1,881 research outputs found
Intra-day Variability of Sagittarius A* at 3 Millimeters
We report observations and analysis of flux monitoring of Sagittarius A* at
3-mm wavelength using the OVRO millimeter interferometer over a period of eight
days (2002 May 23-30). Frequent phase and flux referencing (every 5 minutes)
with the nearby calibrator source J1744-312 was employed to control for
instrumental and atmospheric effects. Time variations are sought by computing
and subtracting, from each visibility in the database, an average visibility
obtained from all the data acquired in our monitoring program having similar uv
spacings. This removes the confusing effects of baseline-dependent, correlated
flux interference caused by the static, thermal emission from the extended
source Sgr A West. Few-day variations up to ~20% and intra-day variability of
\~20% and in some cases up to ~40% on few-hour time scales emerge from the
differenced data on SgrA*. Power spectra of the residuals indicate the presence
of hourly variations on all but two of the eight days. Monte Carlo simulation
of red-noise light curves indicates that the hourly variations are well
described by a red-noise power spectrum with P(f) ~ f^(-1). Of particular
interest is a ~2.5 hour variation seen prominently on two consecutive days. An
average power spectrum from all eight days of data reveals noteworthy power on
this time scale. There is some indication that few-hour variations are more
pronounced on days when the average daily flux is highest. We briefly discuss
the possibility that these few-hour variations are due to the dynamical
modulation of accreting gas around the central supermassive black hole, as well
as the implications for the structure of the SgrA* photosphere at 3 mm.
Finally, these data have enabled us to produce a high sensitivity 3-mm map of
the extended thermal emission surrounding SgrA*.Comment: Accepted for publication in The Astrophysical Journal Letters, 8
pages, 4 figure
X-ray vs. Optical Variations in the Seyfert 1 Nucleus NGC 3516: A Puzzling Disconnectedness
We present optical broadband (B and R) observations of the Seyfert 1 nucleus
NGC 3516, obtained at Wise Observatory from March 1997 to March 2002,
contemporaneously with X-ray 2-10 keV measurements with RXTE. With these data
we increase the temporal baseline of this dataset to 5 years, more than triple
to the coverage we have previously presented for this object. Analysis of the
new data does not confirm the 100-day lag of X-ray behind optical variations,
tentatively reported in our previous work. Indeed, excluding the first year's
data, which drive the previous result, there is no significant correlation at
any lag between the X-ray and optical bands. We also find no correlation at any
lag between optical flux and various X-ray hardness ratios. We conclude that
the close relation observed between the bands during the first year of our
program was either a fluke, or perhaps the result of the exceptionally bright
state of NGC 3516 in 1997, to which it has yet to return. Reviewing the results
of published joint X-ray and UV/optical Seyfert monitoring programs, we
speculate that there are at least two components or mechanisms contributing to
the X-ray continuum emission up to 10 keV: a soft component that is correlated
with UV/optical variations on timescales >1 day, and whose presence can be
detected when the source is observed at low enough energies (about 1 keV), is
unabsorbed, or is in a sufficiently bright phase; and a hard component whose
variations are uncorrelated with the UV/optical.Comment: 9 pages, AJ, in pres
Exact numerical simulation of power-law noises
Many simulations of stochastic processes require colored noises: I describe
here an exact numerical method to simulate power-law noises: the method can be
extended to more general colored noises, and is exact for all time steps, even
when they are unevenly spaced (as may often happen for astronomical data, see
e.g. N. R. Lomb, Astrophys. Space Sci. {\bf 39}, 447 (1976)). The algorithm has
a well-behaved computational complexity, it produces a nearly perfect Gaussian
noise, and its computational efficiency depends on the required degree of noise
Gaussianity.Comment: 14 postscript figures, accepted for publication on Phys. Rev.
Correlated X-ray and Optical Variability in Mkn 509
We present results of a 3 year monitoring campaign of the Seyfert 1 galaxy
Markarian 509, using X-ray data from the Rossi X-ray Timing Explorer (RXTE) and
optical data taken by the SMARTS consortium. Both light curves show significant
variations, and are strongly correlated with the optical flux leading the X-ray
flux by 15 days. The X-ray power spectrum shows a steep high-frequency slope of
-2.0, breaking to a slope of -1.0 at at timescale of 34 days. The lag from
optical to X-ray emission is most likely caused by variations in the accretion
disk propagating inward.Comment: 13 pages, 3 figures. Accepted for publication in the Astrophysical
Journa
Talking quiescence: a rigorous theory that supports parallel composition, action hiding and determinisation
The notion of quiescence - the absence of outputs - is vital in both
behavioural modelling and testing theory. Although the need for quiescence was
already recognised in the 90s, it has only been treated as a second-class
citizen thus far. This paper moves quiescence into the foreground and
introduces the notion of quiescent transition systems (QTSs): an extension of
regular input-output transition systems (IOTSs) in which quiescence is
represented explicitly, via quiescent transitions. Four carefully crafted rules
on the use of quiescent transitions ensure that our QTSs naturally capture
quiescent behaviour.
We present the building blocks for a comprehensive theory on QTSs supporting
parallel composition, action hiding and determinisation. In particular, we
prove that these operations preserve all the aforementioned rules.
