6,418 research outputs found
The Optical Counterpart of the Isolated Neutron Star RX J1605.3+3249
We have detected the optical counterpart to the nearby isolated neutron star
RX J1605.3+3249 using observations from the Space Telescope Imaging
Spectrometer aboard the Hubble Space Telescope. The counterpart, with
m_{50CCD}=26.84+/-0.07 mag and very blue colors, lies close to the ROSAT HRI
error circle and within the Chandra error circle. The spectrum is consistent
with a Rayleigh-Jeans tail whose emission is a factor of ~14 above the
extrapolation of the X-ray blackbody, and the source has an unabsorbed
X-ray-to-optical flux ratio of log(fX/fopt)=4.4, similar to that of other
isolated neutron stars. This confirms the classification of RX J1605.3+3249 as
a neutron star.Comment: 5 pages, 4 figures. Uses emulateapj5.sty, onecolfloat5.sty. Accepted
by ApJL. Small changes from ver. 1 following referee's repor
A Probable Optical Counterpart for the Isolated Neutron Star RX J1308.6+2127
Using a very deep observation with HST/STIS, we have searched for an optical
counterpart to the nearby radio-quiet isolated neutron star RX J1308.6+2127
(RBS 1223). We have identified a single object in the 90% Chandra error circle
that we believe to be the optical counterpart. This object has
mag, which translates approximately to an unabsorbed
flux of ergs/s/cm^2/A at 5150 A or an
X-ray-to-optical flux ratio of . This flux is a factor of
above the extrapolation of the black-body fit to the X-ray
spectrum, consistent with the optical spectra of other isolated neutron stars.
Without color information we cannot conclude that this source is indeed the
counterpart of RX J1308.6+2127. If not, then the counterpart must have
mag, corresponding to a flux that is barely consistent with
the extrapolation of the black-body fit to the X-ray spectrum.Comment: 4 pages, 2 figures. Uses emulateapj5.sty, onecolfloat5.sty. Accepted
by ApJ Letter
Discrete-time rewards model-checked
This paper presents a model-checking approach for analyzing discrete-time Markov reward models. For this purpose, the temporal logic probabilistic CTL is extended with reward constraints. This allows to formulate complex measures â involving expected as well as accumulated rewards â in a precise and succinct way. Algorithms to efficiently analyze such formulae are introduced. The approach is illustrated by model-checking a probabilistic cost model of the IPv4 zeroconf protocol for distributed address assignment in ad-hoc networks
Probabilistic Model Checking for Energy Analysis in Software Product Lines
In a software product line (SPL), a collection of software products is
defined by their commonalities in terms of features rather than explicitly
specifying all products one-by-one. Several verification techniques were
adapted to establish temporal properties of SPLs. Symbolic and family-based
model checking have been proven to be successful for tackling the combinatorial
blow-up arising when reasoning about several feature combinations. However,
most formal verification approaches for SPLs presented in the literature focus
on the static SPLs, where the features of a product are fixed and cannot be
changed during runtime. This is in contrast to dynamic SPLs, allowing to adapt
feature combinations of a product dynamically after deployment. The main
contribution of the paper is a compositional modeling framework for dynamic
SPLs, which supports probabilistic and nondeterministic choices and allows for
quantitative analysis. We specify the feature changes during runtime within an
automata-based coordination component, enabling to reason over strategies how
to trigger dynamic feature changes for optimizing various quantitative
objectives, e.g., energy or monetary costs and reliability. For our framework
there is a natural and conceptually simple translation into the input language
of the prominent probabilistic model checker PRISM. This facilitates the
application of PRISM's powerful symbolic engine to the operational behavior of
dynamic SPLs and their family-based analysis against various quantitative
queries. We demonstrate feasibility of our approach by a case study issuing an
energy-aware bonding network device.Comment: 14 pages, 11 figure
NICMOS Imaging of Molecular Hydrogen Emission in Seyfert Galaxies
We present NICMOS imaging of broad band and molecular hydrogen emission in
Seyfert galaxies. In 6 of 10 Seyferts we detect resolved or extended emission
in the 1-0 S(1) 2.121 or 1-0 S(3) 1.9570 micron molecular hydrogen lines. We
did not detect emission in the most distant galaxy or in the 2 Seyfert 1
galaxies in our sample because of the luminosity of the nuclear point sources.
In NGC 5643, NGC 2110 and MKN 1066, molecular hydrogen emission is detected in
the extended narrow line region on scales of a few hundred pc from the nucleus.
