102 research outputs found
Implications of kHz QPOs for the spin frequencies and magnetic fields of neutron stars: new results from Circinus X-1
Detection of paired kilohertz quasi-periodic oscillations (kHz QPOs) in the
X-ray emission of a compact object is compelling evidence that the object is an
accreting neutron star. In many neutron stars, the stellar spin rate is equal
or roughly equal to Delta-nu, the frequency separation of the QPO pair, or to
2Delta-nu. Hence, if the mechanism that produces the kilohertz QPOs is similar
in all stars, measurement of Delta-nu can provide an estimate of the star's
spin rate. The involvement of the stellar spin in producing Delta-nu indicates
that the magnetic fields of these stars are dynamically important.
We focus here on the implications of the paired kHz QPOs recently discovered
in the low-mass X-ray binary (LMXB) system Cir X-1 (Boutloukos et al. 2006).
The kHz QPOs discovered in Cir X-1 are generally similar to those seen in other
stars, establishing that the compact object in the Cir X-1 system is a neutron
star. However, the frequency nu-u of its upper kHz QPO is up to a factor of
three smaller than is typical, and Delta-nu varies by about a factor 2 (167 Hz,
the largest variation so far observed). Periodic oscillations have not yet been
detected from Cir X-1, so its spin rate has not yet been measured directly. The
low values of nu-u and the large variation of Delta-nu challenge current models
of the generation of kHz QPOs. Improving our understanding of Cir X-1 will
improve our knowledge of the spin rates and magnetic fields of all neutron
stars.Comment: 3 pages, 3 figures, a table with all known accreting millisecond
X-ray pulsars up to October 2007. To appear in "40 Years of Pulsars,
Millisecond Pulsars, Magnetars and More" conference proceeding
Origin of intermittent accretion-powered X-ray oscillations in neutron stars with millisecond spin periods
We have shown previously that many of the properties of persistent
accretion-powered millisecond pulsars can be understood if their X-ray emitting
areas are near their spin axes and move as the accretion rate and structure of
the inner disk vary. Here we show that this "nearly aligned moving spot model"
may also explain the intermittent accretion-powered pulsations that have been
detected in three weakly magnetic accreting neutron stars. We show that
movement of the emitting area from very close to the spin axis to about 10
degrees away can increase the fractional rms amplitude from less than about 0.5
percent, which is usually undetectable with current instruments, to a few
percent, which is easily detectable. The second harmonic of the spin frequency
usually would not be detected, in agreement with observations. The model
produces intermittently detectable oscillations for a range of emitting area
sizes and beaming patterns, stellar masses and radii, and viewing directions.
Intermittent oscillations are more likely in stars that are more compact. In
addition to explaining the sudden appearance of accretion-powered millisecond
oscillations in some neutron stars with millisecond spin periods, the model
explains why accretion-powered millisecond oscillations are relatively rare and
predicts that the persistent accretion-powered millisecond oscillations of
other stars may become undetectable for brief intervals. It suggests why
millisecond oscillations are frequently detected during the X-ray bursts of
some neutron stars but not others and suggests mechanisms that could explain
the occasional temporal association of intermittent accretion-powered
oscillations with thermonuclear X-ray bursts.Comment: 5 pages, 1 figure; includes additional discussion and updated
references; accepted for publication in ApJ
Spectral Learning of Refinement HMMs
We derive a spectral algorithm for learning the parameters of a refinement HMM. This method is simple, efficient, and can be applied to a wide range of supervised sequence labeling tasks. Like other spectral methods, it avoids the problem of local optima and provides a consistent estimate of the parameters. Our experiments on a phoneme recognition task show that when equipped with informative feature functions, it performs significantly better than a supervised HMM and competitively with EM
Business Analytics in (a) Blink
The Blink project’s ambitious goal is to answer all Business Intelligence (BI) queries in mere seconds,
regardless of the database size, with an extremely low total cost of ownership. Blink is a new DBMS
aimed primarily at read-mostly BI query processing that exploits scale-out of commodity multi-core
processors and cheap DRAM to retain a (copy of a) data mart completely in main memory. Additionally,
it exploits proprietary compression technology and cache-conscious algorithms that reduce memory
bandwidth consumption and allow most SQL query processing to be performed on the compressed data.
