5,812 research outputs found
The Transverse Proximity Effect: A Probe to the Environment, Anisotropy, and Megayear Variability of QSOs
The transverse proximity effect is the expected decrease in the strength of
the Lya forest absorption in a QSO spectrum when another QSO lying close to the
line of sight enhances the photoionization rate above that due to the average
cosmic ionizing background. We select three QSOs from the Early Data Release of
the Sloan Digital Sky Survey that have nearby foreground QSOs, with proper line
of sight tangential separations of 0.50, 0.82, and 1.10 h^{-1} Mpc. We estimate
that the ionizing flux from the foreground QSO should increase the
photoionization rate by a factor (94, 13, 13) in these three cases, which would
be clearly detectable in the first QSO and marginally so in the other two. We
do not detect the transverse proximity effect. Three possible explanations are
provided: an increase of the gas density in the vicinity of QSOs, time
variability, and anisotropy of the QSO emission. We find that the increase of
gas density near QSOs can be important if they are located in the most massive
halos present at high redshift, but is not enough to fully explain the absence
of the transverse proximity effect. Anisotropy requires an unrealistically
small opening angle of the QSO emission. Variability demands that the
luminosity of the QSO with the largest predicted effect was much lower 10^6
years ago, whereas the transverse proximity effect observed in the HeII Lya
absorption in QSO 0302-003 by Jakobsen et al. (2003) implies a lifetime longer
than 10^7 years. A combination of all three effects may better explain the lack
of Lya absorption reduction. A larger sample of QSO pairs may be used to
diagnose the environment, anisotropy and lifetime distribution of QSOs.Comment: 27 pages, 13 figures, accepted by Ap
The clock paradox in a static homogeneous gravitational field
The gedanken experiment of the clock paradox is solved exactly using the
general relativistic equations for a static homogeneous gravitational field. We
demonstrate that the general and special relativistic clock paradox solutions
are identical and in particular that they are identical for finite
acceleration. Practical expressions are obtained for proper time and coordinate
time by using the destination distance as the key observable parameter. This
solution provides a formal demonstration of the identity between the special
and general relativistic clock paradox with finite acceleration and where
proper time is assumed to be the same in both formalisms. By solving the
equations of motion for a freely falling clock in a static homogeneous field
elapsed times are calculated for realistic journeys to the stars.Comment: Revision: Posted with the caption included with the figure
The Very Highly Ionized Broad Absorption Line System of the QSO SBS1542+541
We have analyzed the broad absorption line system of the bright (V=16.5)
high-redshift (z=2.361) QSO SBS1542+541 using UV spectra from the HST FOS along
with optical data from the MMT and the Steward Observatory 2.3m telescope.
These spectra give continuous wavelength coverage from 1200 to 8000 Angstroms,
corresponding to 340 to 2480 Angstroms in the QSO rest frame. This object
therefore offers a rare opportunity to study broad absorption lines in the
rest-frame extreme UV. We find that the absorption system is dominated by very
high-ionization species, including O VI, NeVIII, and SiXII. We also identify
apparently saturated broad Lyman-series lines of order Ly-gamma and higher.
There is strong evidence for partial occultation of the QSO emission source,
particularly from the higher-order Lyman lines which indicate a covered
fraction less than 0.2. Overall, the data suggest a correlation between a
larger covered fraction and a higher state of ionization. We suggest that the
different covered fractions can be explained by either a special line of sight
through a disk-like geometry or by the existence of density fluctuations of a
factor >2 in the BAL gas. Our photoionization models of the system indicate a
large column density and high ionization state similar to that found in X-ray
``warm absorbers''.Comment: 31 pages, 13 figures, to be published in Ap
Heating in the Accreted Neutron Star Ocean: Implications for Superburst Ignition
We perform a self-consistent calculation of the thermal structure in the
crust of a superbursting neutron star. In particular, we follow the
nucleosynthetic evolution of an accreted fluid element from its deposition into
the atmosphere down to a depth where the electron Fermi energy is 20 MeV. We
include temperature-dependent continuum electron capture rates and realistic
sources of heat loss by thermal neutrino emission from the crust and core. We
show that, in contrast to previous calculations, electron captures to excited
states and subsequent gamma-emission significantly reduce the local heat loss
due to weak-interaction neutrinos. Depending on the initial composition these
reactions release up to a factor of 10 times more heat at densities < 10^{11}
g/cc than obtained previously. This heating reduces the ignition depth of
superbursts. In particular, it reduces the discrepancy noted by Cumming et al.
