1,229 research outputs found
Infinite compressibility states in the Hierarchical Reference Theory of fluids. II. Numerical evidence
Continuing our investigation into the Hierarchical Reference Theory of fluids
for thermodynamic states of infinite isothermal compressibility kappa[T] we now
turn to the available numerical evidence to elucidate the character of the
partial differential equation: Of the three scenarios identified previously,
only the assumption of the equations turning stiff when building up the
divergence of kappa[T] allows for a satisfactory interpretation of the data. In
addition to the asymptotic regime where the arguments of part I
(cond-mat/0308467) directly apply, a similar mechanism is identified that gives
rise to transient stiffness at intermediate cutoff for low enough temperature.
Heuristic arguments point to a connection between the form of the Fourier
transform of the perturbational part of the interaction potential and the
cutoff where finite difference approximations of the differential equation
cease to be applicable, and they highlight the rather special standing of the
hard-core Yukawa potential as regards the severity of the computational
difficulties.Comment: J. Stat. Phys., in press. Minor changes to match published versio
A search for Galactic transients disguised as gamma-ray bursts
A significant fraction of cosmological gamma-ray bursts (GRBs) are
characterised by a fast rise and exponential decay (FRED) temporal structure.
This is not a distinctive feature of this class, since it is observed in many
Galactic transients and is likely descriptive of a sudden release of energy
followed by a diffusion process. Possible evidence has recently been reported
by Tello et al. (2012) for a Galactic contamination in the sample of FRED GRBs
discovered with Swift. We searched for possible Galactic intruders disguised as
FRED GRBs in the Swift catalogue up to September 2014. We selected 181 FRED
GRBs (2/3 with unknown redshift) and considered different subsamples. We tested
the degree of isotropy through the dipole and the quadrupole moment
distributions, both with reference to the Galaxy and in a
coordinate-system-independent way, as well as with the two-point angular
autocovariance function. In addition, we searched for possible indicators of a
Galactic origin among the spectral and temporal properties of individual GRBs.
We found marginal (~3 sigma) evidence for an excess of FREDs with unknown
redshift towards the Galactic plane compared with what is expected for an
isotropic distribution corrected for the non-uniform sky exposure. However,
when we account for the observational bias against optical follow-up
observations of low-Galactic latitude GRBs, the evidence for anisotropy
decreases to ~2 sigma. In addition, we found no statistical evidence for
different spectral or temporal properties from the bulk of cosmological GRBs.
We found marginal evidence for the presence of a disguised Galactic population
among Swift GRBs with unknown redshift. The estimated fraction is f=(19 +-
11)%, with an upper limit of 34% (90% confidence).Comment: 6 pages, 4 figures, accepted by A&
Implementation of the Hierarchical Reference Theory for simple one-component fluids
Combining renormalization group theoretical ideas with the integral equation
approach to fluid structure and thermodynamics, the Hierarchical Reference
Theory is known to be successful even in the vicinity of the critical point and
for sub-critical temperatures. We here present a software package independent
of earlier programs for the application of this theory to simple fluids
composed of particles interacting via spherically symmetrical pair potentials,
restricting ourselves to hard sphere reference systems. Using the hard-core
Yukawa potential with z=1.8/sigma for illustration, we discuss our
implementation and the results it yields, paying special attention to the core
condition and emphasizing the decoupling assumption's role.Comment: RevTeX, 16 pages, 2 figures. Minor changes, published versio
Competitive density waves in quasi-one-dimensional electron systems
We investigate the nature of the ground state of the one-dimensional t-J
model coupled to adiabatic phonons by use of the Lanczos technique at quarter
filling. Due to the interplay between electron-electron and electron-phonon
interactions, the model undergoes instabilities toward the formation of lattice
and charge modulations. Moderate on-site and intra-site electron-phonon
couplings lead to a competition of different spin-Peierls and dimerized states.
