast between the central gas accumulation and the bar dust lanes is an
order of magnitude larger than observed. The discrepancies are not due to
unexplored model parameter space or a nuclear bar but are probably due to the
vigorous (7 solar masses per year) star formation activity in the center.
As is common in similar bars, the star formation rate in the bar between the
bar ends and the central region is low (~0.5 solar masses per yr), despite the
high gas column density in the bar dust lanes; this is generally attributed to
shear and shocks. We note a tendency for the HII regions to be associated with
the spurs feeding the main bar dust lanes, but these are located on the leading
side of the bar. We propose that stars form in the spurs, which provide a high
column density but low shear environment. HII regions can therefore be found
even on the leading side of the bar because the ionizing stars pass
ballistically through the dust laneComment: Accepted for publication in The Astrophysical Journal, 33 pages
(includes 10 figures
Production of neutron-rich nuclei in fragmentation reactions of 132Sn projectiles at relativistic energies
The fragmentation of neutron-rich 132Sn nuclei produced in the fission of
238U projectiles at 950 MeV/u has been investigated at the FRagment Separator
(FRS) at GSI. This work represents the first investigation of fragmentation of
medium-mass radioactive projectiles with a large neutron excess. The measured
production cross sections of the residual nuclei are relevant for the possible
use of a two-stage reaction scheme (fission+fragmentation) for the production
of extremely neutron-rich medium-mass nuclei in future rare-ion-beam
facilities. Moreover, the new data will provide a better understanding of the
"memory" effect in fragmentation reactions.Comment: 5 pages, 3 figure
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