Additionally, we provide a way to transform existing IOTSs into QTSs, allowing
even IOTSs as input that already contain some quiescent transitions. As an
important application, we show how our QTS framework simplifies the fundamental
model-based testing theory formalised around ioco.Comment: In Proceedings MBT 2012, arXiv:1202.582
A Multi-Wavelength Study of Sgr A*: The Role of Near-IR Flares in Production of X-ray, Soft -ray and Sub-millimeter Emission
(abridged) We describe highlights of the results of two observing campaigns
in 2004 to investigate the correlation of flare activity in Sgr A* in different
wavelength regimes, using a total of nine ground and space-based telescopes. We
report the detection of several new near-IR flares during the campaign based on
{\it HST} observations. The level of near-IR flare activity can be as low as
mJy at 1.6 m and continuous up to about 40% of the total
observing time. Using the NICMOS instrument on the {\it HST}, the {\it
XMM-Newton} and CSO observatories, we also detect simultaneous bright X-ray and
near-IR flare in which we observe for the first time correlated substructures
as well as simultaneous submillimeter and near-IR flaring. X-ray emission is
arising from the population of near-IR-synchrotron-emitting relativistic
particles which scatter submillimeter seed photons within the inner 10
Schwarzschild radii of Sgr A* up to X-ray energies. In addition, using the
inverse Compton scattering picture, we explain the high energy 20-120 keV
emission from the direction toward Sgr A*, and the lack of one-to-one X-ray
counterparts to near-IR flares, by the variation of the magnetic field and the
spectral index distributions of this population of nonthermal particles. In
this picture, the evidence for the variability of submillimeter emission during
a near-IR flare is produced by the low-energy component of the population of
particles emitting synchrotron near-IR emission. Based on the measurements of
the duration of flares in near-IR and submillimeter wavelengths, we argue that
the cooling could be due to adiabatic expansion with the implication that flare
activity may drive an outflow.Comment: 48 pages, 12 figures, ApJ (in press
Neuron dynamics in the presence of 1/f noise
Interest in understanding the interplay between noise and the response of a
non-linear device cuts across disciplinary boundaries. It is as relevant for
unmasking the dynamics of neurons in noisy environments as it is for designing
reliable nanoscale logic circuit elements and sensors. Most studies of noise in
non-linear devices are limited to either time-correlated noise with a
Lorentzian spectrum (of which the white noise is a limiting case) or just white
noise. We use analytical theory and numerical simulations to study the impact
of the more ubiquitous "natural" noise with a 1/f frequency spectrum.
Specifically, we study the impact of the 1/f noise on a leaky integrate and
fire model of a neuron. The impact of noise is considered on two quantities of
interest to neuron function: The spike count Fano factor and the speed of
neuron response to a small step-like stimulus. For the perfect (non-leaky)
integrate and fire model, we show that the Fano factor can be expressed as an
integral over noise spectrum weighted by a (low pass) filter function. This
result elucidates the connection between low frequency noise and disorder in
neuron dynamics. We compare our results to experimental data of single neurons
in vivo, and show how the 1/f noise model provides much better agreement than
the usual approximations based on Lorentzian noise. The low frequency noise,
however, complicates the case for information coding scheme based on interspike
intervals by introducing variability in the neuron response time. On a positive
note, the neuron response time to a step stimulus is, remarkably, nearly
optimal in the presence of 1/f noise. An explanation of this effect elucidates
how the brain can take advantage of noise to prime a subset of the neurons to
respond almost instantly to sudden stimuli.Comment: Phys. Rev. E in pres
Disorder-Induced Shift of Condensation Temperature for Dilute Trapped Bose Gases
We determine the leading shift of the Bose-Einstein condensation temperature
for an ultracold dilute atomic gas in a harmonic trap due to weak disorder by
treating both a Gaussian and a Lorentzian spatial correlation for the quenched
disorder potential. Increasing the correlation length from values much smaller
than the geometric mean of the trap scale and the mean particle distance to
much larger values leads first to an increase of the positive shift to a
maximum at this critical length scale and then to a decrease.Comment: Author information under
http://www.theo-phys.uni-essen.de/tp/ags/pelster_di
Phase synchronization from noisy univariate signals
We present methods for detecting phase synchronization of two
unidirectionally coupled, self-sustained noisy oscillators from a signal of the
driven oscillator alone. One method detects soft, another hard phase locking.
Both are applied to the problem of detecting phase synchronization in von
Karman vortex flow meters.Comment: 4 pages, 4 figure
Transformation of char carbon during bubbling fluidized bed gasification of biomass
This study focuses on the fate of carbon in the char generated by devolatilization of biomass during fluidized bed gasification. A carbon balance model was developed to distinguish between char transformed to carbon-bearing gases and its comminution and elutriation as fine char during gasification. The model accurately predicts the transient accumulation of char carbon in the reactor. Experiments revealed steady state reactor char carbon loadings were achieved after multiple hours of gasification. The model formed the basis of an experimental methodology that assesses the transformation of char carbon based on collection of elutriated solids from the reactor and assessment of the steady state char carbon loading in the reactor. Experiments were performed to distinguish the relative contributions of chemical reaction and physical comminution toward conversion of char to gaseous and solid products. The effects of equivalence ratio, gasification temperature, superficial gas velocity, biomass particle size, and the addition of steam on the partitioning of char carbon between gaseous and solid products during gasification of ground seed corn in a bubbling fluidized bed were investigated. This study revealed that char conversion during gasification of biomass was limited by elutriation of fine char particles arising from fragmentation or attrition of primary char product. Additionally, increased chemical reaction of char to form gases was usually accompanied by increased elutriation of fine char, which suggests that chemical reaction increased the porosity of the char and its susceptibility to fragmentation and attrition. Finally, decreasing superficial gas velocity, increasing equivalence ratio, and decreasing particle size led to increased carbon conversion, while increasing temperature and steam concentration in the reactor had negligible effect
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