arXiv subjects

L. Wen

Publications and source records attributed to L. Wen.

26 records · Page 2Linked to original sources

arXiv2012

Controlling phase separation of a two-component Bose-Einstein condensate by confinement

We point out that the widely accepted condition g11g22<g122 for phase separation of a two-component Bose-Einstein condensate is insufficient if kinetic energy is taken into account, which competes against the intercomponent interaction and favors phase mixing. Here g11, g22, and g12 are the intra- and intercomponent interaction strengths, respectively. Taking a d-dimensional infinitely deep square well potential of width L as an example, a simple scaling analysis shows that if d=1 (d=3), phase separation will be suppressed as L\rightarrow0 (L\rightarrow\infty) whether the condition g11g22<g122 is satisfied or not. In the intermediate case of d=2, the width L is irrelevant but again phase separation can be partially or even completely suppressed even if g11g22<g122. Moreover, the miscibility-immiscibility transition is turned from a first-order one into a second-order one by the kinetic energy. All these results carry over to d-dimensional harmonic potentials, where the harmonic oscillator length ξho plays the role of L. Our finding provides a scenario of controlling the miscibility-immiscibility transition of a two-component condensate by changing the confinement, instead of the conventional approach of changing the values of the g's.

cond-mat.quant-gas↗

arXiv2010

The Commensal Real-time ASKAP Fast Transients (CRAFT) survey

We are developing a purely commensal survey experiment for fast (<5s) transient radio sources. Short-timescale transients are associated with the most energetic and brightest single events in the Universe. Our objective is to cover the enormous volume of transients parameter space made available by ASKAP, with an unprecedented combination of sensitivity and field of view. Fast timescale transients open new vistas on the physics of high brightness temperature emission, extreme states of matter and the physics of strong gravitational fields. In addition, the detection of extragalactic objects affords us an entirely new and extremely sensitive probe on the huge reservoir of baryons present in the IGM. We outline here our approach to the considerable challenge involved in detecting fast transients, particularly the development of hardware fast enough to dedisperse and search the ASKAP data stream at or near real-time rates. Through CRAFT, ASKAP will provide the testbed of many of the key technologies and survey modes proposed for high time resolution science with the SKA.

astro-ph.HE↗

arXiv2008

Freely-suspended matrix-free metamaterials showing THz left-handed pass bands

Featuring dense spatial distributions of engineered metallic particles, electromagnetic metamaterials exhibit simultaneously negative values of both, dielectric permittivity and magnetic permeability, within a resonance frequency band called left-handed passband. Unusual electromagnetic properties are found resulting in promising applications such as sub-wavelength resolution imaging. State-of-the-art micro/nanomanufacturing has led to resonance frequencies reaching the visible red. The common embedding of the metal particles in plastic matrices or deposition on dielectric substrates within a small area severely limits the usefulness of the materials. Here, we use UV or X-ray lithography to build comparably large areas and quantities of the first freely-suspended matrix-free metamaterials in which the metallic structures are S-string-like with their ends held by a window-frame. In vacuo spectral characterization combined with simulation reveals left-handed passbands from 1.6 to 2.2 THz. Owing to their size, the devices can be easily handled. They offer a straightforward way of making them tunable and two-dimensionally isotropic.

cond-mat.mtrl-sci↗

arXiv2007

The superorbital variability and triple nature of the X-ray source 4U 1820-303

We perform a comprehensive analysis of the superorbital modulation in the ultracompact X-ray source 4U 1820-303, consisting of a white dwarf accreting onto a neutron star. Based on RXTE data, we measure the fractional amplitude of the source superorbital variability (with a 170-d quasi-period) in the folded and averaged light curves, and find it to be by a factor of about 2. As proposed before, the superorbital variability can be explained by oscillations of the binary eccentricity. We now present detailed calculations of the eccentricity-dependent flow through the inner Lagrangian point, and find a maximum of the eccentricity of about 0.004 is sufficient to explain the observed fractional amplitude. We then study hierarchical triple models yielding the required quasi-periodic eccentricity oscillations through the Kozai process. We find the resulting theoretical light curves to match well the observed ones. We constrain the ratio of the semimajor axes of the outer and inner systems, the component masses, and the inclination angle between the inner and outer orbits. Last but not least, we discover a remarkable and puzzling synchronization between the observed period of the superorbital variability (equal to the period of the eccentricity oscillations in our model) and the period of the general-relativistic periastron precession of the binary.

