6,488 research outputs found
Steady shocks around black holes produced by sub-keplerian flows with negative energy
We discuss a special case of formation of axisymmetric shocks in the
accretion flow of ideal gas onto a Schwarzschild black hole: when the total
energy of the flow is negative. The result of our analysis enlarges the
parameter space for which these steady shocks are exhibited in the accretion of
gas rotating around relativistic stellar objects. Since keplerian disks have
negative total energy, we guess that, in this energy range, the production of
the shock phenomenon might be easier than in the case of positive energy. So
our outcome reinforces the view that sub-keplerian flows of matter may
significantly affect the physics of the high energy radiation emission from
black hole candidates. We give a simple procedure to obtain analytically the
position of the shocks. The comparison of the analytical results with the data
of 1D and 2D axisymmetric numerical simulations confirms that the shocks form
and are stable.Comment: 5 pages, 5 figures, accepted by MNRAS on 10 November 200
Zeros of Dedekind zeta functions under GRH
Assuming GRH, we prove an explicit upper bound for the number of zeros of a
Dedekind zeta function having imaginary part in . We also prove a
bound for the multiplicity of the zeros.Comment: Some misprints corrected, simplified proof for a lemma. This version
will appear in Mathematics of Computatio
Explicit versions of the prime ideal theorem for Dedekind zeta functions under GRH
Let \psi_\K be the Chebyshev function of a number field \K. Under GRH we
prove an explicit upper bound for |\psi_\K(x)-x| in terms of the degree and
the discriminant of \K. The new bound improves significantly on previous
known results.Comment: Some misprints corrected. This is the final version which will appear
in Mathematics of Computatio
Hydrodynamic Simulations of Oscillating Shock Waves in a Sub-Keplerian Accretion Flow Around Black Holes
We study the accretion processes on a black hole by numerical simulation. We
use a grid based finite difference code for this purpose. We scan the parameter
space spanned by the specific energy and the angular momentum and compare the
time-dependent solutions with those obtained from theoretical considerations.
We found several important results (a) The time dependent flow behaves close to
a constant height model flow in the pre-shock region and a flow with vertical
equilibrium in the post-shock region. (c) The infall time scale in the
post-shock region is several times higher than the free-fall time scale. (b)
There are two discontinuities in the flow, one being just outside of the inner
sonic point. Turbulence plays a major role in determining the locations of
these discontinuities. (d) The two discontinuities oscillate with two different
frequencies and behave as a coupled harmonic oscillator. A Fourier analysis of
the variation of the outer shock location indicates higher power at the lower
frequency and lower power at the higher frequency. The opposite is true when
the analysis of the inner shock is made. These behaviours will have
implications in the spectral and timing properties of black hole candidates.Comment: 19 pages, 13 figures, 1 Table MNRAS (In press
Visual illusions: An interesting tool to investigate developmental dyslexia and autism spectrum disorder
A visual illusion refers to a percept that is different in some aspect from the physical stimulus. Illusions are a powerful non-invasive tool for understanding the neurobiology of vision, telling us, indirectly, how the brain processes visual stimuli. There are some neurodevelopmental disorders characterized by visual deficits. Surprisingly, just a few studies investigated illusory perception in clinical populations. Our aim is to review the literature supporting a possible role for visual illusions in helping us understand the visual deficits in developmental dyslexia and autism spectrum disorder. Future studies could develop new tools – based on visual illusions – to identify an early risk for neurodevelopmental disorders
The free energy landscape of GABA binding to a pentameric ligand-gated ion channel and its disruption by mutations
Pentameric ligand-gated ion channels (pLGICs) of the Cys-loop superfamily are
important neuroreceptors that mediate fast synaptic transmission. They are
activated by the binding of a neurotransmitter, but the details of this process
are still not fully understood. As a prototypical pLGIC, here we choose the
insect resistance to dieldrin (RDL) receptor, involved in the resistance to
insecticides, and investigate the binding of the neurotransmitter GABA to its
extracellular domain at the atomistic level. We achieve this by means of
-sec funnel-metadynamics simulations, which efficiently enhance the
sampling of bound and unbound states by using a funnel-shaped restraining
potential to limit the exploration in the solvent. We reveal the sequence of
events in the binding process, from the capture of GABA from the solvent to its
pinning between the charged residues Arg111 and Glu204 in the binding pocket.
We characterize the associated free energy landscapes in the wild-type RDL
receptor and in two mutant forms, where the key residues Arg111 and Glu204 are
mutated to Ala. Experimentally these mutations produce non-functional channels,
which is reflected in the reduced ligand binding affinities, due to the loss of
essential interactions. We also analyze the dynamical behaviour of the crucial
loop C, whose opening allows the access of GABA to the binding site, while its
closure locks the ligand into the protein. The RDL receptor shares structural
and functional features with other pLGICs, hence our work outlines a valuable
protocol to study the binding of ligands to pLGICs beyond conventional docking
and molecular dynamics techniques.Comment: accepted (May 2016); 27 pages, 6 figures, Table of contents graphic,
Journal of Chemical Theory and Computation (2016
Ab initio simulations of accretion disks instability
We show that accretion disks, both in the subcritical and supercritical
accretion rate regime, may exhibit significant amplitude luminosity
oscillations. The luminosity time behavior has been obtained by performing a
set of time-dependent 2D SPH simulations of accretion disks with different
values of alpha and accretion rate. In this study, to avoid any influence of
the initial disk configuration, we produced the disks injecting matter from an
outer edge far from the central object. The period of oscillations is 2 - 50 s
respectively for the two cases, and the variation amplitude of the disc
luminosity is 10^38 - 10^39 erg/s. An explanation of this luminosity behavior
is proposed in terms of limit cycle instability: the disk oscillates between a
radiation pressure dominated configuration (with a high luminosity value) and a
gas pressure dominated one (with a low luminosity value). The origin of this
instability is the difference between the heat produced by viscosity and the
energy emitted as radiation from the disk surface (the well-known thermal
instability mechanism). We support this hypothesis showing that the limit cycle
behavior produces a sequence of collapsing and refilling states of the
innermost disk region.Comment: 11 pages, 15 Postscript figures, uses natbib.sty, accepted for
publication in MNRA
- …