192 research outputs found
Hard X-ray emitting black hole fed by accretion of low angular momentum matter
Observed spectra of Active Galactic Nuclei (AGN) and luminous X-ray binaries
in our Galaxy suggest that both hot (~10^9 K) and cold (~10^6 K) plasma
components exist close to the central accreting black hole. Hard X-ray
component of the spectra is usually explained by Compton upscattering of
optical/UV photons from optically thick cold plasma by hot electrons.
Observations also indicate that some of these objects are quite efficient in
converting gravitational energy of accretion matter into radiation. Existing
theoretical models have difficulties in explaining the two plasma components
and high intensity of hard X-rays. Most of the models assume that the hot
component emerges from the cold one due to some kind of instability, but no one
offers a satisfactory physical explanation for this. Here we propose a solution
to these difficulties that reverses what was imagined previously: in our model
the hot component forms first and afterward it cools down to form the cold
component. In our model, accretion flow has initially a small angular momentum,
and thus it has a quasi-spherical geometry at large radii. Close to the black
hole, the accreting matter is heated up in shocks that form due to the action
of the centrifugal force. The hot post-shock matter is very efficiently cooled
down by Comptonization of low energy photons and condensates into a thin and
cold accretion disk. The thin disk emits the low energy photons which cool the
hot component.Comment: 15 pages, 2 figures, submitted to ApJ Let
Deuteron distribution in nuclear matter
We analyze the properties of deuteron-like structures in infinite, correlated
nuclear matter, described by a realistic hamiltonian containing the Urbana
two-nucleon and the Urbana TNI many-body potentials. The distribution
of neutron-proton pairs, carrying the deuteron quantum numbers, is obtained as
a function of the total momentum by computing the overlap between the nuclear
matter in its ground state and the deuteron wave functions in correlated basis
functions theory. We study the differences between the S- and D-wave components
of the deuteron and those of the deuteron-like pair in the nuclear medium. The
total number of deuteron type pairs is computed and compared with the
predictions of Levinger's quasideuteron model. The resulting Levinger's factor
in nuclear matter at equilibrium densityis 11.63. We use the local density
approximation to estimate the Levinger's factor for heavy nuclei, obtaining
results which are consistent with the available experimental data from
photoreactions.Comment: 22 pages, 7 figures, typeset using REVTe
Fracture surface analysis of a quenched (α+β)-metastable titanium alloy
Fracture surface analysis is conducted by means of SEM for VT16 titanium alloy specimens solution-treated at temperatures ranging from 700 to 875 °C, water-quenched and subjected to tensile testing. A cup and cone shape failure and dimple microstructure of the fracture surface indicates the ductile behavior of the alloy. Dimple dimensions correlated with the β-grain size of the alloy in quenched condition. The fracture area (namely, the size; the cup and cone shape) depends on the volume fraction of the primary α-phase in the quenched sample. However, the fracture surface changes considerably when the strain-induced β-α''-transformation takes place during tensile testing, resulting in the increase of alloy ductility. © 2017 Author(s).Russian Foundation for Basic Research, RFBR: 15-08-08299 АCouncil on grants of the President of the Russian Federation: MK-6311.2016.8The work was supported by the grant of the President of Russian Federation, No. MK-6311.2016.8, and RFBR grant No. 15-08-08299 А
Can the anomalous X-ray pulsars be powered by accretion?
The nature of the 5-12 s "anomalous" X-ray pulsars remains a mystery. Among
the models that have been proposed to explain the properties of AXPs, the most
likely ones are: (1) isolated accreting neutron stars evolved from the
Thorne-\.{Z}ytkow objects due to complete spiral-in during the common envelope
evolution of high-mass X-ray binaries, and (2) magnetars, which are neutron
stars with ultra-high ( G) surface magnetic fields. We
have critically examined the predicted change of neutron star's spin in the
accretion model, and found that it is unable to account for the steady
spin-down observed in AXPs. A simple analysis also shows that any accretion
disk around an isolated neutron star has extremely limited lifetime. A more
promising explanation for such objects is the magnetar model.Comment: 9 pages, accepted for publication in Ap
Magnetically-dominated jets inside collapsing stars as a model for gamma-ray bursts and supernova explosions
It has been suggested that magnetic fields play a dynamically-important role
in core-collapse explosions of massive stars. In particular, they may be
important in the collapsar scenario for gamma-ray bursts (GRB), where the
central engine is a hyper-accreting black hole or a millisecond magnetar. The
present paper is focussed on the magnetar scenario, with a specific emphasis on
the interaction of the magnetar magnetosphere with the infalling stellar
envelope. First, the ``Pulsar-in-a-Cavity'' problem is introduced as a paradigm
for a magnetar inside a collapsing star. The basic set-up of this fundamental
plasma-physics problem is described, outlining its main features, and simple
estimates are derived for the evolution of the magnetic field. In the context
of a collapsing star, it is proposed that, at first, the ram pressure of the
infalling plasma acts to confine the magnetosphere, enabling a gradual build-up
of the magnetic pressure. At some point, the growing magnetic pressure
overtakes the (decreasing) ram pressure of the gas, resulting in a
magnetically-driven explosion. The explosion should be highly anisotropic, as
the hoop-stress of the toroidal field, confined by the surrounding stellar
matter, collimates the magnetically-dominated outflow into two beamed
magnetic-tower jets. This creates a clean narrow channel for the escape of
energy from the central engine through the star, as required for GRBs. In
addition, the delayed onset of the collimated-explosion phase can explain the
production of large quantities of Nickel-56, as suggested by the GRB-Supernova
connection. Finally, the prospects for numerical simulations of this scenario
are discussed.Comment: Invited paper in the "Physics of Plasmas" (May 2007 special issue),
based on an invited talk at the 48th Annual Meeting of the APS Division of
Plasma Physics (Oct. 30 - Nov. 3, 2006, Philadelphia, PA); 24 pages, 7
figure
Trans-sonic propeller stage
We follow the approach used by Davies and Pringle (1981) and discuss the
trans-sonic substage of the propeller regime. This substage is intermediate
between the supersonic and subsonic propeller substages. In the trans-sonic
regime an envelope around a magnetosphere of a neutron star passes through a
kind of a reorganization process. The envelope in this regime consists of two
parts. In the bottom one turbulent motions are subsonic. Then at some distance
the turbulent velocity becomes equal to the sound velocity.
