1,658 research outputs found
Testing the accuracy of reflection-based supermassive black hole spin measurements in AGN
X-ray reflection is a very powerful method to assess the spin of supermassive
black holes (SMBHs) in active galactic nuclei (AGN), yet this technique is not
universally accepted. Indeed, complex reprocessing (absorption, scattering) of
the intrinsic spectra along the line of sight can mimic the relativistic
effects on which the spin measure is based. In this work, we test the
reliability of SMBH spin measurements that can currently be achieved through
the simulations of high-quality XMM-Newton and NuSTAR spectra. Each member of
our group simulated ten spectra with multiple components that are typically
seen in AGN, such as warm and (partial-covering) neutral absorbers,
relativistic and distant reflection, and thermal emission. The resulting
spectra were blindly analysed by the other two members. Out of the 60 fits, 42
turn out to be physically accurate when compared to the input model. The SMBH
spin is retrieved with success in 31 cases, some of which (9) are even found
among formally inaccurate fits (although with looser constraints). We show
that, at the high signal-to-noise ratio assumed in our simulations, neither the
complexity of the multi-layer, partial-covering absorber nor the input value of
the spin are the major drivers of our results. The height of the X-ray source
(in a lamp-post geometry) instead plays a crucial role in recovering the spin.
In particular, a success rate of 16 out of 16 is found among the accurate fits
for a dimensionless spin parameter larger than 0.8 and a lamp-post height lower
than five gravitational radii.Comment: 20 pages, 9 figures, 4 tables. Accepted for publication in A&
Towards an informed quest for accretion disc winds in quasars: the intriguing case of Ton 28
We report on the detection of a blueshifted Fe K absorption feature in two consecutive XMM–Newton observations of the luminous blue quasar Ton 28, at the 4σ cumulative significance. The rest energy of 9.2 keV implies the presence of an accretion disc wind with bulk outflow velocity of ∼0.28c, while the kinetic power is most likely a few per cent of the quasar luminosity. Remarkably, Ton 28 had been specifically selected as an optimal target to reveal an ultra-fast X-ray wind based on its total luminosity (Lbol > 1046 erg s−1) and [O III] λ5007 Å equivalent width (EW < 6 Å), suggestive of high accretion rate and low inclination, respectively. Other peculiar optical/UV emission-line properties include narrow Hβ, strong Fe II, and blueshifted C IV . These are key parameters in the Eigenvector 1 formalism, and are frequently found in active galaxies with ongoing accretion disc winds, hinting at a common physical explanation. Provided that the effectiveness of our selection method is confirmed with similar sources, this result could represent the first step towards the characterization of black hole winds through multiwavelength indicators in the absence of high-quality X-ray spectra
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
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Experimental and numerical investigation on forced convection in circular tubes with nanofluids
This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu.In this paper an experimental and numerical study to investigate the convective heat transfer
characteristics of fully developed turbulent flow of a water–Al2O3 nanofluid in a circular tube is presented.
The numerical simulations are accomplished on the experimental test section configuration. In the analysis,
the fluid flow and the thermal field are assumed axial-symmetric, two-dimensional and steady state. The
single-phase model is employed to model the nanofluid mixture and k-ε model is used to describe the
turbulent fluid flow. Experimental and numerical results are carried out for different volumetric flow rates
and nanoparticles concentration values. Heat transfer convective coefficients as a function of flow rates and
Reynolds numbers are presented. The results indicate that the heat transfer coefficients increase for all
nanofluids concentrations compared to pure water at increasing volumetric flow rate. Heat transfer
coefficient increases are observed at assigned volumetric flow rate for nanofluid mixture with higher
concentrations whereas Nusselt numbers present lower values than the ones for pure water
The environment of the SN-less GRB 111005A at z = 0.0133
The collapsar model has proved highly successful in explaining the properties
of long gamma-ray bursts (GRBs), with the most direct confirmation being the
detection of a supernova (SN) coincident with the majority of nearby long GRBs.
Within this model, a long GRB is produced by the core-collapse of a metal-poor,
rapidly rotating, massive star. The detection of some long GRBs in metal-rich
environments, and more fundamentally the three examples of long GRBs (GRB
060505, GRB 060614 and GRB 111005A) with no coincident SN detection down to
very deep limits is in strong contention with theoretical expectations. In this
paper we present MUSE observations of the host galaxy of GRB 111005A, which is
the most recent and compelling example yet of a SN-less, long GRB. At
z=0.01326, GRB 111005A is the third closest GRB ever detected, and second
closest long duration GRB, enabling the nearby environment to be studied at a
resolution of 270 pc. From the analysis of the MUSE data cube, we find GRB
111005A to have occurred within a metal-rich environment with little signs of
ongoing star formation. Spectral analysis at the position of the GRB indicates
the presence of an old stellar population (tau > 10 Myr), which limits the mass
of the GRB progenitor to M_ZAMS<15 Msolar, in direct conflict with the
collapsar model. Our deep limits on the presence of any SN emission combined
with the environmental conditions at the position of GRB 111005A necessitate
the exploration of a novel long GRB formation mechanism that is unrelated to
massive stars.Comment: Now accepted by A&A. Manuscript replaced to match accepted version.
