132 research outputs found
Solitons and Black Holes in a Generalized Skyrme Model with Dilaton-Quarkonium field
Skyrme theory is among the viable effective theories which emerge from
low-energy limit of quantum chromodynamics. Many of its generalizations include
also a dilaton. Here we find new self-gravitating solutions, both solitons and
black holes, in a Generalized Skyrme Model (GSM) in which a dilaton is present.
The investigation of the properties of the solutions is done numerically. We
find that the introduction of the dilaton in the theory does not change the
picture qualitatively, only quantitatively. The model considered here has one
free parameter more than the Einstein-Skyrme model which comes from the
potential of the dilaton. We have applied also the turning point method to
establish that one of the black-hole branches of solutions is unstable. The
turning point method here is based on the first law of black-hole
thermodynamics a detailed derivation of which is given in the Appendix of the
paper.Comment: 19 pages, 10 figures; v2: typos corrected, comments adde
Time Evolution of the Radial Perturbations and Linear Stability of Solitons and Black Holes in a Generalized Skyrme Model
We study the time evolution of the radial perturbation for self-gravitating
soliton and black-hole solutions in a generalized Skyrme model in which a
dilaton is present. The background solutions were obtained recently by some of
the authors. For both the solitons and the black holes two branches of
solutions exist which merge at some critical value of the corresponding
parameter. The results show that, similar to the case without a scalar field,
one of the branches is stable against radial perturbations and the other is
unstable. The conclusions for the linear stability of the black holes in the
generalized Skyrme model are also in agreement with the results from the
thermodynamical stability analysis based on the turning point method.Comment: 18 pages, 12 figures; v2: typos corrected, comments adde
Non-radial oscillations of anisotropic neutron stars in the Cowling approximation
One of the most common assumptions in the study of neutron star models and
their oscillations is that the pressure is isotopic, however there are
arguments that this may not be correct. Thus in the present paper we make a
first step towards studying the nonradial oscillations of neutron stars with an
anisotropic pressure. We adopt the so-called Cowling approximation where the
spacetime metric is kept fixed and the oscillation spectrum for the first few
fluid modes is obtained. The effect of the anisotropy on the frequencies is
apparent, although with the present results it might be hard to distinguish it
from the changes in the frequencies caused by different equations of state.Comment: 17 pages, 8 figures; title changed, comments adde
Uniqueness and nonuniqueness of the stationary black holes in 5D Einstein-Maxwell and Einstein-Maxwell-dilaton gravity
In the present paper we investigate the general problem of uniqueness of the
stationary black solutions in 5D Einstein-Maxwell-dilaton gravity with
arbitrary dilaton coupling parameter containing the Einstein-Maxwell gravity as
a particular case. We formulate and prove uniqueness theorems classifying the
stationary black hole solutions in terms of their interval structure, electric
and magnetic charges and the magnetic fluxes. The proofs are based on the
nonpositivity of the Riemann curvature operator on the space of the potentials
which imposes restrictions on the dilaton coupling parameter.Comment: 21 pages, LaTe
Subclinical reactivation of varicella zoster virus after covid-19 as a possible cause of stroke in young patient
Previous studies have observed an association between Varicella Zoster Virus (VZV) infection and stroke. Here we discuss possible causes of Herpes Zoster (HZ) due to COVID-19. Reactivation of VZV caused by decline of cellular immune response has been noted during the convalescent period or after recovery from COVID-19. Whether s troke can be a late sequel of a COVID-19 due to different inflammatory and coagulation mechanisms is currently uncertain. We present the case of a young patient with acute ischemic stroke, two months after mild COVID19 infection associated with subclinical reactivation of VZV infection and discuss possible causes of stroke.