During this substage the boundary propagates outwards till it
reaches the outer boundary, and so the subsonic regime starts.
We found that the trans-sonic substage is unstable, so the transition between
supersonic and subsonic substages proceeds on the dynamical time scale. For
realistic parameters this time is in the range from weeks to years.Comment: 8 pages with figures, submitted to Astron. Astroph. Transaction
Recent progress on the accurate determination of the equation of state of neutron and nuclear matter
The problem of accurately determining the equation of state of nuclear and
neutron matter at density near and beyond saturation is still an open
challenge. In this paper we will review the most recent progress made by means
of Quantum Monte Carlo calculations, which are at present the only ab-inito
method capable to treat a sufficiently large number of particles to give
meaningful estimates depending only on the choice of the nucleon-nucleon
interaction. In particular, we will discuss the introduction of
density-dependent interactions, the study of the temperature dependence of the
equation of state, and the possibility of accurately studying the effect of the
onset of hyperons by developing an accurate hyperon-nucleon and
hyperon-nucleon-nucleon interaction.Comment: 3 figures, 1 table, to appear in the Proceedings of "XIII Convegno di
Cortona su Problemi di Fisica Nucleare Teorica", Cortona (Italy), April 6-8,
201
Effect of Thermomechanical Treatment Parameters on Structure, Phase Composition and Mechanical Properties of Ti-3Al-5Mo-4.5V Titanium Alloy
The structure, phase composition and mechanical properties of (α+β) - titanium alloy solution treated at 850 C, cold-rolled at the reduction ratio in the range of 0... 45%, followed by ageing at 450, 500, 550 C for 0.5, 1.5, 3 hours was studied using XRD, microindentation and tensile testing. The influence of strain level at cold rolling and time-temperature parameters of ageing on the formation of structure and phase composition of solution treated and water quenched Ti-3Al-5Mo-4.5V alloy was investigated and discussed in terms of tensile properties and microhardness. The parameters of low temperature thermomechanical treatment (LTMT) of the (α+β) - alloy were proposed to obtain a high-strength state. © Published under licence by IOP Publishing Ltd.This study was financially supported by the State Assignment, grant number 0836-2020-0020. The research equipment was purchased with the support from Act 211 of the Government of the Russian Federation, contract No. 02.A03.21.0006
Nonlinear variations in axisymmetric accretion
We subject the stationary solutions of inviscid and axially symmetric
rotational accretion to a time-dependent radial perturbation, which includes
nonlinearity to any arbitrary order. Regardless of the order of nonlinearity,
the equation of the perturbation bears a form that is similar to the metric
equation of an analogue acoustic black hole. We bring out the time dependence
of the perturbation in the form of a Li\'enard system, by requiring the
perturbation to be a standing wave under the second order of nonlinearity. We
perform a dynamical systems analysis of the Li\'enard system to reveal a saddle
point in real time, whose implication is that instabilities will develop in the
accreting system when the perturbation is extended into the nonlinear regime.
We also model the perturbation as a high-frequency travelling wave, and carry
out a Wentzel-Kramers-Brillouin analysis, treating nonlinearity iteratively as
a very feeble effect. Under this approach both the amplitude and the energy
flux of the perturbation exhibit growth, with the acoustic horizon segregating
the regions of stability and instability.Comment: 15 pages, ReVTeX. Substantially revised with respect to the previous
version. One figure and a new section on travelling waves (Sec. VI) have been
added. The bibliography has been revised. arXiv admin note: substantial text
overlap with arXiv:1207.107
On the Nature of Part Time Radio Pulsars
The recent discovery of rotating radio transients and the quasi-periodicity
of pulsar activity in the radio pulsar PSR B193124 has challenged the
conventional theory of radio pulsar emission. Here we suggest that these
phenomena could be due to the interaction between the neutron star
magnetosphere and the surrounding debris disk. The pattern of pulsar emission
depends on whether the disk can penetrate the light cylinder and efficiently
quench the processes of particle production and acceleration inside the
magnetospheric gap. A precessing disk may naturally account for the
switch-on/off behavior in PSR B193124.Comment: 9 pages, accepted to ApJ
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