586 research outputs found
Optical phase coherent timing of the Crab nebula pulsar with Iqueye at the ESO New Technology Telescope
The Crab nebula pulsar was observed in 2009 January and December with a novel
very fast optical photon counter, Iqueye, mounted at the ESO 3.5 m New
Technology Telescope. Thanks to the exquisite quality of the Iqueye data, we
computed accurate phase coherent timing solutions for the two observing runs
and over the entire year 2009. Our statistical uncertainty on the determination
of the phase of the main pulse and the rotational period of the pulsar for
short (a few days) time intervals are s and ~0.5 ps,
respectively. Comparison with the Jodrell Bank radio ephemerides shows that the
optical pulse leads the radio one by ~240 s in January and ~160 s in
December, in agreement with a number of other measurements performed after
1996. A third-order polynomial fit adequately describes the spin-down for the
2009 January plus December optical observations. The phase noise is consistent
with being Gaussian distributed with a dispersion of s in most observations, in agreement with theoretical expectations for
photon noise-induced phase variability.Comment: 10 pages, 5 figures. Accepted for publication in Monthly Notices of
the Royal Astronomical Societ
Aqueye+: a new ultrafast single photon counter for optical high time resolution astrophysics
Aqueye+ is a new ultrafast optical single photon counter, based on single
photon avalanche photodiodes (SPAD) and a 4-fold split-pupil concept. It is a
completely revisited version of its predecessor, Aqueye, successfully mounted
at the 182 cm Copernicus telescope in Asiago. Here we will present the new
technological features implemented on Aqueye+, namely a state of the art timing
system, a dedicated and optimized optical train, a high sensitivity and high
frame rate field camera and remote control, which will give Aqueye plus much
superior performances with respect to its predecessor, unparalleled by any
other existing fast photometer. The instrument will host also an optical
vorticity module to achieve high performance astronomical coronography and a
real time acquisition of atmospheric seeing unit. The present paper describes
the instrument and its first performances.Comment: Proceedings of the SPIE, Volume 9504, id. 95040C 14 pp. (2015
Apparent digestibility of compound diets with increasing levels of perilla (Perilla frutescens L.) seeds in rabbit.
The aim of this study was to determine the effects of three levels (0, 5 and 10%) of perilla (Perilla frutescens L.) seeds (PFS), included in isonitrogenous and isocaloric diets, on the apparent digestibility in rabbit aged of 73 days. The trial was carried out on 30 crossbred (Carmagnola Grey x New Zealand) rabbits randomly divided in three groups of ten animals each (five male and five female rabbits). Each of them was kept in individual cages. The faeces were collected during the last week of a growing trial that lasted 50 days. No obvious health problems were encountered during the experiment and no rabbits died during the trial. The measured parameters were digestibility of dry matter, organic matter, crude protein, ether extract, neutral detergent fibre, acid detergent fibre and gross energy. The only parameter that was modified by the inclusion of PFS was the ether extract digestibility; it resulted higher in the 10% PFS diet (83.9%) than in the other two diets. Perilla seed may be used satisfactorily as a nutrient supplement for rabbits at levels of up to 10% in the diet with a better digestibility of ether extract than in the other two diets
Estimate of a spatially variable reservoir compressibility by assimilation of ground surface displacement data
Abstract.
