426 research outputs found
Optical pulsations in HZ Herculis. 5. Pulse-resolved spectrophotometry
Digital spectra of HZ Herculis were obtained with 10 A resolution in the 3,600 - 6,000 A region, synchronously dividing the 1.24-s optical pulsation period into eight 155-ms phase bins. The optical pulses are detected in the data, but their fractional amplitude is only 0.08 percent, a factor of 4 less than typically observed. The separate spectra of each one-eighth of the pulse are identical to within the statistics of the observation. If the X-ray to optical pulse reprocessing mechanism concentrates the optical pulsations into discrete spectral line features, data require the pulses to be distributed among more than four such lines
Attractor and Basin Entropies of Random Boolean Networks Under Asynchronous Stochastic Update
We introduce a numerical method to study random Boolean networks with
asynchronous stochas- tic update. Each node in the network of states starts
with equal occupation probability and this probability distribution then
evolves to a steady state. Nodes left with finite occupation probability
determine the attractors and the sizes of their basins. As for synchronous
update, the basin entropy grows with system size only for critical networks,
where the distribution of attractor lengths is a power law. We determine
analytically the distribution for the number of attractors and basin sizes for
frozen networks with connectivity K = 1.Comment: 5 pages, 3 figures, in submissio
Random sampling vs. exact enumeration of attractors in random Boolean networks
We clarify the effect different sampling methods and weighting schemes have
on the statistics of attractors in ensembles of random Boolean networks (RBNs).
We directly measure cycle lengths of attractors and sizes of basins of
attraction in RBNs using exact enumeration of the state space. In general, the
distribution of attractor lengths differs markedly from that obtained by
randomly choosing an initial state and following the dynamics to reach an
attractor. Our results indicate that the former distribution decays as a
power-law with exponent 1 for all connectivities in the infinite system
size limit. In contrast, the latter distribution decays as a power law only for
K=2. This is because the mean basin size grows linearly with the attractor
cycle length for , and is statistically independent of the cycle length
for K=2. We also find that the histograms of basin sizes are strongly peaked at
integer multiples of powers of two for
Rapid convergence of time-averaged frequency in phase synchronized systems
Numerical and experimental evidence is presented to show that many phase
synchronized systems of non-identical chaotic oscillators, where the chaotic
state is reached through a period-doubling cascade, show rapid convergence of
the time-averaged frequency. The speed of convergence toward the natural
frequency scales as the inverse of the measurement period. The results also
suggest an explanation for why such chaotic oscillators can be phase
synchronized.Comment: 6 pages, 9 figure
Evaluation of Specific Symptoms of Bacterial Vaginosis Among Pregnant Women
Objective: Identification of the symptoms of bacterial vaginosis (BV) in pregnancy might be rational in order to identify a possible BV-associated group at risk of preterm delivery
Far-UV Observations of NGC 4151 during the ORFEUS-SPAS II Mission
We observed the Seyfert 1 galaxy NGC 4151 on eleven occasions at 1-2 day
intervals using the Berkeley spectrometer during the ORFEUS-SPAS II mission in
1996 November. The mean spectrum covers 912-1220 A at ~0.3 A resolution with a
total exposure of 15,658 seconds. The mean flux at 1000 A was 4.7e-13
erg/cm^2/s/A. We identify the neutral hydrogen absorption with a number of
components that correspond to the velocity distribution of \ion{H}{1} seen in
our own Galaxy as well as features identified in the CIV 1549 absorption
profile by Weymann et al. The main component of neutral hydrogen in NGC 4151
has a total column density of log N_HI = 18.7 +/- 1.5 cm^{-2} for a Doppler
parameter b=250 +/- 50 km/s, and it covers 84 +/- 6% of the source. This is
consistent with previous results obtained with the Hopkins Ultraviolet
Telescope. Other intrinsic far-UV absorption features are not resolved, but the
CIII* 1176 absorption line has a significantly higher blueshift relative to NGC
4151 than the CIII 977 resonance line. This implies that the highest velocity
region of the outflowing gas has the highest density. Variations in the
equivalent width of the CIII* 1176 absorption line anticorrelate with continuum
variations on timescales of days. For an ionization timescale <1 day, we set an
upper limit of 25 pc on the distance of the absorbing gas from the central
source. The OVI 1034 and HeII 1085 emission lines also vary on timescales of
1-2 days, but their response to the continuum variations is complex. For some
continuum variations they show no response, while for others the response is
instantaneous to the limit of our sampling interval.Comment: 4 pages, 2 PostScript figures, uses emulateapj.sty, apjfonts.sty. To
appear in the Astrophysical Journal (Letters) special issue for ORFEU
Extreme value statistics and return intervals in long-range correlated uniform deviates
We study extremal statistics and return intervals in stationary long-range
correlated sequences for which the underlying probability density function is
bounded and uniform. The extremal statistics we consider e.g., maximum relative
to minimum are such that the reference point from which the maximum is measured
is itself a random quantity. We analytically calculate the limiting
distributions for independent and identically distributed random variables, and
use these as a reference point for correlated cases. The distributions are
different from that of the maximum itself i.e., a Weibull distribution,
reflecting the fact that the distribution of the reference point either
dominates over or convolves with the distribution of the maximum. The
functional form of the limiting distributions is unaffected by correlations,
although the convergence is slower. We show that our findings can be directly
generalized to a wide class of stochastic processes. We also analyze return
interval distributions, and compare them to recent conjectures of their
functional form
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