Some additional discussion added, and velocity map of the host galaxy now
include
Numerical solution of the two-dimensional Helmholtz equation with variable coefficients by the radial integration boundary integral and integro-differential equation methods
This is the author's accepted manuscript. The final published article is available from the link below. Copyright @ 2012 Taylor & Francis.This paper presents new formulations of the boundary–domain integral equation (BDIE) and the boundary–domain integro-differential equation (BDIDE) methods for the numerical solution of the two-dimensional Helmholtz equation with variable coefficients. When the material parameters are variable (with constant or variable wave number), a parametrix is adopted to reduce the Helmholtz equation to a BDIE or BDIDE. However, when material parameters are constant (with variable wave number), the standard fundamental solution for the Laplace equation is used in the formulation. The radial integration method is then employed to convert the domain integrals arising in both BDIE and BDIDE methods into equivalent boundary integrals. The resulting formulations lead to pure boundary integral and integro-differential equations with no domain integrals. Numerical examples are presented for several simple problems, for which exact solutions are available, to demonstrate the efficiency of the proposed methods
A Numerical Analysis on Nanofluid Mixed Convection in Triangular Cross-Sectioned Ducts Heated by a Uniform Heat Flux:
In this paper, results obtained by the numerical investigation on laminar mixed convection in triangular ducts, filled with nanofluids, are presented in order to evaluate the fluid dynamic and thermal features of the considered geometry by considering Al 2 O 3 /water based nanofluids. The system is heated by a constant and uniform heat flux also along the perimeter of the triangular duct section in H2 mode as thermal boundary condition and the single-phase model has been assigned for a Reynolds number value equal to 100. Results are given for different nanoparticle volume concentrations and Richardson number values ranging from 0% to 5% and from 0 to 5, respectively. Results, presented for the fully developed regime flow, show the enhancement of average convective heat transfer coefficients values for increasing values of Richardson number and particle fractions. However, wall shear stress and required pumping power profiles increase as expected. The PEC analysis showed that the use of nanofluids in mixed convection seems slightly convenient. It should be underlined that, at the moment, experimental data are not available to compare the numerical proposed model for mixed convection in horizontal triangular ducts with nanofluids
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
A gravitational lensing explanation for the excess of strong Mg-II absorbers in GRB afterglow spectra
GRB afterglows offer a probe of the intergalactic medium out to high redshift
which complements observations along more abundant quasar lines-of-sight.
Although both quasars and GRB afterglows should provide a-priori random
sight-lines through the intervening IGM, it has been observed that strong Mg-II
absorbers are twice as likely to be found along sight-lines toward GRBs.
Several proposals to reconcile this discrepancy have been put forward, but none
has been found sufficient to explain the magnitude of the effect. In this paper
we estimate the effect of gravitational lensing by galaxies and their
surrounding mass distributions on the statistics of Mg-II absorption. We find
that the multi-band magnification bias could be very strong in the
spectroscopic GRB afterglow population and that gravitational lensing can
explain the discrepancy in density of absorbers, for plausibly steep luminosity
functions. The model makes the prediction that approximately 20%-60% of the
spectroscopic afterglow sample (i.e. ~ 5-15 of 26 sources) would have been
multiply imaged, and hence result in repeating bursts. We show that despite
this large lensing fraction it is likely that none would yet have been
identified by chance owing to the finite sky coverage of GRB searches. We
predict that continued optical monitoring of the bright GRB afterglow locations
in the months and years following the initial decay would lead to
identification of lensed GRB afterglows. A confirmation of the lensing
hypothesis would allow us to constrain the GRB luminosity function down to
otherwise inaccessibly faint levels, with potential consequences for GRB
models.Comment: 8 pages, 3 figures. Submitted to MNRAS
Learning Early Exit Strategies for Additive Ranking Ensembles
Modern search engine ranking pipelines are commonly based on large machine-learned ensembles of regression trees. We propose LEAR, a novel - learned - technique aimed to reduce the average number of trees traversed by documents to accumulate the scores, thus reducing the overall query response time. LEAR exploits a classifier that predicts whether a document can early exit the ensemble because it is unlikely to be ranked among the final top-k results. The early exit decision occurs at a sentinel point, i.e., after having evaluated a limited number of trees, and the partial scores are exploited to filter out non-promising documents. We evaluate LEAR by deploying it in a production-like setting, adopting a state-of-the-art algorithm for ensembles traversal. We provide a comprehensive experimental evaluation on two public datasets. The experiments show that LEAR has a significant impact on the efficiency of the query processing without hindering its ranking quality. In detail, on a first dataset, LEAR is able to achieve a speedup of 3x without any loss in NDCG@10, while on a second dataset the speedup is larger than 5x with a negligible NDCG@10 loss (< 0.05%)
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