1,463 research outputs found
Solvable model of a self-gravitating system
We introduce and discuss an effective model of a self-gravitating system
whose equilibrium thermodynamics can be solved in both the microcanonical and
the canonical ensemble, up to a maximization with respect to a single variable.
Such a model can be derived from a model of self-gravitating particles confined
on a ring, referred to as the self-gravitating ring (SGR) model, allowing a
quantitative comparison between the thermodynamics of the two models. Despite
the rather crude approximations involved in its derivation, the effective model
compares quite well with the SGR model. Moreover, we discuss the relation
between the effective model presented here and another model introduced by
Thirring forty years ago. The two models are very similar and can be considered
as examples of a class of minimal models of self-gravitating systems.Comment: 21 pages, 6 figures; submitted to JSTAT for the special issue on
long-range interaction
Optical afterglow luminosities in the Swift epoch: confirming clustering and bimodality
We show that Gamma Ray Bursts (GRBs) of known redshift and rest frame optical
extinction detected by the Swift satellite fully confirm earlier results
concerning the distribution of the optical afterglow luminosity at 12 hours
after trigger (rest frame time). This distribution is bimodal and relatively
narrow, especially for the high luminosity branch. This is intriguing, given
that Swift GRBs have, on average, a redshift larger than pre-Swift ones, and is
unexpected in the common scenario explaining the GRB afterglow. We investigate
if the observed distribution can be the result of selection effects affecting a
unimodal parent luminosity distribution, and find that either the distribution
is intrinsically bimodal, or most (60 per cent) of the bursts are absorbed by a
substantial amount of grey dust. In both cases we suggest that most dark bursts
should belong to the underluminous optical family.Comment: 5 pages 3 figures, minor revision, added reference, accepted for
publication in MNRAS Letter
Classical Nucleation Theory for Active Fluid Phase Separation
Classical nucleation theory (CNT), linking rare nucleation events to the free
energy landscape of a growing nucleus, is central to understanding phase-change
kinetics in passive fluids. Nucleation in non-equilibrium systems is much
harder to describe because there is no free energy, but instead a
dynamics-dependent quasi-potential that typically must be found numerically.
Here we extend CNT to a class of active phase separating systems governed by a
minimal field-theoretic model (Active Model B+). In the small noise and
supersaturation limits that CNT assumes, we compute analytically the
quasi-potential, and hence nucleation barrier, for liquid-vapor phase
separation. Crucially to our results, detailed balance, although broken
microscopically by activity, is restored along the instanton trajectory, which
in CNT involves the nuclear radius as the sole reaction coordinate
Clinical significance of epithelial-to-mesenchymal transition in laryngeal carcinoma: Its role in the different subsites
Background: During epithelial-to-mesenchymal transition, cancer cells lose adhesion capacity gaining migratory properties. The role of the process on prognosis has been evaluated in 50 cases of laryngeal carcinoma. Methods: E-cadherin, N-cadherin, β-catenin, α-catenin, γ-catenin, caveolin-1, and vimentin immunohistochemical expression were evaluated using a double score based on staining intensity and cellular localization. Results: Cytoplasmic E-cadherin and α/γ catenin staining were associated with a decrease in survival, cytoplasmic β-catenin was associated with advanced stage, and N-cadherin and vimentin expression were associated with poor differentiation and tumor relapse. On the basis of cancer cells, epithelial or mesenchymal morphological and immunophenotypic similarity we identified 4 main subgroups correlated with a transition to a more undifferentiated phenotype, which have a different pattern of relapse and survival. Conclusion: The negative prognostic role of epithelial-to-mesenchymal transition has been confirmed and a predictive role in glottic tumors has been suggested, leading us to propose epithelial-to-mesenchymal transition as an additional adverse feature in laryngeal carcinoma
Extracting maximum power from active colloidal heat engines
© 2018 EPLA. Colloidal heat engines extract power out of a fluctuating bath by manipulating a confined tracer. Considering a self-propelled tracer surrounded by a bath of passive colloids, we optimize the engine performances based on the maximum available power. Our approach relies on an adiabatic mean-field treatment of the bath particles which reduces the many-body description into an effective tracer dynamics. It leads us to reveal that, when operated at constant activity, an engine can only produce less maximum power than its passive counterpart. In contrast, the output power of an isothermal engine, operating with cyclic variations of the self-propulsion without any passive equivalent, exhibits an optimum in terms of confinement and activity. Direct numerical simulations of the microscopic dynamics support the validity of these results even beyond the mean-field regime, with potential relevance to the design of experimental engines
