9,151 research outputs found
Fast Transition between High-soft and Low-soft States in GRS 1915+105: Evidence for a Critically Viscous Accretion Flow
We present the results of a detailed analysis of RXTE observations of class
which show an unusual state transition between high-soft and low-soft
states in the microquasar GRS 1915+105. Out of about 600 pointed RXTE
observations, the source was found to exhibit such state transition only on 16
occasions. An examination of the RXTE/ASM data in conjunction with the pointed
observations reveals that these events appeared as a series of quasi-regular
dips in two stretches of long duration (about 20 days during each occasions)
when hard X-ray and radio flux were very low. The X-ray light curve and
color-color diagram of the source during these observations are found to be
different from any reported so far. The duration of these dips is found to be
of the order of a few tens of seconds with a repetition time of a few hundred
seconds. The transition between these dips and non-dips which differ in
intensity by a factor of ~ 3.5, is observed to be very fast (~ a few seconds).
It is observed that the low-frequency narrow QPOs are absent in the power
density spectrum (PDS) of the dip and non-dip regions of class and the
PDS is a power law in 0.1 - 10 Hz frequency range. There is a remarkable
similarity in the spectral and timing properties of the source during the dip
and non-dip regions in these set of observations. These properties of the
source are distinctly different from those seen in the observations of other
classes. This indicates that the basic accretion disk structure during both dip
and non-dip regions of class is similar, but differ only in intensity.
To explain these observations, we invoke a model in which the viscosity is very
close to critical viscosity and the shock wave is weak or absent.Comment: Replaced with correct figures, Jour. of Astrophysics and Astronomy
(accepted
An Optically Plugged Quadrupole Trap for Bose-Einstein Condensates
We created sodium Bose-Einstein condensates in an optically plugged
quadrupole magnetic trap (OPT). A focused, 532nm laser beam repelled atoms from
the coil center where Majorana loss is significant. We produced condensates of
up to atoms, a factor of 60 improvement over previous work [1],
a number comparable to the best all-magnetic traps, and transferred up to atoms into a purely optical trap. Due to the tight axial
confinement and azimuthal symmetry of the quadrupole coils, the OPT shows
promise for creating Bose-Einstein condensates in a ring geometry
On the origin of the various types of radio emission in GRS 1915+105
We investigate the association between the radio ``plateau'' states and the
large superluminal flares in GRS 1915+105 and propose a qualitative scenario to
explain this association. We identify several candidate superluminal flare
events from available monitoring data on this source and analyze the
contemporaneous RXTE pointed observations. We detect a strong correlation
between the average X-ray flux during the ``plateau'' state and the total
energy emitted in radio during the subsequent radio flare. We find that the
sequence of events is similar for all large radio flares with a fast rise and
exponential decay morphology. Based on these results, we propose a qualitative
scenario in which the separating ejecta during the superluminal flares are
observed due to the interaction of the matter blob ejected during the X-ray
soft dips, with the steady jet already established during the ``plateau''
state. This picture can explain all types of radio emission observed from this
source in terms of its X-ray emission characteristics.Comment: Corrected typo in the author names, contents unchanged, accepted in
Ap
Isodrastic Magnetic fields for suppressing transitions in guiding-centre motion
In a magnetic field, transitions between classes of guiding-centre motion can
lead to cross-field diffusion and escape. We say a magnetic field is isodrastic
if guiding centres make no transitions between classes of motion. Therefore,
this is an important ideal for enhancing confinement. First, we present a weak
formulation, based on the longitudinal adiabatic invariant, generalising
omnigenity. To demonstrate that isodrasticity is strictly more general than
omnigenity, we construct weakly isodrastic mirror fields that are not
omnigenous. Then we present a strong formulation that is exact for
guiding-centre motion. We develop a first-order treatment of the strong version
via a Melnikov function and show that it recovers the weak version. The theory
provides quantification of deviations from isodrasticity that can be used as
objective functions in optimal design. The theory is illustrated on some simple
examples.Comment: 72 pages, 31 Figure
Corridor Roof Causing Almost Decapitation: a Rare Case Report
Background: Cut throat injury can be homicidal, suicidal or accidental in manner. However, in the absence of tentative cuts, deep cut throat injury usually raised suspicion towards homicide, even against the circumstantial history provided by close relatives or eye witness. Decapitation or almost decapitation is usually noticed in homicidal assault using any heavy cutting weapon like sword and axe or in suicidal railway injury.Case Report: A 91-years old, otherwise healthy lady (empress of one high profile Company) died due to almost decapitation as a result of accidental fall of wheel chair and trapping of neck underneath roof of corridor while moving up in an elevator (lift). With suspicion of property dispute, the investigating police agency was not ready to accept it as a case of accident till the neck injury was clarified demonstrating associated injuries during autopsy and later during visit to scene of death.Conclusion: In high profile case, video recording of autopsy findings, even if not mandatory as per the law, should be made to remain in safer side. Although police should not be allowed during conduction of autopsy, it will be healthy practice to clarify any doubts then and there over autopsy table, if the case warrants so
Bragg Spectroscopy of Vortex Lattices in Bose-Einstein condensates
We have measured the velocity field of a vortex lattice within a sodium
Bose-Einstein condensate using Bragg scattering. The phase gradient of the
macroscopic wavefunction was mapped into the spatial structure of the
diffracted atom cloud, allowing for single shot measurement of the rotation
parameters. A combination of spectral and spatial information yields a complete
description of the superfluid flow, coarse-grained over the lattice structure,
including direct and independent measurements of the rate and sense of
rotation. Signatures of the microscopic quantum rotation have also been
observed.Comment: 5 pages, 5 Figures, A movie built from the CM data is available in
our Webpage: http://www.physics.gatech.edu/chandra/index.htm; added Fig.5
presents new data, showing signatures of the microscopic vortex structure in
the diffracted clou
The effect of metformin on biomarkers associated with breast cancer outcomes: a systematic review, meta-analysis, and dose–response of randomized clinical trials
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