Searcharxiv⌕ Search

arXiv subjects

W. Zhang

Publications and source records attributed to W. Zhang.

At least 217 records · Page 12Linked to original sources
arXiv2006

Calibration of the Rossi X-ray Timing Explorer Proportional Counter Array

We present the calibration and background model for the Proportional Counter Array (PCA) aboard the Rossi X-ray Timing Explorer (RXTE). The energy calibration is systematics limited below 10 keV with deviations from a power-law fit to the Crab nebula plus pulsar less than 1%. Unmodelled variations in the instrument background amount to less than 2% of the observed background below 10 keV and less than 1% between 10 and 20 keV. Individual photon arrival times are accurate to 4.4 micro-seconds at all times during the mission and to 2.5 micro-seconds after 29 April 1997. The peak pointing direction of the five collimators is known to a precision of a few arc-seconds

astro-ph↗
arXiv2005

High field x-ray diffraction study on a magnetic-field-induced valence transition in YbInCu4

We report the first high-field x-ray diffraction experiment using synchrotron x-rays and pulsed magnetic fields exceeding 30 T. Lattice deformation due to a magnetic-field-induced valence transition in YbInCu4 is studied. It has been found that the Bragg reflection profile at 32 K changes significantly at around 27 T due to the structural transition. In the vicinity of the transition field the low-field and the high-field phases are observed simultaneously as the two distinct Bragg reflection peaks: This is a direct evidence of the fact that the field-induced valence state transition is the first order phase transition. The field-dependence of the low-field-phase Bragg peak intensity is found to be scaled with the magnetization.

cond-mat.str-el↗
arXiv2005

The Island state of the Atoll Source 4U 1820--30

We study the rapid X-ray time variability in all public data available from the \textit{Rossi X-ray Timing Explorer's} Proportional Counter Array on the atoll source 4U 1820--30 in the low-luminosity island state. A total of $\sim46$ ks of data were used. We compare the frequencies of the variability components of 4U 1820--30 with those in other atolls sources. These frequencies were previously found to follow a universal scheme of correlations. We find that 4U 1820--30 shows correlations that are shifted by factors of $1.13\pm0.01$ and $1.21\pm0.02$ with respect to those in other atoll sources. These shifts are similar to, but smaller than the shift factor $\sim1.45$ previously reported for some accreting millisecond pulsars. Therefore, 4U 1820--30 is the first atoll source which shows no significant pulsations but has a significant shift in the frequency correlations compared with other 3 non-pulsating atoll sources.

astro-ph↗
arXiv2004

Radio and X-ray Observations of PSR B0540-69

PSR B0540-69 is one of a small handful of pulsars known to emit giant pulses and the only extra-galactic pulsar known to do so. We observed the pulsar for a total of 72 hr over a 6 month interval and detected 141 giant pulses. We have obtained correct phasing between the radio arrival times of the giant pulses and the X-ray pulse profile. The giant pulses occur in two phase windows, located 6.7 ms before and 5.0 ms after the midpoint of the X-ray profile. We have detected the integrated profile of the pulsar at 1.4 GHz with a flux density of only 24 micro-Jy. The statistics of the giant pulses are clearly power-law and it is likely that the giant pulses contribute only a few per cent to the integrated pulse flux density, similar to other pulsars with giant pulse emission. Simultaneous X-ray and radio observations show no significant increase in the X-ray flux at the time of the radio giants. The relative enhancements in the X-ray emission must be at least 5.5 times smaller than in the radio.

astro-ph↗
arXiv2004

Limits to phase resolution in matter wave interferometry

We study the quantum dynamics of a two-mode Bose-Einstein condensate in a time-dependent symmetric double-well potential using analytical and numerical methods. The effects of internal degrees of freedom on the visibility of interference fringes during a stage of ballistic expansion are investigated varying particle number, nonlinear interaction sign and strength as well as tunneling coupling. Expressions for the phase resolution are derived and the possible enhancement due to squeezing is discussed. In particular, the role of the superfluid - Mott insulator cross-over and its analog for attractive interactions is recognized.

quant-ph↗
arXiv2004

Magic numbers for superheavy nuclei in relativistic continuum Hartree-Bogoliubov theory

The magic proton and neutron numbers are searched in the superheavy region with proton number $Z$=100 - 140 and neutron number $N$= ($Z$+30) - (2$Z$+32) by the relativistic continuum Hartree-Bogoliubov (RCHB) theory with interactions NL1, NL3, NLSH, TM1, TW99, DD-ME1, PK1, and PK1R. Based on the two-nucleon separation energies $S_{2p}$ and $S_{2n}$, the two-nucleon gaps $δ_{2p}$ and $δ_{2n}$, the shell correction energies $E_{shell}^{p}$ and $E_{shell}^{n}$, the pairing energies $E_{pair}^{p}$ and $E_{pair}^{n}$, and the pairing gaps $Δ_{p}$ and $Δ_{n}$, $Z$=120, 132, and 138 and $N$=172, 184, 198, 228, 238, and 258 are suggested to be the magic numbers within the present approach. The $α$-decay half-lives are also discussed. In addition, the potential energy surfaces of possible doubly magic nuclei are obtained by the deformation-constrained relativistic mean field (RMF) theory, and the shell effects stabilizing the nuclei are investigated. Furthermore, the formation cross sections of $^{292}_{172}$120 and $^{304}_{184}$120 at the optimal excitation energy are estimated by a phenomenological cold fusion reactions model with the structure information extracted from the constrained RMF calculation.