Kinetic theory for non-equilibrium stationary states in long-range interacting systems
We study long-range interacting systems perturbed by external stochastic
forces. Unlike the case of short-range systems, where stochastic forces usually
act locally on each particle, here we consider perturbations by external
stochastic fields. The system reaches stationary states where external forces
balance dissipation on average. These states do not respect detailed balance
and support non-vanishing fluxes of conserved quantities. We generalize the
kinetic theory of isolated long-range systems to describe the dynamics of this
non-equilibrium problem. The kinetic equation that we obtain applies to
plasmas, self-gravitating systems, and to a broad class of other systems. Our
theoretical results hold for homogeneous states, but may also be generalized to
apply to inhomogeneous states. We obtain an excellent agreement between our
theoretical predictions and numerical simulations. We discuss possible
applications to describe non-equilibrium phase transitions.Comment: 11 pages, 2 figures; v2: small changes, close to the published
versio
STATISTICS IN CLINICAL TRIALS: OUT OF CONDITION. SOME PROBLEMS OF UNCONDITIONAL INFERENCEAT THE CROSSROADS OF METHODOLOGY AND ETHICS
Randomized controlled trials are experiments for the evaluation of a new treatment option, currently representing the "gold standard" in health care assessment. Clinical trials fulfill a double role of evidence production and of regulatory oversight in sanctioning new drugs' approval into the drug market.
For this reason trials are large and tightly regulated enterprises that have to comply with ethical requirements while at the same time maintaining high epistemic standards, in a balance that becomes increasingly difficult to strike as research questions become more and more sophisticated.
The statistical framework adopted for designing and analysing trials represents a relevant part of this architecture. Statististical methodology influences such aspects as which inferences are licensed on the basis of data and what is the degree of support granted to an hypothesis. Thus, statistics plays a fundamental role as a gatekeeper both in warranting the ethical permissibility of a trial, and in licensing conclusions about the most effective treatment.
Certain widely-accepted statistical principles have an impact on the way results from medical studies are evaluated.
One such principle is conditioning, i.e. the possibility to incorporate an assessment of strength of evidence in inferential statements of confidence. Currently, conditioning is not part of the statistical method in use, although it is upheld by alternative statistical paradigms such as the Bayesian. In my thesis I analyze the impact of conditioning upon the ethical, epistemic and regulatory facets of trials and I suggest the possibility of incorporating conditioning within the current statistical paradigm of clinical research
Suzaku observations of ‘bare’ active galactic nuclei
We present an X-ray spectral analysis of a large sample of 25 ‘bare’ active galactic nuclei (AGN), sources with little or no complicating intrinsic absorption, observed with Suzaku. Our work focuses on studying the potential contribution from relativistic disc reflection and examining the implications of this interpretation for the intrinsic spectral complexities frequently displayed by AGN in the X-ray bandpass. During the analysis, we take the unique approach of attempting to simultaneously undertake a systematic analysis of the whole sample, as well as a detailed treatment of each individual source, and find that disc reflection has the required flexibility to successfully reproduce the broad-band spectrum observed for all of the sources considered. Where possible, we use the reflected emission to place constraints on the black hole spin for this sample of sources. Our analysis suggests a general preference for rapidly rotating black holes, which if taken at face value is most consistent with the scenario in which supermassive black hole growth is dominated by prolonged, ordered accretion. However, there may be observational biases towards AGN with high spin in the compiled sample, limiting our ability to draw strong conclusions for the general population at this stage. Finally, contrary to popular belief, our analysis also implies that the dichotomy between radio-loud/radio-quiet AGN is not solely related to black hole spin
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