33,178 research outputs found
Optical and X-ray Variability in The Least Luminous AGN, NGC4395
We report the detection of optical and X-ray variability in the least
luminous known Seyfert galaxy, NGC4395. The featureless continuum changed by a
factor of 2 in 6 months, which is typical of more luminous AGN. The largest
variation was seen at shorter wavelengths, so that the spectrum becomes
`harder' during higher activity states. In a one week optical broad band
monitoring program, a 20% change was seen between successive nights. In a 1
month period the spectral shape changed from a power law with spectral index
alpha ~0 (characteristic of quasars) to a spectral index alpha ~2 (as observed
in other dwarf AGN). ROSAT HRI and PSPC archive data show a variable X-ray
source coincident with the galactic nucleus. A change in X-ray flux by a factor
\~2 in 15 days has been observed. When compared with more luminous AGN, NGC4395
appears to be very X-ray quiet. The hardness ratio obtained from the PSPC data
suggests that the spectrum could be absorbed. We also report the discovery of
weak CaIIK absorption, suggesting the presence of a young stellar cluster
providing of the order of 10% of the blue light. Using HST UV archive data,
together with the optical and X-ray observations, we examine the spectral
energy distribution for NGC4395 and discuss the physical conditions implied by
the nuclear activity under the standard AGN model. The observations can be
explained by either an accreting massive black hole emitting at about 10^(-3)
L_(Edd) or by a single old compact SNR with an age of 50 to 500 yr generated by
a small nuclear starburst.Comment: 19 pages, 9 figures, to appear in MNRA
Effective forecasting for supply-chain planning: an empirical evaluation and strategies for improvement
Demand forecasting is a crucial aspect of the planning process in supply-chain companies. The most common approach to forecasting demand in these companies involves the use of a simple univariate statistical method to produce a forecast and the subsequent judgmental adjustment of this by the company's demand planners to take into account market intelligence relating to any exceptional circumstances expected over the planning horizon. Based on four company case studies, which included collecting more than 12,000 forecasts and outcomes, this paper examines: i) the extent to which the judgmental adjustments led to improvements in accuracy, ii) the extent to which the adjustments were biased and inefficient, iii) the circumstances where adjustments were detrimental or beneficial, and iv) methods that could lead to greater levels of accuracy. It was found that the judgmentally adjusted forecasts were both biased and inefficient. In particular, market intelligence that was expected to have a positive impact on demand was used far less effectively than intelligence suggesting a negative impact. The paper goes on to propose a set of improvements that could be applied to the forecasting processes in the companies and to the forecasting software that is used in these processes
Short time-scale optical variability of the dwarf Seyfert nucleus in NGC 4395
We present optical spectroscopic observations of the least-luminous known
Seyfert 1 galaxy, NGC 4395, which was monitored every half-hour over the course
of 3 nights. The continuum emission varied by ~35 per cent over the course of 3
nights, and we find marginal evidence for greater variability in the blue
continuum than the red. A number of diagnostic checks were performed on the
data in order to constrain any systematic or aperture effects. No correlations
were found that adequately explained the observed variability, hence we
conclude that we have observed real intrinsic variability of the nuclear
source. No simultaneous variability was measured in the broad H-beta line,
although given the difficulty in deblending the broad and narrow components it
is difficult to comment on the significance of this result. The observed short
time-scale continuum variability is consistent with NGC 4395 having an
intermediate-mass (~10^5 solar masses) central supermassive black hole, rather
than a very low accretion rate. Comparison with the Seyfert 1 galaxy NGC 5548
shows that the observed variability seems to scale with black hole mass in
roughly the manner expected in accretion models. However the absolute
time-scale of variability differs by several orders of magnitude from that
expected in simple accretion disc models in both cases.Comment: 16 pages, 14 figures, 5 tables, accepted for publication in MNRA
Cell wall microstructure, pore size distribution and absolute density of hemp shiv