Diffusio
Thermodynamic analysis of black hole solutions in gravitating nonlinear electrodynamics
We perform a general study of the thermodynamic properties of static
electrically charged black hole solutions of nonlinear electrodynamics
minimally coupled to gravitation in three space dimensions. The Lagrangian
densities governing the dynamics of these models in flat space are defined as
arbitrary functions of the gauge field invariants, constrained by some
requirements for physical admissibility. The exhaustive classification of these
theories in flat space, in terms of the behaviour of the Lagrangian densities
in vacuum and on the boundary of their domain of definition, defines twelve
families of admissible models. When these models are coupled to gravity, the
flat space classification leads to a complete characterization of the
associated sets of gravitating electrostatic spherically symmetric solutions by
their central and asymptotic behaviours. We focus on nine of these families,
which support asymptotically Schwarzschild-like black hole configurations, for
which the thermodynamic analysis is possible and pertinent. In this way, the
thermodynamic laws are extended to the sets of black hole solutions of these
families, for which the generic behaviours of the relevant state variables are
classified and thoroughly analyzed in terms of the aforementioned boundary
properties of the Lagrangians. Moreover, we find universal scaling laws (which
hold and are the same for all the black hole solutions of models belonging to
any of the nine families) running the thermodynamic variables with the electric
charge and the horizon radius. These scale transformations form a one-parameter
multiplicative group, leading to universal "renormalization group"-like
first-order differential equations. The beams of characteristics of these
equations generate the full set of black hole states associated to any of these
gravitating nonlinear electrodynamics...Comment: 51 single column pages, 19 postscript figures, 2 tables, GRG tex
style; minor corrections added; final version appearing in General Relativity
and Gravitatio
Gravitational waves from single neutron stars: an advanced detector era survey
With the doors beginning to swing open on the new gravitational wave
astronomy, this review provides an up-to-date survey of the most important
physical mechanisms that could lead to emission of potentially detectable
gravitational radiation from isolated and accreting neutron stars. In
particular we discuss the gravitational wave-driven instability and
asteroseismology formalism of the f- and r-modes, the different ways that a
neutron star could form and sustain a non-axisymmetric quadrupolar "mountain"
deformation, the excitation of oscillations during magnetar flares and the
possible gravitational wave signature of pulsar glitches. We focus on progress
made in the recent years in each topic, make a fresh assessment of the
gravitational wave detectability of each mechanism and, finally, highlight key
problems and desiderata for future work.Comment: 39 pages, 12 figures, 2 tables. Chapter of the book "Physics and
Astrophysics of Neutron Stars", NewCompStar COST Action 1304. Minor
corrections to match published versio
A Compressed Sensing Framework for Magnetic Resonance Fingerprinting
Inspired by the recently proposed Magnetic Resonance Fingerprinting (MRF) technique we develop a principled compressed sensing framework for quantitative MRI. The three key components are: a random pulse excitation sequence following the MRF technique; a random EPI subsampling strategy and an iterative projection algorithm that imposes consistency with the Bloch equations. We show that, as long as the excitation sequence possesses an appropriate form of persistent excitation, we are able to achieve accurate recovery the proton density, T1, T2 and off-resonance maps simultaneously from a limited number of samples
Signatures of de-domestication in autochthonous pig breeds and of domestication in wild boar populations from MC1R and NR6A1 allele distribution
Autochthonous pig breeds are usually reared in extensive or semi-extensive production systems that might facilitate contact with wild boars and, thus, reciprocal genetic exchanges. In this study, we analysed variants in the melanocortin 1 receptor (MC1R) gene (which cause different coat colour phenotypes) and in the nuclear receptor subfamily 6 group A member 1 (NR6A1) gene (associated with increased vertebral number) in 712 pigs of 12 local pig breeds raised in Italy (Apulo-Calabrese, Casertana, Cinta Senese, Mora Romagnola, Nero Siciliano and Sarda) and south-eastern European countries (Krskopolje from Slovenia, Black Slavonian and Turopolje from Croatia, Mangalitsa and Moravka from Serbia and East Balkan Swine from Bulgaria) and compared the data with the genetic variability at these loci investigated in 229 wild boars from populations spread in the same macro-geographic areas. None of the autochthonous pig breeds or wild boar populations were fixed for one allele at both loci. Domestic and wild-type alleles at these two genes were present in both domestic and wild populations. Findings of the distribution of MC1R alleles might be useful for tracing back the complex genetic history of autochthonous breeds. Altogether, these results indirectly demonstrate that bidirectional introgression of wild and domestic alleles is derived and affected by the human and naturally driven evolutionary forces that are shaping the Sus scrofa genome: autochthonous breeds are experiencing a sort of 'de-domestication' process, and wild resources are challenged by a 'domestication' drift. Both need to be further investigated and managed
- …