Emission is coincident with [OIII] and H alpha+[NII] line emission. This
emission is also near dust lanes observed in the visible to near-infrared color
maps suggesting that a multiphase medium exists near the ionization cones and
that the morphology of the line emission is dependent on the density of the
ambient media. The high 1-0 S(1) or S(3) H2 to H alpha flux ratio suggests that
shock excitation of molecular hydrogen (rather than UV fluorescence) is the
dominant excitation process in these extended features. In NGC 2992 and NGC
3227 the molecular hydrogen emission is from 800 and 100 pc diameter `disks'
(respectively) which are not directly associated with [OIII] emission and are
near high levels of extinction (AV > 10). In NGC 4945 the molecular hydrogen
emission appears to be from the edge of a 100 pc superbubble. In these 3
galaxies the molecular gas could be excited by processes associated with local
star formation. We confirm previous spectroscopic studies finding that no
single mechanism is likely to be responsible for the molecular hydrogen
excitation in Seyfert galaxies.Comment: submitted to Ap
A Massive White Dwarf Companion to the Eccentric Binary Pulsar System PSR B2303+46
Pulsars in close, eccentric binary systems are usually assumed to have
another neutron star as a companion. These double neutron star binaries have
proven to be the best laboratories for experimental General Relativity and are
the most secure candidates for gravitational wave interferometers. We present
deep B, V, and R images of the field containing the eccentric binary pulsar
system PSR B2303+46. We find a faint, blue object (B=26.60+/-0.09;
(B-R)_0=-0.4+/-0.2) coincident with the timing position. We suggest this object
is the optical counterpart to the PSR B2303+46 system. The counterpart is too
bright to reflect emission from the pulsar or a neutron star companion. Most
likely, the companion of PSR B2303+46 is not a neutron star but a massive white
dwarf. We show that the observations are consistent with a hot white dwarf
companion (T_eff>~5*10^4 K) with cooling age equal the characteristic age of
the pulsar (t_cool~30 Myr) and mass within the range set by timing observations
and the Chandrasekhar mass (1.2<M_C<1.4 M_sun). Given the eccentric orbit, the
white dwarf must have formed before the neutron star, from what was originally
the more massive star in the binary. Due to mass transfer, the originally less
massive star could become sufficiently massive to end its life in a supernova
explosion and form the radio pulsar. We constrain the mass of the pulsar to be
in the range 1.24<M_psr<1.44 M_sun.Comment: Revised version, with only very small changes. 6 pages, 1 figure,
uses aas4pp2.sty, psfig.sty. Accepted for publication in Astrophysical
Journal Letter
Shear-induced lamellar ionic liquid-crystal foam
n a recent paper we reported an experimental study of two N-alkylimidazolium salts. These ionic compounds exhibit liquid crystalline behaviour with melting points above 50 degrees C in bulk. However, if they are sheared, a (possibly non-equilibrium) lamellar phase forms at room temperature. Upon shearing a thin film of the material between microscope slides, textures were observed that are strikingly similar to liquid (wet) foams. The images obtained from polarising optical microscopy (POM) were found to share many of the known quantitative properties of a two-dimensional foam coarsening process. Here we report an experimental study of this foam using a shearing system coupled with POM. The structure and evolution of the foam are investigated through the image analysis of time sequences of micrographs obtained for well-controlled sets of physical parameters (sample thickness, shear rate and temperature). In particular, we find that there is a threshold shear rate below which no foam can form. Above this threshold, a steady-state foam pattern is obtained where the mean cell area generally decreases with increasing shear rate. Furthermore, the steady-state internal cell angles and distribution of the cell number of sides deviate from their equilibrium (i.e. zero-shear) values
The ESO UVES Advanced Data Products Quasar Sample - VI. Sub-Damped Lyman- Metallicity Measurements and the Circum-Galactic Medium
The Circum-Galactic Medium (CGM) can be probed through the analysis of
absorbing systems in the line-of-sight to bright background quasars. We present
measurements of the metallicity of a new sample of 15 sub-damped Lyman-
absorbers (sub-DLAs, defined as absorbers with 19.0 < log N(H I) < 20.3) with
redshift 0.584 < < 3.104 from the ESO Ultra-Violet Echelle
Spectrograph (UVES) Advanced Data Products Quasar Sample (EUADP). We combine
these results with other measurements from the literature to produce a
compilation of metallicity measurements for 92 sub-DLAs as well as a sample of
362 DLAs. We apply a multi-element analysis to quantify the amount of dust in
these two classes of systems. We find that either the element depletion
patterns in these systems differ from the Galactic depletion patterns or they
have a different nucleosynthetic history than our own Galaxy. We propose a new
method to derive the velocity width of absorption profiles, using the modeled
Voigt profile features. The correlation between the velocity width delta_V90 of
the absorption profile and the metallicity is found to be tighter for DLAs than
for sub-DLAs. We report hints of a bimodal distribution in the [Fe/H]
metallicity of low redshift (z < 1.25) sub-DLAs, which is unseen at higher
redshifts. This feature can be interpreted as a signature from the metal-poor,
accreting gas and the metal-rich, outflowing gas, both being traced by sub-DLAs
at low redshifts.Comment: 64 pages, 31 figures, 27 tables. Submitted to MNRA
The Nearby Neutron Star RX J0720.4-3125 from Radio to X-rays
We present radio, optical, ultraviolet, and X-ray observations of the
isolated, thermally-emitting neutron star RX J0720.4-3125 using the Parkes
radio telescope, the Very Large Array, the Hubble Space Telescope, and the
Chandra X-ray Observatory. From these data we show that the optical/UV spectrum
of RX J0720.4-3125 is not well fit by a Rayleigh-Jeans tail as previously
thought, but is instead best fit by either a single non-thermal power-law or a
combination of a Rayleigh-Jeans tail and a non-thermal power-law. Taken
together with the X-ray spectrum, we find the best model for RX J0720.4-3125 to
be two blackbodies plus a power-law, with the cool blackbody implying a radius
of 11-13 km at an assumed distance of 300 pc. This is similar to many middle
aged (10^{5-6} yr) radio pulsars such as PSR B0656+14, evidence supporting the
hypothesis that RX J0720.4-3125 is likely to be an off-beam radio pulsar. The
radio data limit the flux at 1.4 GHz to be <0.24 mJy, or a luminosity limit of
4*pi*d^2*F < 3e25*d_300^2 ergs/s, and we see no sign of extended nebulosity,
consistent with expectations for a pulsar like RX J0720.4-3125.Comment: 13 pages, 9 figures. Uses emulateapj5.sty and onecolfloat5.sty.
Accepted for publication in Ap
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