between the temperatures needed for unstable 12C ignition on timescales
consistent with observations and the reduction in crust temperature from Cooper
pair neutrino emission.Comment: 10 pages, 11 figures, the Astrophysical Journal, in press (scheduled
for v. 662). Revised from v1 in response to referee's comment
Saturation properties and incompressibility of nuclear matter: A consistent determination from nuclear masses
Starting with a two-body effective nucleon-nucleon interaction, it is shown
that the infinite nuclear matter model of atomic nuclei is more appropriate
than the conventional Bethe-Weizsacker like mass formulae to extract saturation
properties of nuclear matter from nuclear masses. In particular, the saturation
density thus obtained agrees with that of electron scattering data and the
Hartree-Fock calculations. For the first time using nuclear mass formula, the
radius constant =1.138 fm and binding energy per nucleon = -16.11
MeV, corresponding to the infinite nuclear matter, are consistently obtained
from the same source. An important offshoot of this study is the determination
of nuclear matter incompressibility to be 288 28 MeV using
the same source of nuclear masses as input.Comment: 14 latex pages, five figures available on request ( to appear in Phy.
Rev. C
The red stellar population in NGC 1569
We present HST NICMOS photometry of the resolved stellar population in NGC
1569. The CMD in the F110W and F160W photometric bands contains ~2400 stars
with a formal photometric error < 0.1 mag down to J~23.5 and H~22.5. We
describe the data processing which is required to calibrate the instrumental
peculiarities of NICMOS. Two different packages for PSF-fitting photometry are
used to strengthen the photometric results in the crowded stellar field of NGC
1569. The resulting CMD is discussed in terms of the major evolutionary
properties of the resolved stellar populations. For a distance modulus of 26.71
and a reddening E(B-V)=0.56, our CMD samples stars down to ~0.8 Mo,
corresponding to look-back times > 15 Gyr. This is clear indication of SF
activity spanning an entire Hubble time. The metallicity of the reddest RGB
stars is in better agreement with Z=0.004 as measured in HII regions, than with
Z=0.0004 as expected from the stellar ages. The presence of - yet undetected -
very metal-poor stars embedded in the stellar distribution around J=22.75 and
J-H=1.15 is, however, not ruled out. The youngest stars (< 50 Myr) are
preferentially found around the two central super star clusters, whereas the
oldest population has a more uniform spatial distribution. A SFR per unit area
of 1 Mo yr*(-1) kpc*(-2) and a mass formed in stars of ~ 1.4x10*6 Mo in the
last 50 Myr are derived from the CMD. The NIR CMD places strong constraints on
the lower limit of the onset of SF in NGC 1569. The exceptionally high crowding
in the NICMOS images of NGC 1569 is a challenge for the photometric analysis.
As a result, optical and NIR images of NGC 1569 sample different populations
and cannot be cross-correlated. Nevertheless, we demonstrate the consistency of
the SF histories derived from the optical and NIR CMDs.Comment: 41 pages including 1 table (Latex) and 14 figures (postscript).
Accepted for publication in the Astronomical Journal, March 2001 issu
On Teaching Discrete Mathematics to Freshman Computer Science Students
Discrete Mathematics is an inevitable part of any undergraduate computer science degree programme. However, today's computer science student typically finds this to be at best a necessary evil with which they struggle to engage. Twenty years ago, we started to address this issue seriously in our university, and we have instituted a number of innovations throughout the years which have had a positive effect on engagement and, thus, attainment. In this paper, we describe and motivate the innovations which we introduced, and provide a detailed analysis of how and why engagement and attainment levels varied over two decades as a direct result of these innovation
Mathisson's helical motions for a spinning particle --- are they unphysical?
It has been asserted in the literature that Mathisson's helical motions are
unphysical, with the argument that their radius can be arbitrarily large. We
revisit Mathisson's helical motions of a free spinning particle, and observe
that such statement is unfounded. Their radius is finite and confined to the
disk of centroids. We argue that the helical motions are perfectly valid and
physically equivalent descriptions of the motion of a spinning body, the
difference between them being the choice of the representative point of the
particle, thus a gauge choice. We discuss the kinematical explanation of these
motions, and we dynamically interpret them through the concept of hidden
momentum. We also show that, contrary to previous claims, the frequency of the
helical motions coincides, even in the relativistic limit, with the
zitterbewegung frequency of the Dirac equation for the electron
Towards a Notion of Distributed Time for Petri Nets
We set the ground for research on a timed extension of Petri nets where time parameters are associated with tokens and arcs carry constraints that qualify the age of tokens required for enabling. The novelty is that, rather than a single global clock, we use a set of unrelated clocks --- possibly one per place --- allowing a local timing as well as distributed time synchronisation. We give a formal definition of the model and investigate properties of local versus global timing, including decidability issues and notions of processes of the respective models
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