11,218 research outputs found
Fluxon analogues and dark solitons in linearly coupled Bose-Einstein condensates
Two effectively one-dimensional parallel coupled Bose-Einstein condensates in
the presence of external potentials are studied. The system is modelled by
linearly coupled Gross-Pitaevskii equations. In particular, grey-soliton-like
solutions representing analogues of superconducting Josephson fluxons as well
as coupled dark solitons are discussed. Theoretical approximations based on
variational formulations are derived. It is found that the presence of a
magnetic trap can destabilize the fluxon analogues. However, stabilization is
possible by controlling the effective linear coupling between the condensates.Comment: 14 pages, 7 figures, The paper is to appear in Journal of Physics
The origin of optical emission lines in the soft state of X-ray binary outbursts: the case of MAXI J1820+070
The optical emission line spectra of X-ray binaries (XRBs) are thought to be
produced in an irradiated atmosphere, possibly the base of a wind, located
above the outer accretion disc. However, the physical nature of - and physical
conditions in - the line-forming region remain poorly understood. Here, we test
the idea that the optical spectrum is formed in the transition region between
the cool, geometrically thin part of the disc near the mid-plane and a hot,
vertically extended atmosphere or outflow produced by X-ray irradiation. We
first present a VLT X-Shooter spectrum of XRB MAXI J1820+070 in the soft state
associated with its 2018 outburst, which displays a rich set of double-peaked
hydrogen and helium recombination lines. Aided by ancillary X-ray spectra and
reddening estimates, we then model this spectrum with the Monte Carlo radiative
transfer code Python, using a simple biconical disc wind model inspired by
radiation-hydrodynamic simulations of irradiation-driven outflows from XRB
discs. Such a model can qualitatively reproduce the observed features; nearly
all of the optical emission arising from the transonic 'transition region' near
the base of the wind. In this region, characteristic electron densities are on
the order of 10 cm, in line with the observed flat Balmer
decrement (H/H). We conclude that strong irradiation
can naturally give rise to both the optical line-forming layer in XRB discs and
an overlying outflow/atmosphere that produces X-ray absorption lines.Comment: 17 pages, 11 figures. This article has been accepted for publication
in MNRAS published by Oxford University Press on behalf of the Royal
Astronomical Societ
Structural insight into the TRIAP1/PRELI-like domain family of mitochondrial phospholipid transfer complexes
The composition of the mitochondrial membrane is important for its architecture and proper function. Mitochondria depend on a tightly regulated supply of phospholipid via intra-mitochondrial synthesis and by direct import from the endoplasmic reticulum. The Ups1/PRELI-like family together with its mitochondrial chaperones (TRIAP1/Mdm35) represent a unique heterodimeric lipid transfer system that is evolutionary conserved from yeast to man. Work presented here provides new atomic resolution insight into the function of a human member of this system. Crystal structures of free TRIAP1 and the TRIAP1âSLMO1 complex reveal how the PRELI domain is chaperoned during import into the intermembrane mitochondrial space. The structural resemblance of PRELI-like domain of SLMO1 with that of mammalian phoshatidylinositol transfer proteins (PITPs) suggest that they share similar lipid transfer mechanisms, in which access to a buried phospholipid-binding cavity is regulated by conformationally adaptable loops
Independent sexual violence advisers (ISVAs) in England, Wales and Northern Ireland: a study of impacts, effects, coping mechanisms and effective support systems for people working as ISVAs and ISVA Managers
This study is the first of its kind in England, Wales and Northern Ireland and was carried out to gather information about the impact of working as Independent Sexual Violence Advisers (ISVAs) or ISVA Managers. The ISVA role is relatively new, and as such, little is known about this difficult and emotionally charged work. Although some literature looks at the impact of working in the sexual violence field, there is little from England, Wales or Northern Ireland, and even less focused on the direct, ongoing, face-to-face work ISVAs do. This report focuses on the impacts, effects, coping mechanisms and effective support systems for people working as ISVAs and ISVA Managers in supporting victim-survivors of sexual violence across England, Wales and Northern Ireland. Since its introduction in 2005, the ISVA role has been invaluable in providing continuous support to victim-survivors of sexual violence and working to meet their emotional and practical needs during the criminal justice process. As conviction rates are at an all-time low, ISVAs can also provide support to people who may otherwise feel let down by the Criminal Justice System. ISVAs can also support people who do not want to report the crime to the police â helping people at, potentially, the lowest point of their life. This survey focused on providing an overview of the ISVA role (e.g. context they work in, caseloads, supervision and support available) and determining which factors predict the likelihood of experiencing negative and positive impacts as a result of the work they do
Faint Infrared Flares from the Microquasar GRS 1915+105
We present simultaneous infrared and X-ray observations of the Galactic
microquasar GRS 1915+105 using the Palomar 5-m telescope and Rossi X-ray Timing
Explorer on July 10, 1998 UT. Over the course of 5 hours, we observed 6 faint
infrared (IR) flares with peak amplitudes of mJy and durations
of seconds. These flares are associated with X-ray
soft-dip/soft-flare cycles, as opposed to the brighter IR flares associated
with X-ray hard-dip/soft-flare cycles seen in August 1997 by Eikenberry et al.
