793 research outputs found
Oscillations of Thick Accretion Discs Around Black Holes - II
We present a numerical study of the global modes of oscillation of thick
accretion discs around black holes. We have previously studied the case of
constant distributions of specific angular momentum. In this second paper, we
investigate (i) how the size of the disc affects the oscillation
eigenfrequencies, and (ii) the effect of power-law distributions of angular
momentum on the oscillations. In particular, we compare the oscillations of the
disc with the epicyclic eigenfrequencies of a test particle with different
angular momentum distributions orbiting around the central object. We find that
there is a frequency shift away from the epicyclic eigenfrequency of the test
particle to lower values as the size of the tori is increased. We have also
studied the response of a thick accretion disc to a localized external
perturbation using non constant specific angular momentum distributions within
the disc. We find that in this case it is also possible (as reported previously
for constant angular momentum distributions) to efficiently excite internal
modes of oscillation. In fact we show here that the local perturbations excite
global oscillations (acoustic p modes) closely related to the epicyclic
oscillations of test particles. Our results are particularly relevant in the
context of low mass X-ray binaries and microquasars, and the high frequency
Quasi-Periodic Oscillations (QPOs) observed in them. Our computations make use
of a Smooth Particle Hydrodynamics (SPH) code in azimuthal symmetry, and use a
gravitational potential that mimics the effects of strong gravity.Comment: 10 pages, 8 figures, accepted for publication as a paper in the
Monthly Notices of the Royal Astronomical Societ
Oscillations of Thick Accretion Discs Around Black Holes
We present a numerical study of the response of a thick accretion disc to a
localized, external perturbation with the aim of exciting internal modes of
oscillation. We find that the perturbations efficiently excite global modes
recently identified as acoustic p--modes, and closely related to the epicyclic
oscillations of test particles. The two strongest modes occur at
eigenfrequencies which are in a 3:2 ratio. We have assumed a constant specific
angular momentum distribution within the disc. Our models are in principle
scale--free and can be used to simulate accretion tori around stellar or super
massive black holes.Comment: 4 pages, 4 figures, accepted for publication as a letter in the
Monthly Notices of the Royal Astronomical Societ
A search for radio pulsars and fast transients in M31 using the WSRT
We present the results of the most sensitive and comprehensive survey yet
undertaken for radio pulsars and fast transients in the Andromeda galaxy (M31)
and its satellites, using the Westerbork Synthesis Radio Telescope (WSRT) at a
central frequency of 328 MHz. We used the WSRT in a special configuration
called 8gr8 (eight-grate) mode, which provides a large instantaneous
field-of-view, about 5 square degrees per pointing, with good sensitivity, long
dwell times (up to 8 hours per pointing), and good spatial resolution (a few
arc minutes) for locating sources. We have searched for both periodicities and
single pulses in our data, aiming to detect bright, persistent radio pulsars
and rotating radio transients (RRATs) of either Galactic or extragalactic
origin. Our searches did not reveal any confirmed periodic signals or bright
single bursts from (potentially) cosmological distances. However, we do report
the detection of several single pulse events, some repeating at the same
dispersion measure, which could potentially originate from neutron stars in
M31. One in particular was seen multiple times, including a burst of six pulses
in 2000 seconds, at a dispersion measure of 54.7 pc cm^-3, which potentially
places the origin of this source outside of our Galaxy. Our results are
compared to a range of hypothetical populations of pulsars and RRATs in M31 and
allow us to constrain the luminosity function of pulsars in M31. They also show
that, unless the pulsar population in M31 is much dimmer than in our Galaxy,
there is no need to invoke any violation of the inverse square law of the
distance for pulsar fluxes.Comment: 18 pages, 14 figures, 8 tables. Accepted for publication in the main
journal of MNRA
12CO and 13CO J=3-2 observations toward N11 in the Large Magellanic Cloud
After 30 Doradus, N11 is the second largest and brightest nebula in the LMC.
This large nebula has several OB associations with bright nebulae at its
surroundings. N11 was previously mapped at the lowest rotational transitions of
CO (J=1--0 and 2--1), and in some particular regions pointings of the
CO J=1--0 and 2--1 lines were also performed. Using ASTE we mapped the
whole extension of the N11 nebula in the CO J=3--2 line, and three
sub-regions in the CO J=3--2 line. The regions mapped in the CO
J=3--2 were selected based on that they may be exposed to the radiation at
different ways: a region lying over the nebula related to the OB association
LH10 (N11B), another one that it is associated with the southern part of the
nebula related to the OB association LH13 (N11D), and finally a farther area at
the southwest without any embedded OB association (N11I). We found that the
morphology of the molecular clouds lying in each region shows some signatures
that could be explained by the expansion of the nebulae and the action of the
radiation. Fragmentation generated in a molecular shell due to the expansion of
the N11 nebula is suggested. The integrated line ratios CO/CO
show evidences of selective photodissociation of the CO, and probably
other mechanisms such as chemical fractionation. The CO contribution to the
continuum at 870 m was directly derived. The distribution of the
integrated line ratios CO J=3--2/2--1 show hints of stellar feedback in
N11B and N11D. The ratio between the virial and LTE mass (M/M) is higher than unity in all analyzed molecular clumps, which suggests
that the clumps are not gravitationally bounded and may be supported by
external pressure. A non-LTE analysis suggests that we are mapping gas with
densities about a few 10 cm.Comment: Accepted to be published in A&A. Figures were degrade
Oscillations of tori in the pseudo-Newtonian potential
Context. The high-frequency quasi-periodic oscillations (HF QPOs) in neutron
star and stellar-mass black hole X-ray binaries may be the result of a
resonance between the radial and vertical epicyclic oscillations in strong
gravity. Aims. In this paper we investigate the resonant coupling between the
epicyclic modes in a torus in a strong gravitational field. Methods. We perform
numerical simulations of axisymmetric constant angular momentum tori in the
pseudo-Newtonian potential. The epicyclic motion is excited by adding a
constant radial velocity to the torus. Results. We verify that slender tori
perform epicyclic motions at the frequencies of free particles, but the
epicyclic frequencies decrease as the tori grow thicker. More importantly, and
in contrast to previous numerical studies, we do not find a coupling between
the radial and vertical epicyclic motions. The appearance of other modes than
the radial epicyclic motion in our simulations is rather due to small numerical
deviations from exact equilibrium in the initial state of our torus.