In the former case two electrons belong to the unit cell and we expect a
paramagnetic band insulator state, while lattice dimerization leads to a Mott
insulating state with quasi long range antiferromagnetic order. The zero
temperature phase diagram is obtained as a function of intra-site and
inter-site electron-phonon couplings, analytically in the limit and
numerically at finite J/t.Comment: 7 pages, 7 figures, to be published in Phys. Rev.
Recent developments of the Hierarchical Reference Theory of Fluids and its relation to the Renormalization Group
The Hierarchical Reference Theory (HRT) of fluids is a general framework for
the description of phase transitions in microscopic models of classical and
quantum statistical physics. The foundations of HRT are briefly reviewed in a
self-consistent formulation which includes both the original sharp cut-off
procedure and the smooth cut-off implementation, which has been recently
investigated. The critical properties of HRT are summarized, together with the
behavior of the theory at first order phase transitions. However, the emphasis
of this presentation is on the close relationship between HRT and non
perturbative renormalization group methods, as well as on recent
generalizations of HRT to microscopic models of interest in soft matter and
quantum many body physics.Comment: 17 pages, 5 figures. Review paper to appear in Molecular Physic
Magneto-elastic effects and magnetization plateaus in two dimensional systems
We show the importance of both strong frustration and spin-lattice coupling
for the stabilization of magnetization plateaus in translationally invariant
two-dimensional systems. We consider a frustrated spin-1/2 Heisenberg model
coupled to adiabatic phonons under an external magnetic field. At zero
magnetization, simple structures with two or at most four spins per unit cell
are stabilized, forming dimers or plaquettes, respectively. A much
richer scenario is found in the case of magnetization , where larger
unit cells are formed with non-trivial spin textures and an analogy with the
corresponding classical Ising model is detectable. Specific predictions on
lattice distortions and local spin values can be directly measured by X-rays
and Nuclear Magnetic Resonance experiments.Comment: 4 pages and 4 figure
Compton-thick AGN in the NuSTAR era II: A deep NuSTAR and XMM-Newton view of the candidate Compton thick AGN in NGC 1358
We present the combined NuSTATR and XMM-Newton 0.6-79 keV spectral analysis
of a Seyfert 2 galaxy, NGC 1358, which we selected as a candidate Compton thick
(CT-) active galactic nucleus (AGN) on the basis of previous Swift/BAT and
Chandra studies. According to our analysis, NGC 1358 is confirmed to be a
CT-AGN using physical motivated models, at >3 confidence level. Our
best-fit shows that the column density along the 'line-of-sight' of the
obscuring material surrounding the accreting super-massive black hole is N = [1.96--2.80] 10 cm. The high-quality data from
NuSTAR gives the best constraints on the spectral shape above 10 keV to
date on NGC 1358. Moreover, by combining NuSTAR and XMM-Newton data, we find
that the obscuring torus has a low covering factor ( <0.17), and the
obscuring material is distributed in clumps, rather than uniformly. We also
derive an estimate of NGC 1358's Eddington ratio, finding it to be
10, which is in
acceptable agreement with previous measurements. Finally, we find no evidence
of short-term variability, over a 100 ks time-span, in terms of both
'line-of-sight' column density and flux.Comment: 12 pages, 6 figure
Heavily obscured AGN in the local Universe
We present here a new powerful diagnostic plot to select heavily obscured AGN
in the local universe by combining infrared (Spitzer, IRAS) and X-ray (XMM)
information. On the basis of this plot, we selected a sample of X-ray obscured
sources in the 2XMM catalogue and found seven newly discovered Compton-thick
AGN candidates.Comment: 2 pages, 2 figures, To appear in refereed Proceedings of "X-ray
Astronomy 2009: Present Status, Multi-Wavelength Approach and Future
Perspectives", Bologna, Italy, September 7-11, 2009, AIP, eds. A. Comastri,
M. Cappi, and L. Angelin
- …