66,207 research outputs found
Dependence of X-Ray Burst Models on Nuclear Reaction Rates
X-ray bursts are thermonuclear flashes on the surface of accreting neutron
stars and reliable burst models are needed to interpret observations in terms
of properties of the neutron star and the binary system. We investigate the
dependence of X-ray burst models on uncertainties in (p,),
(,), and (,p) nuclear reaction rates using fully
self-consistent burst models that account for the feedbacks between changes in
nuclear energy generation and changes in astrophysical conditions. A two-step
approach first identified sensitive nuclear reaction rates in a single-zone
model with ignition conditions chosen to match calculations with a
state-of-the-art 1D multi-zone model based on the {\Kepler} stellar evolution
code. All relevant reaction rates on neutron deficient isotopes up to mass 106
were individually varied by a factor of 100 up and down. Calculations of the 84
highest impact reaction rate changes were then repeated in the 1D multi-zone
model. We find a number of uncertain reaction rates that affect predictions of
light curves and burst ashes significantly. The results provide insights into
the nuclear processes that shape X-ray burst observables and guidance for
future nuclear physics work to reduce nuclear uncertainties in X-ray burst
models.Comment: 24 pages, 13 figures, 4 tables, submitte
Nucleosynthesis in Type I X-ray Bursts
Type I X-ray bursts are thermonuclear explosions that occur in the envelopes
of accreting neutron stars. Detailed observations of these phenomena have
prompted numerous studies in theoretical astrophysics and experimental nuclear
physics since their discovery over 35 years ago. In this review, we begin by
discussing key observational features of these phenomena that may be sensitive
to the particular patterns of nucleosynthesis from the associated thermonuclear
burning. We then summarize efforts to model type I X-ray bursts, with emphasis
on determining the nuclear physics processes involved throughout these bursts.
We discuss and evaluate limitations in the models, particularly with regard to
key uncertainties in the nuclear physics input. Finally, we examine recent,
relevant experimental measurements and outline future prospects to improve our
understanding of these unique environments from observational, theoretical and
experimental perspectives.Comment: Accepted by Prog. Part. Nucl. Phys., 45 pages, 14 figure
Perception of non-verbal emotional listener feedback
This paper reports on a listening test assessing the perception of short non-verbal emotional vocalisations emitted by a listener as feedback to the speaker. We clarify the concepts backchannel and feedback, and investigate the use of affect bursts as a means of giving emotional feedback via the backchannel. Experiments with German and Dutch subjects confirm that the recognition of emotion from affect bursts in a dialogical context is similar to their perception in isolation. We also investigate the acceptability of affect bursts when used as listener feedback. Acceptability appears to be linked to display rules for emotion expression. While many ratings were similar between Dutch and German listeners, a number of clear differences was found, suggesting language-specific affect bursts
Classical novae and type I X-ray bursts: challenges for the 21st century
Classical nova explosions and type I X-ray bursts are the most frequent types
of thermonuclear stellar explosions in the Galaxy. Both phenomena arise from
thermonuclear ignition in the envelopes of accreting compact objects in close
binary star systems. Detailed observations of these events have stimulated
numerous studies in theoretical astrophysics and experimental nuclear physics.
We discuss observational features of these phenomena and theoretical efforts to
better understand the energy production and nucleosynthesis in these
explosions. We also examine and summarize studies directed at identifying
nuclear physics quantities with uncertainties that significantly affect model
predictions.Comment: 40 pages, accepted for AIP Advances: Stardust - Progress and Problems
in Nuclear Astrophysic
The Sensitivity of Nucleosynthesis in Type I X-ray Bursts to Thermonuclear Reaction-Rate Variations
We examine the sensitivity of nucleosynthesis in Type I X-ray bursts to
variations in nuclear rates. As a large number of nuclear processes are
involved in these phenomena -with the vast majority of reaction rates only
determined theoretically due to the lack of any experimental information- our
results can provide a means for determining which rates play significant roles
in the thermonuclear runaway. These results may then motivate new experiments.