astro-ph↗

arXiv2007

Dependence of the orbital modulation of X-rays from 4U 1820-303 on the accretion rate

We report the discovery, using RXTE data, of a dependence of the X-ray orbital modulation depth on the X-ray spectral state in the ultracompact atoll binary \source. This state (measured by us by the position on the X-ray colour-colour diagram) is tightly coupled to the accretion rate, which, in turn, is coupled to the phase of the 170-d superorbital cycle of this source. The modulation depth is much stronger in the high-luminosity, so-called banana, state than in the low-luminosity, island, state. We find the X-ray modulation is independent of energy, which rules out bound-free X-ray absorption in an optically thin medium as the cause of the modulation. We also find a significant dependence of the offset phase of the orbital modulation on the spectral state, which favours the model in which the modulation is caused by scattering in hot gas around a bulge at the disc edge, which both size and the position vary with the accretion rate. Estimates of the source inclination appear to rule out a model in which the bulge itself occults a part of an accretion disc corona. We calculate the average flux of this source over the course of its superorbital variability (which has the period of 170 d), and find it to be fully compatible with the model of accretion due to the angular momentum loss via emission of gravitational radiation. Also, we compare the dates of all X-ray bursts observed from this source by BeppoSAX and RXTE with the RXTE light curve, and find all of them to coincide with deep minima of the flux, confirming previous results based on smaller samples.

astro-ph↗

arXiv1999

Localizations of Thirteen Gamma-ray Bursts by the All-Sky Monitor on RXTE

The All-Sky Monitor (ASM) on the Rossi X-ray Timing Explorer (RXTE) has been used to localize thirteen confirmed X-ray counterparts to Gamma-Ray Bursts (GRBs) detected over three years of operation. We quantify the errors in ASM localizations of brief transient sources by using observations of persistent sources with well-known locations. We apply the results of this analysis to obtain accurate error boxes with reliable confidence levels for the thirteen GRBs. In six of these thirteen cases, multiple detections by the ASM allow the positions to be localized to a diamond of order ~15' x 3'. In five further cases, the Interplanetary Network (IPN) constrains the usually ~3 deg. x 3' (full-width) ASM error box to an area of a few tens of square arcminutes. This work adds eleven burst localizations to the list of ~60 well-localized GRBs.

astro-ph↗

arXiv1997

A Shock-Patching Code for Ultra-Relativistic Fluid Flows

We have developed a one-dimensional code to solve ultra-relativistic hydrodynamic problems, using the Glimm method for an accurate treatment of shocks and contact discontinuities. The implementation of the Glimm method is based on an exact Riemann solver and van der Corput sampling sequence. In order to improve computational efficiency, the Glimm method is replaced by a finite differencing scheme in those regions where the fluid flow is sufficiently smooth. The accuracy and convergence of this hybrid method is investigated in tests involving planar, cylindrically and spherically symmetric flows that exhibit strong shocks and Lorentz factors of up to ~2000. This hybrid code has proven to be successful in simulating the interaction between a thin, ultra-relativistic, spherical shell and a low density stationary medium, a situation likely to appear in Gamma-Ray Bursts, supernovae explosions, pulsar winds and AGNs.

astro-ph↗

arXiv1997

Impact of Relativistic Fireballs on External Matter : Numerical Models of Cosmological Gamma-Ray Bursts

We numerically model the interaction between an expanding fireball and a stationary external medium whose density is either homogeneous or varies with distance as a power-law. The evolution is followed until most of the fireball kinetic energy is converted into internal energy. The density, pressure and flow Lorentz factor profiles are shown at different stages, including shock and rarefaction wave reflections, for a fireball of initial bulk Lorentz factor Gamma = 100, both in the adiabatic and non-adiabatic (radiative) regimes. For cooling times shorter than the dynamic time, bolometric light-curves are computed for values of Gamma = 50, 100 and 200. We compare the numerical light-curves with analytic results, and find that for a homogeneous external medium there is a simple scaling relationship among light-curves obtained for different parameters. The light-curves for power-law external densities are similar in shape to those in the homogeneous case. We discuss the implications of a comparison of the results with observed Gamma-Ray Burst time histories.

astro-ph↗