Fluid extraction from producing hydrocarbon reservoirs can cause anthropogenic land subsidence. In
this work, a 3-D finite-element (FE) geomechanical model is used to predict the land surface displacements above
a gas field where displacement observations are available. An ensemble-based data assimilation (DA) algorithm
is implemented that incorporates these observations into the response of the FE geomechanical model, thus re-
ducing the uncertainty on the geomechanical parameters of the sedimentary basin embedding the reservoir. The
calibration focuses on the uniaxial vertical compressibility
c
M
, which is often the geomechanical parameter to
which the model response is most sensitive. The partition of the reservoir into blocks delimited by faults moti-
vates the assumption of a heterogeneous spatial distribution of
c
M
within the reservoir. A preliminary synthetic
test case is here used to evaluate the effectiveness of the DA algorithm in reducing the parameter uncertainty
associated with a heterogeneous
c
M
distribution. A significant improvement in matching the observed data is
obtained with respect to the case in which a homogeneous
c
M
is hypothesized. These preliminary results are
quite encouraging and call for the application of the procedure to real gas fields
Aqueye optical observations of the Crab Nebula pulsar
We observed the Crab pulsar in October 2008 at the Copernico Telescope in
Asiago - Cima Ekar with the optical photon counter Aqueye (the Asiago Quantum
Eye) which has the best temporal resolution and accuracy ever achieved in the
optical domain (hundreds of picoseconds). Our goal was to perform a detailed
analysis of the optical period and phase drift of the main peak of the Crab
pulsar and compare it with the Jodrell Bank ephemerides. We determined the
position of the main peak using the steepest zero of the cross-correlation
function between the pulsar signal and an accurate optical template. The pulsar
rotational period and period derivative have been measured with great accuracy
using observations covering only a 2 day time interval. The error on the period
is 1.7 ps, limited only by the statistical uncertainty. Both the rotational
frequency and its first derivative are in agreement with those from the Jodrell
Bank radio ephemerides archive. We also found evidence of the optical peak
leading the radio one by ~230 microseconds. The distribution of phase-residuals
of the whole dataset is slightly wider than that of a synthetic signal
generated as a sequence of pulses distributed in time with the probability
proportional to the pulse shape, such as the average count rate and background
level are those of the Crab pulsar observed with Aqueye. The counting
statistics and quality of the data allowed us to determine the pulsar period
and period derivative with great accuracy in 2 days only. The time of arrival
of the optical peak of the Crab pulsar leads the radio one in agreement with
what recently reported in the literature. The distribution of the phase
residuals can be approximated with a Gaussian and is consistent with being
completely caused by photon noise (for the best data sets).Comment: 7 pages, 7 figures. Accepted for publication in Astronomy and
Astrophysic
On the importance of the heterogeneity assumption in the characterization of reservoir geomechanical properties
The geomechanical analysis of a highly compartmentalized reservoir is performed to simulate
the seafloor subsidence due to gas production. The available observations over the hydrocarbon
reservoir consist of bathymetric surveys carried out before and at the end of a 10-yr
production life. The main goal is the calibration of the reservoir compressibility cM, that is,
the main geomechanical parameter controlling the surface response. Two conceptual models
are considered: in one (i) cM varies only with the depth and the vertical effective stress
(heterogeneity due to lithostratigraphic variability); in another (ii) cM varies also in the horizontal
plane, that is, it is spatially distributed within the reservoir stratigraphic units. The latter
hypothesis accounts for a possible partitioning of the reservoir due to the presence of sealing
faults and thrusts that suggests the idea of a block heterogeneous system with the number of
reservoir blocks equal to the number of uncertain parameters. The method applied here relies
on an ensemble-based data assimilation (DA) algorithm (i.e. the ensemble smoother, ES),
which incorporates the information from the bathymetric measurements into the geomechanical
model response to infer and reduce the uncertainty of the parameter cM. The outcome from
conceptual model (i) indicates that DA is effective in reducing the cM uncertainty. However,
the maximum settlement still remains underestimated, while the areal extent of the subsidence
bowl is overestimated. We demonstrate that the selection of the heterogeneous conceptual
model (ii) allows to reproduce much better the observations thus removing a clear bias of
the model structure. DA allows significantly reducing the cM uncertainty in the five blocks
(out of the seven) characterized by large volume and large pressure decline. Conversely, the
assimilation of land displacements only partially constrains the prior cM uncertainty in the
reservoir blocks marginally contributing to the cumulative seafloor subsidence, that is, blocks
with low pressure
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