2,022 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 Tonā28
We report on the detection of a blueshifted FeāKāabsorption feature in two consecutive XMMāNewtonā observations of the luminous blue quasar Tonā28, at the 4Ļ cumulative significance. The rest energy of 9.2 keV implies the presence of an accretion disc wind with bulk outflow velocity of ā¼0.28c, while the kinetic power is most likely a few perācent of the quasar luminosity. Remarkably, Tonā28āhad been specifically selected as an optimal target to reveal an ultra-fast X-ray wind based on its total luminosity (Lbol > 1046 erg sā1) and [OāIII]āĪ»5007āĆ
āequivalent width (EW < 6 Ć
), suggestive of high accretion rate and low inclination, respectively. Other peculiar optical/UV emission-line properties include narrow HĪ², strong FeāII,āand blueshifted CāIVā. 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
Energy landscape and phase transitions in the self-gravitating ring model
We apply a recently proposed criterion for the existence of phase
transitions, which is based on the properties of the saddles of the energy
landscape, to a simplified model of a system with gravitational interactions,
referred to as the self-gravitating ring model. We show analytically that the
criterion correctly singles out the phase transition between a homogeneous and
a clustered phase and also suggests the presence of another phase transition,
not previously known. On the basis of the properties of the energy landscape we
conjecture on the nature of the latter transition
Recommended from our members
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 waterāAl2O3 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-Īµ 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
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, Ī²-catenin, Ī±-catenin, Ī³-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 Ī±/Ī³ catenin staining were associated with a decrease in survival, cytoplasmic Ī²-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
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 boundaryādomain integral equation (BDIE) and the boundaryādomain 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
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 boundaryādomain integral equation (BDIE) and the boundaryādomain 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
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