nucl-th↗
arXiv2003

First order shape transition and critical point nuclei in Sm isotopes from relativistic mean field approach

The critical point nuclei in Sm isotopes, which marks the first order phase transition between spherical U(5) and axially deformed shapes SU(3), have been investigated in the microscopic quadrupole constrained relativistic mean field (RMF) model plus BCS method with all the most used interactions, i.e., NL1, NL3, NLSH and TM1. The calculated potential energy surfaces show a clear shape transition for the even-even Sm isotopes with $N = 82\sim 96$ and the critical point nuclei are found to be $^{148}$Sm, $^{150}$Sm and $^{152}$Sm. Similar conclusions can also be drawn from the microscopic neutron and proton single particle spectra.

nucl-th↗
arXiv2003

Observation of spinor dynamics in optically trapped 87Rb Bose-Einstein Condensates

We measure spin mixing of F=1 and F=2 spinor condensates of 87Rb atoms confined in an optical trap. We determine the spin mixing time to be typically less than 600 ms and observe spin population oscillations. The equilibrium spin configuration in the F=1 manifold is measured for different magnetic fields and found to show ferromagnetic behavior for low field gradients. An F=2 condensate is created by microwave excitation from F=1 manifold, and this spin-2 condensate is observed to decay exponentially with time constant 250 ms. Despite the short lifetime in the F=2 manifold, spin mixing of the condensate is observed within 50 ms.

cond-mat.soft↗
arXiv2003

The STAR Time Projection Chamber: A Unique Tool for Studying High Multiplicity Events at RHIC

The STAR Time Projection Chamber (TPC) is used to record collisions at the Relativistic Heavy Ion Collider (RHIC). The TPC is the central element in a suite of detectors that surrounds the interaction vertex. The TPC provides complete coverage around the beam-line, and provides complete tracking for charged particles within +- 1.8 units of pseudo-rapidity of the center-of-mass frame. Charged particles with momenta greater than 100 MeV/c are recorded. Multiplicities in excess of 3,000 tracks per event are routinely reconstructed in the software. The TPC measures 4 m in diameter by 4.2 m long, making it the largest TPC in the world.

nucl-ex↗
arXiv2002

Gamma-Ray Bursts and Jet-Powered Supernovae

The last five years have seen growing challenges to the traditional paradigm of a core collapse supernova powered by the neutrino emission of a young proto-neutron star. Chief among these challenges are gamma-ray bursts (GRBs) and the supernovae that seem to accompany them. Here we review some recent - and not so recent - models for GRBs and supernovae in which strong magnetic fields, rotation, or accretion into a black hole play a role. The conditions for these energetic explosions are special and, at this point, there is no compelling reason to invoke them in the general case. That is, 99% of supernovae may still operate in the traditional fashion.

astro-ph↗
arXiv2002

The Central Engines of Gamma-Ray Bursts

Leading models for the "central engine" of long, soft gamma-ray bursts (GRBs) are briefly reviewed with emphasis on the collapsar model. Growing evidence supports the hypothesis that GRBs are a supernova-like phenomenon occurring in star forming regions, differing from ordinary supernovae in that a large fraction of their energy is concentrated in highly relativistic jets. The possible progenitors and physics of such explosions are discussed and the important role of the interaction of the emerging relativistic jet with the collapsing star is emphasized. This interaction may be responsible for most of the time structure seen in long, soft GRBs. What we have called "GRBs" may actually be a diverse set of phenomena with a key parameter being the angle at which the burst is observed. GRB 980425/SN 1988bw and the recently discovered hard x-ray flashes may be examples of this diversity.

astro-ph↗
arXiv2002

Input-output theory for fermions in an atom cavity

We generalize the quantum optical input-output theory developed for optical cavities to ultracold fermionic atoms confined in a trapping potential, which forms an "atom cavity". In order to account for the Pauli exclusion principle, quantum Langevin equations for all cavity modes are derived. The dissipative part of these multi-mode Langevin equations includes a coupling between cavity modes. We also derive a set of boundary conditions for the Fermi field that relate the output fields to the input fields and the field radiated by the cavity. Starting from a constant uniform current of fermions incident on one side of the cavity, we use the boundary conditions to calculate the occupation numbers and current density for the fermions that are reflected and transmitted by the cavity.