This paper, for the first time, fully characterizes the intrinsic physical parameters of hemp shiv including cell wall microstructure, pore size distribution and absolute density. Scanning electron microscopy revealed microstructural features similar to hardwoods. Confocal microscopy revealed three major layers in the cell wall: middle lamella, primary cell wall and secondary cell wall. Computed tomography improved the visualization of pore shape and pore connectivity in three dimensions. Mercury intrusion porosimetry (MIP) showed that the average accessible porosity was 76.67 ± 2.03% and pore size classes could be distinguished into micropores (3–10 nm) and macropores (0.1–1 µm and 20–80 µm). The absolute density was evaluated by helium pycnometry, MIP and Archimedes’ methods. The results show that these methods can lead to misinterpretation of absolute density. The MIP method showed a realistic absolute density (1.45 g cm−3) consistent with the density of the known constituents, including lignin, cellulose and hemi-cellulose. However, helium pycnometry and Archimedes’ methods gave falsely low values owing to 10% of the volume being inaccessible pores, which require sample pretreatment in order to be filled by liquid or gas. This indicates that the determination of the cell wall density is strongly dependent on sample geometry and preparation
Contrasting the beam interaction characteristics of selected lasers with a partially stabilised zirconia (PSZ) bio-ceramic
Differences in the beam interaction characteristics of a CO2 laser, a Nd:YAG laser, a high power diode laser (HPDL) and an excimer laser with a partially stabilised zirconia (PSZ) bio-ceramic have been studied. A derivative of Beer-Lambert’s law was applied and the laser beam absorption lengths of the four lasers were calculated as 33.55 x 10-3 cm for the CO2 laser, 18.22 x 10-3 cm for the Nd:YAG laser, 17.17 x 10-3 cm for the HPDL and 8.41 x 10-6 cm for the excimer laser. It was determined graphically that the fluence threshold values at which significant material removal was effected by the CO2 laser, the Nd:YAG laser, the HPDL and the excimer laser were 52 J/cm2, 97 J/cm2, 115 J/cm2 and 0.48 J/cm2 respectively. The thermal loading value for the CO2 laser, the Nd:YAG laser, the HPDL and the excimer laser were calculated as being 1.55 kJ/cm3, 5.32 kJ/cm3, 6.69 kJ/cm3 and 57.04 kJ/cm3 respectively
Entanglement Patterns in Mutually Unbiased Basis Sets for N Prime-state Particles
A few simply-stated rules govern the entanglement patterns that can occur in
mutually unbiased basis sets (MUBs), and constrain the combinations of such
patterns that can coexist (ie, the stoichiometry) in full complements of p^N+1
MUBs. We consider Hilbert spaces of prime power dimension (as realized by
systems of N prime-state particles, or qupits), where full complements are
known to exist, and we assume only that MUBs are eigenbases of generalized
Pauli operators, without using a particular construction. The general rules
include the following: 1) In any MUB, a particular qupit appears either in a
pure state, or totally entangled, and 2) in any full MUB complement, each qupit
is pure in p+1 bases (not necessarily the same ones), and totally entangled in
the remaining p^N-p. It follows that the maximum number of product bases is
p+1, and when this number is realized, all remaining p^N-p bases in the
complement are characterized by the total entanglement of every qupit. This
"standard distribution" is inescapable for two qupits (of any p), where only
product and generalized Bell bases are admissible MUB types. This and the
following results generalize previous results for qubits and qutrits. With
three qupits there are three MUB types, and a number of combinations (p+2) are
possible in full complements. With N=4, there are 6 MUB types for p=2, but new
MUB types become possible with larger p, and these are essential to the
realization of full complements. With this example, we argue that new MUB
types, showing new entanglement characteristics, should enter with every step
in N, and when N is a prime plus 1, also at critical p values, p=N-1. Such MUBs
should play critical roles in filling complements.Comment: 27 pages, one figure, to be submitted to Physical Revie
Simulation at Dryden Flight Research Facility from 1957 to 1982
The Dryden Flight Research Facility has been a leader in developing simulation as an integral part of flight test research. The history of that effort is reviewed, starting in 1957 and continuing to the present time. The contributions of the major program activities conducted at Dryden during this 25-year period to the development of a simulation philosophy and capability is explained
A Nearly Scale Invariant Spectrum of Gravitational Radiation from Global Phase Transitions
Using a large N sigma model approximation we explicitly calculate the power
spectrum of gravitational waves arising from a global phase transition in the
early universe and we confirm that it is scale invariant, implying an
observation of such a spectrum may not be a unique feature of inflation.
Moreover, the predicted amplitude can be over 3 orders of magnitude larger than
the naive dimensional estimate, implying that even a transition that occurs
after inflation may dominate in Cosmic Microwave Background polarization or
other gravity wave signals.Comment: 4 pages, PRL published versio
Cis-regulatory basis of sister cell type divergence in the vertebrate retina
Multicellular organisms evolved via repeated functional divergence of transcriptionally related sister cell types, but the mechanisms underlying sister cell type divergence are not well understood. Here, we study a canonical pair of sister cell types, retinal photoreceptors and bipolar cells, to identify the ke
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