15,511 research outputs found
Dynamic scaling and universality in evolution of fluctuating random networks
We found that models of evolving random networks exhibit dynamic scaling
similar to scaling of growing surfaces. It is demonstrated by numerical
simulations of two variants of the model in which nodes are added as well as
removed [Phys. Rev. Lett. 83, 5587 (1999)]. The averaged size and connectivity
of the network increase as power-laws in early times but later saturate.
Saturated values and times of saturation change with paramaters controlling the
local evolution of the network topology. Both saturated values and times of
saturation obey also power-law dependences on controlling parameters. Scaling
exponents are calculated and universal features are discussed.Comment: 7 pages, 6 figures, Europhysics Letters for
Transient jets in V617 Sagittarii
Some of the luminous Compact Binary Supersoft X-Ray sources (CBSS) have shown
indications of jets, also called satellites due to their appearance in the
spectra. In V Sagittae (V Sge) stars, the galactic counterparts of the CBSS,
such features have been reported only for WX Cen. If V Sge stars are indeed the
analogs of CBSS, one may expect transient jet emission in other objects of this
class. Spectroscopic observations of the V Sge star V617 Sgr have been made,
both at high photometric state and at decline. We show that V617 Sgr presents
Halpha satellites at high photometric state with velocities of +/-780 km/s.
This feature confirms, once more, the CBSS nature of the V Sge stars, however
the details of the spectral characteristics also suggest that the two groups of
stars display some intrinsic spectroscopic differences, which are likely to be
due to a selection effect related to chemical abundance.Comment: Four pages, accepted to be published as a Letter in A&
The equation of state of neutron matter, symmetry energy, and neutron star structure
We review the calculation of the equation of state of pure neutron matter
using quantum Monte Carlo (QMC) methods. QMC algorithms permit the study of
many-body nuclear systems using realistic two- and three-body forces in a
nonperturbative framework. We present the results for the equation of state of
neutron matter, and focus on the role of three-neutron forces at supranuclear
density. We discuss the correlation between the symmetry energy, the neutron
star radius and the symmetry energy. We also combine QMC and theoretical models
of the three-nucleon interactions, and recent neutron star observations to
constrain the value of the symmetry energy and its density dependence.Comment: 11 pages, 11 figure
Stellar archeology of the nearby LINER galaxies NGC 4579 and NGC 4736
Stellar archeology of nearby LINER galaxies may reveal if there is a stellar
young population that may be responsible for the LINER phenomenon. We show
results for the classical LINER galaxies NGC 4579 and NGC 4736 and find no
evidence of such populations.Comment: 2 pages, 2 figures, to be published in the Proceedings of the IAU
Symposium no. 26
Neutron star mass and radius measurements from atmospheric model fits to X-ray burst cooling tail spectra
Observations of thermonuclear X-ray bursts from accreting neutron stars (NSs)
in low-mass X-ray binary systems can be used to constrain NS masses and radii.
Most previous work of this type has set these constraints using Planck function
fits as a proxy: both the models and the data are fit with diluted blackbody
functions to yield normalizations and temperatures which are then compared
against each other. Here, for the first time, we fit atmosphere models of X-ray
bursting NSs directly to the observed spectra. We present a hierarchical
Bayesian fitting framework that uses state-of-the-art X-ray bursting NS
atmosphere models with realistic opacities and relativistic exact Compton
scattering kernels as a model for the surface emission. We test our approach
against synthetic data, and find that for data that are well-described by our
model we can obtain robust radius, mass, distance, and composition
measurements. We then apply our technique to Rossi X-ray Timing Explorer
observations of five hard-state X-ray bursts from 4U 1702-429. Our joint fit to
all five bursts shows that the theoretical atmosphere models describe the data
well but there are still some unmodeled features in the spectrum corresponding
to a relative error of 1-5% of the energy flux. After marginalizing over this
intrinsic scatter, we find that at 68% credibility the circumferential radius
of the NS in 4U 1702-429 is R = 12.4+-0.4 km, the gravitational mass is
M=1.9+-0.3 Msun, the distance is 5.1 < D/kpc < 6.2, and the hydrogen mass
fraction is X < 0.09.Comment: 15 pages, 11 figures, submitted to A&
Escape Orbits for Non-Compact Flat Billiards
It is proven that, under some conditions on , the non-compact flat
billiard
has no orbits going {\em directly} to . The relevance of such
sufficient conditions is discussed.Comment: 9 pages, LaTeX, 3 postscript figures available at
http://www.princeton.edu/~marco/papers/ . Minor changes since previously
posted version. Submitted to 'Chaos
Mapping low and high density clouds in astrophysical nebulae by imaging forbidden line emission
Emission line ratios have been essential for determining physical parameters
such as gas temperature and density in astrophysical gaseous nebulae. With the
advent of panoramic spectroscopic devices, images of regions with emission
lines related to these physical parameters can, in principle, also be produced.
We show that, with observations from modern instruments, it is possible to
transform images taken from density sensitive forbidden lines into images of
emission from high and low-density clouds by applying a transformation matrix.
In order to achieve this, images of the pairs of density sensitive lines as
well as the adjacent continuum have to be observed and combined. We have
computed the critical densities for a series of pairs of lines in the infrared,
optical, ultraviolet and X-rays bands, and calculated the pair line intensity
ratios in the high and low-density limit using a 4 and 5 level atom
approximation. In order to illustrate the method we applied it to GMOS-IFU data
of two galactic nuclei. We conclude that this method provides new information
of astrophysical interest, especially for mapping low and high-density clouds;
for this reason we call it "the ld/hd imaging method".Comment: 7 pages, 4 figures, accepted for publication on MNRA
WR 7a: a V Sagittae or a qWR star?
The star WR 7a, also known as SPH 2, has a spectrum that resembles that of V
Sagittae stars although no O VI emission has been reported. The Temporal
Variance Spectrum - TVS - analysis of our data shows weak but strongly variable
emission of O VI lines which is below the noise level in the intensity
spectrum. Contrary to what is seen in V Sagittae stars, optical photometric
monitoring shows very little, if any, flickering. We found evidence of periodic
variability. The most likely photometric period is P(phot) = 0.227(14) d, while
radial velocities suggest a period of P(spec) = 0.204(13) d. One-day aliases of
these periods can not be ruled out. We call attention to similarities with HD
45166 and DI Cru (= WR 46), where multiple periods are present. They may be
associated to the binary motion or to non-radial oscillations. In contrast to a
previous conclusion by Pereira et al. (1998), we show that WR 7a contains
hydrogen. The spectrum of the primary star seems to be detectable as the N V
4604A absorption line is visible. If so, it means that the wind is optically
thin in the continuum and that it is likely to be a helium main sequence star.
Given the similarity to HD 45166, we suggests that WR 7a may be a qWR - quasi
Wolf-Rayet - star. Its classification is WN4h/CE in the Smith et al. (1996)
three dimensional classification system.Comment: 6 pages, 6 figures, preprint of an article accepted for publication
in Monthly Notices of The Royal Astronomical Societ
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