For our studies, we have performed a comprehensive series of one-zone
post-processing calculations in conjunction with various representative X-ray
burst thermodynamic histories. We present those reactions whose rate variations
have the largest effects on yields in our studies.Comment: 8 pages, accepted for publication in New Astronomy Reviews, Special
Issue on "Astronomy with Radioactivities VI" workshop, Ringberg Castle,
Germany, Jan. 200
Cosmic rays and TeV photons as probes of quantum properties of space-time
It has been recently observed that small violations of Lorentz invariance, of
a type which may arise in quantum gravity, could explain both the observations
of cosmic rays above the GZK cutoff and the observations of 20-TeV gamma rays
from Markarian 501. We show here that different pictures of the short-distance
structure of space-time would lead to different manifestations of
Lorentz-invariance violation. Specifically, the deformation of Lorentz
invariance needed to resolve these observational paradoxes can only arise
within commutative short-distance pictures of space-time. In noncommutative
space-times there is no anomalous effect, at least at leading order. Also
exploiting the fact that arrival-time delays between high energy photons with
different energies would arise in both the commutative and the noncommutative
Lorentz-violation pictures, we describe an experimental programme, based on
time-of-arrival analysis of high energy photons and searches of violations of
GZK and TeV-photon limits, which could discriminate between alternative
scenarios of Lorentz-invariance breakdown and could provide and unexpected
window on the (quantum) nature of space-time at very short distances.Comment: 8 pages, LaTe
Phenomenology of Particle Production and Propagation in String-Motivated Canonical Noncommutative Spacetime
We outline a phenomenological programme for the search of effects induced by
(string-motivated) canonical noncommutative spacetime. The tests we propose are
based, in analogy with a corresponding programme developed over the last few
years for the study of Lie-algebra noncommutative spacetimes, on the role of
the noncommutativity parameters in the dispersion relation. We focus on
the role of deformed dispersion relations in particle-production collision
processes, where the noncommutativity parameters would affect the threshold
equation, and in the dispersion of gamma rays observed from distant
astrophysical sources. We emphasize that the studies here proposed have the
advantage of involving particles of relatively high energies, and may therefore
be less sensitive to "contamination" (through IR/UV mixing) from the UV sector
of the theory. We also explore the possibility that the relevant deformation of
the dispersion relations could be responsible for the experimentally-observed
violations of the GZK cutoff for cosmic rays and could have a role in the
observation of hard photons from distant astrophysical sources.Comment: With respect to the experimental information available at the time of
writing version 1 of this manuscript (hep-th/0109191v1) the situation has
evolved significantly. Our remarks on the benefits of high-energy
observations found additional encouragement from the results reported in
hep-th/020925
Searching for plasticity in dissociated cortical cultures on multi-electrode arrays
We attempted to induce functional plasticity in dense cultures of cortical cells using stimulation through extracellular electrodes embedded in the culture dish substrate (multi-electrode arrays, or MEAs). We looked for plasticity expressed in changes in spontaneous burst patterns, and in array-wide response patterns to electrical stimuli, following several induction protocols related to those used in the literature, as well as some novel ones. Experiments were performed with spontaneous culture-wide bursting suppressed by either distributed electrical stimulation or by elevated extracellular magnesium concentrations as well as with spontaneous bursting untreated. Changes concomitant with induction were no larger in magnitude than changes that occurred spontaneously, except in one novel protocol in which spontaneous bursts were quieted using distributed electrical stimulation
Experimental analysis and computational modeling of interburst intervals in spontaneous activity of cortical neuronal culture
Rhythmic bursting is the most striking behavior of cultured cortical networks and may start in the second week after plating. In this study, we focus on the intervals between spontaneously occurring bursts, and compare experimentally recorded values with model simulations. In the models, we use standard neurons and synapses, with physiologically plausible parameters taken from literature. All networks had a random recurrent architecture with sparsely connected neurons. The number of neurons varied between 500 and 5,000. We find that network models with homogeneous synaptic strengths produce asynchronous spiking or stable regular bursts. The latter, however, are in a range not seen in recordings. By increasing the synaptic strength in a (randomly chosen) subset of neurons, our simulations show interburst intervals (IBIs) that agree better with in vitro experiments. In this regime, called weakly synchronized, the models produce irregular network bursts, which are initiated by neurons with relatively stronger synapses. In some noise-driven networks, a subthreshold, deterministic, input is applied to neurons with strong synapses, to mimic pacemaker network drive. We show that models with such âintrinsically active neuronsâ (pacemaker-driven models) tend to generate IBIs that are determined by the frequency of the fastest pacemaker and do not resemble experimental data. Alternatively, noise-driven models yield realistic IBIs. Generally, we found that large-scale noise-driven neuronal network models required synaptic strengths with a bimodal distribution to reproduce the experimentally observed IBI range. Our results imply that the results obtained from small network models cannot simply be extrapolated to models of more realistic size. Synaptic strengths in large-scale neuronal network simulations need readjustment to a bimodal distribution, whereas small networks do not require such change
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