3,950 research outputs found
Dynamics of gap solitons in a dipolar Bose-Einstein condensate on a three-dimensional optical lattice
We suggest and study the stable disk- and cigar-shaped gap solitons of a
dipolar Bose-Einstein condensate of Cr atoms localized in the lowest
band gap by three optical-lattice (OL) potentials along orthogonal directions.
The one-dimensional version of these solitons of experimental interest confined
by an OL along the dipole moment direction and harmonic traps in transverse
directions is also considered. Important dynamics of (i) breathing oscillation
of a gap soliton upon perturbation and (ii) dragging of a gap soliton by a
moving lattice along axial direction demonstrates the stability of gap
solitons. A movie clip of dragging of three-dimensional gap soliton is
included.Comment: To see the dragging movie clip please download sourc
Dynamics of quasi-one-dimensional bright and vortex solitons of a dipolar Bose-Einstein condensate with repulsive atomic interaction
By numerical and variational analysis of the three-dimensional
Gross-Pitaevskii equation we study the formation and dynamics of bright and
vortex-bright solitons in a cigar-shaped dipolar Bose-Einstein condensate for
large repulsive atomic interactions. Phase diagram showing the region of
stability of the solitons is obtained. We also study the dynamics of breathing
oscillation of the solitons as well as the collision dynamics of two solitons
at large velocities. Two solitons placed side-by-side at rest coalesce to form
a stable bound soliton molecule due to dipolar attraction.Comment: To obtain the included video clips S1, S2, S3 and S4, please download
sourc
Intermediate-line Emission in AGNs: The Effect of Prescription of the Gas Density
The requirement of intermediate line component in the recently observed
spectra of several AGNs points to possibility of the existence of a physically
separate region between broad line region (BLR) and narrow line region (NLR).
In this paper we explore the emission from intermediate line region (ILR) by
using the photoionization simulations of the gas clouds distributed radially
from the AGN center. The gas clouds span distances typical for BLR, ILR and
NLR, and the appearance of dust at the sublimation radius is fully taken into
account in our model. Single cloud structure is calculated under the assumption
of the constant pressure. We show that the slope of the power law cloud density
radial profile does not affect the existence of ILR in major types of AGN. We
found that the low ionization iron line, Fe~II, appears to be highly sensitive
for the presence of dust and therefore becomes potential tracer of dust content
in line emitting regions. We show that the use of disk-like cloud density
profile computed at the upper part of the accretion disc atmosphere reproduces
the observed properties of the line emissivities. In particular, the distance
of H line inferred from our model agrees with that obtained from the
reverberation mapping studies in Sy1 galaxy NGC 5548.Comment: 15 pages, 13 figure
The intermediate line region in active galactic nuclei
We show that the recently observed suppression of the gap between the broad
line region (BLR) and the narrow line region (NLR) in some AGN can be fully
explained by an increase of the gas density in the emitting region. Our model
predicts the formation of the intermediate line region (ILR) that is observed
in some Seyfert galaxies by the detection of emission lines with intermediate
velocity full width half maximum (FWHM) 700 - 1200 km s. These
lines are believed to be originating from an ILR located somewhere between the
BLR and NLR. As it was previously proved, the apparent gap is assumed to be
caused by the presence of dust beyond the sublimation radius. Our computations
with the use of {\sc cloudy} photoionization code, show that the differences in
the shape of spectral energy distribution (SED) from the central region of AGN,
do not diminish the apparent gap in the line emission in those objects. A
strong discontinuity in the line emission vs radius exists for all lines at the
dust sublimation radius. However, increasing the gas density to
10 cm at the sublimation radius provides the continuous line
emission vs radius and fully explains the recently observed lack of apparent
gap in some AGN. We show that such a high density is consistent with the
density of upper layers of an accretion disk atmosphere. Therefore, the upper
layers of the disk atmosphere can give rise to the formation of observed
emission line clouds.Comment: 9 pages, 6 figures, accepted for publication in Ap
Black soliton in a quasi-one-dimensional trapped fermion-fermion mixture
Employing a time-dependent mean-field-hydrodynamic model we study the
generation of black solitons in a degenerate fermion-fermion mixture in a
cigar-shaped geometry using variational and numerical solutions. The black
soliton is found to be the first stationary vibrational excitation of the
system and is considered to be a nonlinear continuation of the vibrational
excitation of the harmonic oscillator state. We illustrate the stationary
nature of the black soliton, by studying different perturbations on it after
its formation.Comment: 7 pages, 10 figure
Conditions for the Thermal Instability in the Galactic Centre Mini-spiral region
We explore the conditions for the thermal instability to operate in the
mini-spiral region of the Galactic centre (Sgr A*), where both the hot and cold
media are known to coexist. The photoionisation Cloudy calculations are
performed for different physical states of plasma. We neglect the dynamics of
the material and concentrate on the study of the parameter ranges where the
thermal instability may operate, taking into account the past history of Sgr A*
bolometric luminosity. We show that the thermal instability does not operate at
the present very low level of the Sgr A* activity. However, Sgr A* was much
more luminous in the past. For the highest luminosity states the two-phase
medium can be created up to 1.4 pc from the centre. The presence of dust grains
tends to suppress the instability, but the dust is destroyed in the presence of
strong radiation field and hot plasma. The clumpiness is thus induced in the
high activity period, and the cooling/heating timescales are long enough to
preserve later the past multi-phase structure. The instability enhances the
clumpiness of the mini-spiral medium and creates a possibility of episodes of
enhanced accretion of cold clumps towards Sgr A*. The mechanism determines the
range of masses and sizes of clouds; under the conditions of Sgr A*, the likely
values come out - for the cloud typical mass.Comment: Accepted for publication in MNRAS, 10 pages, 7 figure
Localization of a dipolar Bose-Einstein condensate in a bichromatic optical lattice
By numerical simulation and variational analysis of the Gross-Pitaevskii
equation we study the localization, with an exponential tail, of a dipolar
Bose-Einstein condensate (DBEC) of Cr atoms in a three-dimensional
bichromatic optical-lattice (OL) generated by two monochromatic OL of
incommensurate wavelengths along three orthogonal directions. For a fixed
dipole-dipole interaction, a localized state of a small number of atoms () could be obtained when the short-range interaction is not too attractive
or not too repulsive. A phase diagram showing the region of stability of a DBEC
with short-range interaction and dipole-dipole interaction is given
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