cond-mat.mes-hall↗
arXiv2002

Raman coupler for a trapped two-component quantum-degenerate Fermi gas

We investigate theoretically the Raman coupling between two internal states of a trapped low-density quantum-degenerate Fermi gas. In general, the trap frequencies associated with the two internal states can be different, leading to the onset of collapses and revivals in the population difference of the two internal states. This behavior can be changed drastically by two-body collisions. In particular, we show that under appropriate conditions they can suppress the dephasing leading to the collapse of the population difference, and restore almost full Rabi oscillations between the two internal states. These results are compared and contrasted to those for a quantum-degenerate bosonic gas.

cond-mat.soft↗
arXiv2002

Low and high frequency variability as a function of spectral properties in the bright X-ray binary GX 5-1

We report on a detailed analysis of data obtained over nearly four years with the Rossi X-ray Timing Explorer of the Z source GX 5-1. From a spectral analysis using a hardness-intensity diagram it was found that the source traced out the typical Z-shaped pattern. The study of the power spectral properties showed that when the source moved on the Horizontal Branch towards the Normal Branch the fractional rms amplitudes and timescales of all variability decreased, while their FWHMs increased. The frequency separation of the two kHz QPO peaks decreased from 344+-12 Hz to 232+-13 Hz. At low frequencies, besides the horizontal branch oscillation (HBO) and its second harmonic, two additional broad Lorentzian components were needed to obtain acceptable fits. These broad Lorentzians have Q-values of ~1-2 and have frequencies 0.5 and 1.5 times the HBO frequency. When interpreted as being related to the HBO, they seem to favor disc models for the HBO over the magnetic beat-frequency model. We compare the timing properties and colour-colour diagrams (CDs) of GX 5-1 with those of the back hole candidate XTE J1550-564 and the atoll source 4U 1608-52. The CDs are strikingly similar when a colour scheme is used that is commonly employed in back hole studies. However, this may be a degeneracy as the CDs turn out to be more complicated when colours common in neutron star studies are employed. Apart from some remarkable similarities between the CD of XTE J1550-564 and that of 4U 1608-52, several differences can be seen between these CDs and that of GX 5-1. Conclusions on spectral states or properties based solely on the use of CDs using the ``black hole scheme'' should be regarded with caution.

astro-ph↗
arXiv2001

Quasiparticle spectrum and dynamical stability of an atomic Bose-Einstein condensate coupled to a degenerate Fermi gas

The quasiparticle excitations and dynamical stability of an atomic Bose-Einstein condensate coupled to a quantum degenerate Fermi gas of atoms at zero temperature is studied. The Fermi gas is assumed to be either in the normal state or to have undergone a phase transition to a superfluid state by forming Cooper pairs. The quasiparticle excitations of the Bose-Einstein condensate exhibit a dynamical instability due to a resonant exchange of energy and momentum with quasiparticle excitations of the Fermi gas. The stability regime for the bosons depends on whether the Fermi gas is in the normal state or in the superfluid state. We show that the energy gap in the quasiparticle spectrum for the superfluid state stabilizes the low energy energy excitations of the condensate. In the stable regime, we calculate the boson quasiparticle spectrum, which is modified by the fluctuations in the density of the Fermi gas.

cond-mat.soft↗
arXiv2001

Hiccups in the night: X-ray monitoring of the two Crab-like LMC pulsars

We are undertaking an extensive X-ray monitoring campaign of the two Crab-like pulsars in the Large Magellanic Clouds, PSR B0540-69 and PSR J0537-6910. We present our current phase-connected timing analysis derived from a set of 50 pointed X-ray observations spanning several years. From our initial 1.2 yr monitoring program of the young 50 ms pulsar PSR B0540-69, we find the first compelling evidence for a glitch in its rotation. This glitch is characterized by dv / v = 1.90 +/- 0.05 X 10E-9 and $dvdot/ vdot = (8.5 +/- 0.5 X 10E-5. Taking into account the glitch event, we derive a braking index of n = 1.81 +/- 0.07, significantly lower than previous reported. For the 16 ms pulsar, PSR J0537-6910, we recorded 6 large glitch events during a period of nearly 3 years, the highest rate of all known Crab-like systems. Despite the extreme timing activity, the long term spin-down of this pulsar continues to average -1.9743 X 10E-10 Hz/s.

astro-ph↗
arXiv2001

Manipulating the critical temperature for the superfluid phase transition in trapped atomic Fermi gases

We examine the effect of the trapping potential on the critical temperature, $T_C$, for the BCS transition to a superfluid state in trapped atomic gases of fermions. $T_C$ for an arbitrary power law trap is calculated in the Thomas-Fermi approximation. For anharmonic traps, $T_C$ can be increased by several orders of magnitude in comparison to a harmonic trap. Our theoretical results indicate that, in practice, one could manipulate the critical temperature for the BCS phase transition by shaping the traps confining the atomic Fermi gases.

cond-mat.supr-con↗