Conclusions. We find that there is no pressure coupling between the two
axisymmetric epicyclic modes as long as the torus is symmetric with respect to
the equatorial plane. However we also find that there are other modes in the
disc that may be more attractive for explaining the HF QPOs.Comment: 8 pages, 9 figure
Validity and reliability of the satel 40 HZ stabilometric force platform for measuring quiet stance and dynamic standing balance in healthy subjects
Background: A force platform must have validity and reliability for optimal use. The objective of this study was to analyze the validity and the reliability of the Satel 40 Hz stabilometric force platform.
Methods: A study of instrumental validity and reliability, involving a cross-sectional correlational and comparative analysis was performed. To determine the validity, four certified weights located on three axes were used and the ability of the stabilometric force platform to detect changes in the position of the different axes was observed. A test–retest was performed to analyze the reliability. Forty-two symptom-free volunteers participated in the study. Assessments were taken in a standing static position and in a dynamic position, with the eyes open and closed. Three measurements were taken and the intra-class correlation coefficient (ICC) was calculated.
Results: The validity increased as the weight increased for all the variables measured in the stabilometric parameters (p < 0.05). The reliability was shown to be good to excellent for the two visual conditions. The positional variables obtained a higher ICC. The variable with the best ICC was the Y mean in OE (ICC 0.874 and a p < 0.001). All the values showed an increase in a dynamic situation.
Conclusion: The findings support the reliability and validity of the Satel 40 Hz stabilometric force platform. The platform could be recommended to evaluate static and dynamic standing balance in healthy adult individuals. Guidelines for treatment and the level of quality of stabilometry could be obtained from its use
A New Field Protocol for Monitoring Forest Degradation
Forest degradation leads to the gradual reduction of forest carbon stocks, function, and biodiversity following anthropogenic disturbance. Whilst tropical degradation is a widespread problem, it is currently very under-studied and its magnitude and extent are largely unknown. This is due, at least in part, to the lack of developed and tested methods for monitoring degradation. Due to the relatively subtle and ongoing changes associated with degradation, which can include the removal of small trees for fuelwood or understory clearance for agricultural production, it is very hard to detect using Earth Observation. Furthermore, degrading activities are normally spatially heterogeneous and stochastic, and therefore conventional forest inventory plots distributed across a landscape do not act as suitable indicators: at best only a small proportion of plots (often zero) will actually be degraded in a landscape undergoing active degradation. This problem is compounded because the metal tree tags used in permanent forest inventory plots likely deter tree clearance, biasing inventories toward under-reporting change. We have therefore developed a new forest plot protocol designed to monitor forest degradation. This involves a plot that can be set up quickly, so a large number can be established across a landscape, and easily remeasured, even though it does not use tree tags or other obvious markers. We present data from a demonstration plot network set up in Jalisco, Mexico, which were measured twice between 2017 and 2018. The protocol was successful, with one plot detecting degradation under our definition (losing greater than 10% AGB but remaining forest), and a further plot being deforested for Avocado (Persea americana) production. Live AGB ranged from 8.4 Mg ha–1 to 140.8 Mg ha–1 in Census 1, and from 0 Mg ha–1 to 144.2 Mg ha–1 Census 2, with four of ten plots losing AGB, and the remainder staying stable or showing slight increases. We suggest this protocol has great potential for underpinning appropriate forest plot networks for degradation monitoring, potentially in combination with Earth Observation analysis, but also in isolation
Continuous glucose monitoring versus capillary point-of-care testing for inpatient glycemic control in type 2 diabetes patients hospitalized in the general ward and treated with a basal bolus insulin regimen
Q1ArtÃculo original325-329Background:
Continuous glucose monitoring (CGM) may improve the management of patients with type 2 diabetes hospitalized in the general ward by facilitating the detection of hyper- and hypoglycemic episodes. However, the lack of data on the accuracy and safety of CGM have limited its application.
Methods:
A prospective pilot study was conducted including 38 patients hospitalized in the general ward with a known diagnosis of type 2 diabetes mellitus (DM) and hyperglycemic individuals without a history of DM with a blood sugar of 140-400 mg on admission treated with a basal bolus insulin regimen. Inpatient glycemic control and the incidence of hypoglycemic episodes were compared between detection by CGM of interstitial fluid for up to 6 days and point-of-care (POC) capillary blood glucose monitoring performed pre- and postprandially, before bedtime and at 3 am.
Results:
No differences in average daily glucose levels were observed between CGM and POC (176.2 ± 33.9 vs 176.6 ± 33.7 mg/dl, P = .828). However, CGM detected a higher number of hypoglycemic episodes than POC (55 vs 12, P < .01). Glucose measurements were clinically valid, with 91.9% of patients falling within the Clarke error grid A and B zones.
Conclusions:
Our preliminary results indicate that the use of CGM in type 2 patients hospitalized in the general ward provides accurate estimation of blood sugar levels and is more effective than POC for the detection of hypoglycemic episodes and asymptomatic hypoglycemia
- …