3,856 research outputs found
Spectral Properties of Accretion Disks Around Black Holes II -- Sub-Keplerian Flows With and Without Shocks
Close to a black hole, the density of the sub-Keplerian accreting matter
becomes higher compared to a spherical flow due to the presence of a
centrifugal barrier independent of whether or not a standing shock actually
forms. This hot dense flow intercepts soft photons from a cold Keplerian disk
and reprocesses them to form high energy X-rays and gamma rays. We study the
spectral properties of various models of accretion disks where a Keplerian disk
on the equatorial plane may or may not be flanked by a sub-Keplerian disk and
the sub-Keplerian flow may or may not possess standing shocks. From comparison
with the spectra, we believe that the observed properties could be explained
better when both the components (Keplerian and sub-Keplerian) are
simultaneously present close to a black hole, even though the sub-Keplerian
halo component may have been produced out of the Keplerian disk itself at
larger radii. We are able to understand soft and hard states of black hole
candidates, properties of X-ray novae outbursts, and quasi-periodic
oscillations of black hole candidates using these two component models. We fit
spectra of X-ray novae GS1124-68 and GS2000+25 and satisfactorily reproduce the
light curves of these objects.Comment: 15 Latex pages plus 12 figures. Macros included. Astrophysical
Journal (In press
Satellite observations of thought experiments close to a black hole
Since black holes are `black', methods of their identification must
necessarily be indirect. Due to very special boundary condition on the horizon,
the advective flow behaves in a particular way, which includes formation of
centrifugal pressure dominated boundary layer or CENBOL where much of the
infall energy is released and outflows are generated. The observational aspects
of black holes must depend on the steady and time-dependent properties of this
boundary layer. Several observational results are written down in this review
which seem to support the predictions of thought experiments based on this
advective accretion/outflow model. In future, when gravitational waves are
detected, some other predictions of this model could be tested as well.Comment: Published in Classical and Quantum Gravity, v. 17, No. 12, p. 2427,
200
Quantum spin glass and the dipolar interaction
Systems in which the dipolar energy dominates the magnetic interaction, and
the crystal field generates strong anisotropy favoring the longitudinal
interaction terms, are considered. Such systems in external magnetic field are
expected to be a good experimental realization of the transverse field Ising
model. With random interactions this model yields a spin glass to paramagnet
phase transition as function of the transverse field. Here we show that the
off-diagonal dipolar interaction, although effectively reduced, destroys the
spin glass order at any finite transverse field. Moreover, the resulting
correlation length is shown to be small near the crossover to the paramagnetic
phase, in agreement with the behavior of the nonlinear susceptibility in the
experiments on \LHx. Thus, we argue that the in these experiments a
cross-over to the paramagnetic phase, and not quantum criticality, was
observed.Comment: To appear in Phys. Rev. Let
Ionization of hydrogen atoms by electron impact at 1eV, 0.5eV and 0.3eV above threshold
We present here triple differential cross sections for ionization of hydrogen
atoms by electron impact at 1eV, 0.5eV and 0.3eV energy above threshold,
calculated in the hyperspherical partial wave theory. The results are in very
good agreement with the available semiclassical results of Deb and Crothers
\cite{DC02} for these energies. With this, we are able to demonstrate that the
hyperspherical partial wave theory yields good cross sections from 30 eV
\cite{DPC03} down to near threshold for equal energy sharing kinematics.Comment: 6 pages, 9 figure
Magneto-transport in a mesoscopic ring with Rashba and Dresselhaus spin-orbit interactions
Electronic transport in a one-dimensional mesoscopic ring threaded by a
magnetic flux is studied in presence of Rashba and Dresselhaus spin-orbit
interactions. A completely analytical technique within a tight-binding
formalism unveils the spin-split bands in presence of the spin-orbit
interactions and leads to a method of determining the strength of the
Dresselhaus interaction. In addition to this, the persistent currents for
ordered and disordered rings have been investigated numerically. It is observed
that, the presence of the spin-orbit interaction, in general, leads to an
enhanced amplitude of the persistent current. Numerical results corroborate the
respective analytical findings.Comment: 7 pages, 7 figure
Quantum fluctuation induced ordered phase in the Blume-Capel model
We consider the Blume-Capel model with the quantum tunneling between the
excited states. We find a magnetically ordered phase transition induced by
quantum fluctuation in a model. The model has no phase transition in the
corresponding classical case. Usually, quantum fluctuation breaks ordered phase
as in the case of the transverse field Ising model. However, in present case,
an ordered phase is induced by quantum fluctuation. Moreover, we find a phase
transition between a quantum paramagnetic phase and a classical diamagnetic
phase at zero temperature. We study the properties of the phase transition by
using a mean field approximation (MFA), and then, by a quantum Monte Carlo
method to confirm the result of the MFA.Comment: 7 pages, 6 figures, corrected some typo
A Novel Quantum Transition in a Fully Frustrated Transverse Ising Antiferromagnet
We consider a long-range Ising antiferromagnet (LRIAF) put in a transverse
field. Applying quantum Monte Carlo method, we study the variation of order
parameter (spin correlation in Trotter time direction), susceptibility and
average energy of the system for various values of the transverse field at
different temperatures. The antiferromagnetic order is seen to get immediately
broken as soon as the thermal or quantum fluctuations are added. We also
discuss the phase diagram for the Sherrington-Kirkpatrick (SK) model with the
same LRIAF bias, also in presence of a transverse field. We find that while the
antiferromagnetic order is immediately broken as one adds an infinitesimal
transverse field or thermal fluctuation to the system, an infinitesimal SK spin
glass disorder is enough to induce a stable glass order in the antiferromagnet.
This glass order eventually gets destroyed as the thermal or quantum
fluctuations increased beyond their threshold values and the transition to para
phase occurs. Indications of this novel phase transition are discussed. Because
of the presence of full frustration, this surrogate property of the LRIAF for
incubation of stable spin glass phase in it (induced by addition of a small
disorder) should enable eventually the study of classical and quantum spin
glass phases by using some perturbation theory with respect to the disorder.Comment: 9 pages, 5 figure
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