Blink always scans (portions of) the data mart in parallel on all nodes, without using any indexes or
materialized views, and without any query optimizer to choose among them. The Blink technology has
thus far been incorp
A model for the waveform behavior of accreting millisecond pulsars: Nearly aligned magnetic fields and moving emission regions
We investigate further a model of the accreting millisecond X-ray pulsars we
proposed earlier. In this model, the X-ray-emitting regions of these pulsars
are near their spin axes but move. This is to be expected if the magnetic poles
of these stars are close to their spin axes, so that accreting gas is channeled
there. As the accretion rate and the structure of the inner disk vary, gas is
channeled along different field lines to different locations on the stellar
surface, causing the X-ray-emitting areas to move. We show that this "nearly
aligned moving spot model" can explain many properties of the accreting
millisecond X-ray pulsars, including their generally low oscillation amplitudes
and nearly sinusoidal waveforms; the variability of their pulse amplitudes,
shapes, and phases; the correlations in this variability; and the similarity of
the accretion- and nuclear-powered pulse shapes and phases in some. It may also
explain why accretion-powered millisecond pulsars are difficult to detect, why
some are intermittent, and why all detected so far are transients. This model
can be tested by comparing with observations the waveform changes it predicts,
including the changes with accretion rate.Comment: 21 pages, 6 figures; includes 3 new sections, 14 additional pages, 4
additional figures with 11 new plots, and additional references; accepted for
publication in Ap
The development and validation of a scoring tool to predict the operative duration of elective laparoscopic cholecystectomy
Background: The ability to accurately predict operative duration has the potential to optimise theatre efficiency and utilisation, thus reducing costs and increasing staff and patient satisfaction. With laparoscopic cholecystectomy being one of the most commonly performed procedures worldwide, a tool to predict operative duration could be extremely beneficial to healthcare organisations.
Methods: Data collected from the CholeS study on patients undergoing cholecystectomy in UK and Irish hospitals between 04/2014 and 05/2014 were used to study operative duration. A multivariable binary logistic regression model was produced in order to identify significant independent predictors of long (> 90 min) operations. The resulting model was converted to a risk score, which was subsequently validated on second cohort of patients using ROC curves.
Results: After exclusions, data were available for 7227 patients in the derivation (CholeS) cohort. The median operative duration was 60 min (interquartile range 45–85), with 17.7% of operations lasting longer than 90 min. Ten factors were found to be significant independent predictors of operative durations > 90 min, including ASA, age, previous surgical admissions, BMI, gallbladder wall thickness and CBD diameter. A risk score was then produced from these factors, and applied to a cohort of 2405 patients from a tertiary centre for external validation. This returned an area under the ROC curve of 0.708 (SE = 0.013, p 90 min increasing more than eightfold from 5.1 to 41.8% in the extremes of the score.
Conclusion: The scoring tool produced in this study was found to be significantly predictive of long operative durations on validation in an external cohort. As such, the tool may have the potential to enable organisations to better organise theatre lists and deliver greater efficiencies in care
Better the devil you know:Threat effects and attachment to the European Union
The EU is facing unprecedented challenges and significant threats to its economic and political security. Austerity, the Eurozone crisis, rising immigration and heightened fear of terrorism all present serious challenges to the process of integration. How does this context of insecurity impact on what the EU means to its citizens? Will the public become increasingly Eurosceptic or will they discover a hitherto unrecognised attachment to the EU as the prospect of its collapse becomes real? Psychological research has demonstrated that individual exposure to threat decreases cognitive capacity, inducing a tendency towards rigidity or conservatism - a tendency to cling to the ‘devil you know’. So what might this mean for the European integration process? Using experimental techniques drawn from political psychology, the authors find a dual threat effect. The EU symbol has a negative (anti-EU) effect on EU-related attitudes when presented in neutral context. This is consonant with conceptualisations of the EU as a threat to national cultural and political norms. In contrast, however, visual priming of participants with EU symbols has a positive (pro-EU) effect on related attitudes when these are presented in a context that implies a subtle but imminent threat to the benefits of EU membership
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