(1998). Interestingly, the IR flares begin {\it before} the X-ray oscillations,
implying an ``outside-in'' origin of the IR/X-ray cycle. We also show that the
quasi-steady IR excess in August 1997 is due to the pile-up of similar faint
flares. We discuss the implications of this flaring behavior for understanding
jet formation in microquasars.Comment: 10 pages, 4 figures Accepted for publication in ApJ Letter
Optimal conditions for observing Josephson oscillations in a double-well Bose-gas condensate
The Josephson oscillations between condensates in a double-well trap are
known theoretically to be strongly effected by the mean field interaction in
dilute atomic gases. The most important effect is that the amplitude of
oscillation in the relative population of the two wells is greatly suppressed
due to the mean field interaction, which can make it difficult to observe the
Josephson effect. Starting from the work of Raghavan, Smerzi, Fantoni, and
Shenoy, we calculate the maximum amplitude of oscillation in the relative
population as a function of various physical parameters, such as the trap
aspect ratio, the Gaussian barrier height and width, and the total number of
atoms in the condensate. We also compare results for Na and
Rb. Our main new result is that the maximum amplitude of oscillation
depends strongly on the aspect ratio of the harmonic trap and can be maximized
in a ``pancake'' trap, as used in the experiment of Anderson and Kasevich.Comment: 8 pages with 5 embeded figure
Discrete models of dislocations and their motion in cubic crystals
A discrete model describing defects in crystal lattices and having the
standard linear anisotropic elasticity as its continuum limit is proposed. The
main ingredients entering the model are the elastic stiffness constants of the
material and a dimensionless periodic function that restores the translation
invariance of the crystal and influences the Peierls stress. Explicit
expressions are given for crystals with cubic symmetry: sc, fcc and bcc.
Numerical simulations of this model with conservative or damped dynamics
illustrate static and moving edge and screw dislocations and describe their
cores and profiles. Dislocation loops and dipoles are also numerically
observed. Cracks can be created and propagated by applying a sufficient load to
a dipole formed by two edge dislocations.Comment: 23 pages, 15 figures, to appear in Phys. Rev.
Condensates beyond mean field theory: quantum backreaction as decoherence
We propose an experiment to measure the slow log(N) convergence to mean-field
theory (MFT) around a dynamical instability. Using a density matrix formalism,
we derive equations of motion which go beyond MFT and provide accurate
predictions for the quantum break-time. The leading quantum corrections appear
as decoherence of the reduced single-particle quantum state.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Let
Variability of Quasars at 10 microns
Twenty five low redshift quasars have been monitored for several decades at
five near- and mid-infrared wavelengths to detect rapid variations which would
indicate that a nonthermal component was present in the "10 micron bump". Such
variability has apparently been detected in several radio loud quasars and in
the radio quiet quasar PG1535+547. In addition, the structure function of
PG1226+023 shows that an apparently periodic component is present in its
near-infrared emission.Comment: AAS LaTex, 33 figures in 2 postscript files, AJ, accepte
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