Detection of an X-ray Pulsar Wind Nebula and Tail in SNR N157B

We report Chandra X-ray observations of the supernova remnant N157B in the Large Magellanic Cloud, which are presented together with an archival HST optical image and a radio continuum map for comparison. This remnant contains the recently discovered 16 ms X-ray pulsar PSR J0537-6910, the most rapidly rotating young pulsar known. Using phase-resolved Chandra imaging, we pinpoint the location of the pulsar to within an uncertainty of less than 1 arcsec. PSR J0537-6910 is not detected in any other wavelength band. The X-ray observations resolve three distinct features: the pulsar itself, a surrounding compact wind nebula which is strongly elongated and a feature of large-scale diffuse emission trailing from the pulsar. This latter comet tail-shaped feature coexists with enhanced radio emission and is oriented nearly perpendicular to the major axis of the pulsar wind nebula. We propose the following scenario to explain these features. The bright, compact nebula is likely powered by a toroidal pulsar wind of relativistic particles which is partially confined by the ram-pressure from the supersonic motion of the pulsar. The particles, after being forced out from the compact nebula (the head of the ``comet''), are eventually dumped into a bubble (the tail), which is primarily responsible for the extended diffuse X-ray and radio emission. The ram-pressure confinement also allows a natural explanation for the observed X-ray luminosity of the compact nebula and for the unusually small X-ray to spin-down luminosity ratio, compared to similarly energetic pulsars. We estimate the pulsar wind Lorentz factor of N157B as about 4 times 10^6 (with an uncertainty of a factor about 2, consistent with that inferred from the modeling of the Crab Nebula.Comment: 15 pages plus 4 figures. The postscript file of the whole paper is available at http://xray.astro.umass.edu/wqd/papers/n157b/n157b.ps. accepted for publication in Ap

Pressure-Temperature Phase Diagram of Multiferroic Ni3V2O8Ni_3V_2O_8

The pressure-temperature phase diagram of multiferroic $Ni_3V_2O_8$ is investigated for hydrostatic pressures up to 2 GPa. The stability range of the ferroelectric phase associated with the incommensurate helical spin order is reduced by pressure and ferroelectricity is completely suppressed at the critical pressure of 1.64 GPa at 6.2 K. Thermal expansion measurements at ambient pressure show strong step-like anomalies of the lattice parameters associated with the lock-in transition into the commensurate paraelectric phase. The expansion anomalies are highly anisotropic, the related volume change is consistent with the high-pressure phase diagram

The Suppression and Recovery of the Ferroelectric Phase in Multiferroic MnWO4MnWO_4

We report the discovery of a complete suppression of ferroelectricity in $MnWO_4$ by 10 % iron substitution and its restoration in external magnetic fields. The spontaneous polarization in $Mn_{0.9}Fe_{0.1}WO_4$ arises below 12 K in external fields above 4 T. The magnetic/ferroelectric phase diagram is constructed from the anomalies of the dielectric constant, polarization, magnetization, and heat capacity. The observations are qualitatively described by a mean field model with competing interactions and strong anisotropy. We propose that the magnetic field induces a non-collinear inversion symmetry breaking magnetic structure in $Mn_{0.9}Fe_{0.1}WO_4$

Knowledge discovery for friction stir welding via data driven approaches: Part 2 – multiobjective modelling using fuzzy rule based systems

In this final part of this extensive study, a new systematic data-driven fuzzy modelling approach has been developed, taking into account both the modelling accuracy and its interpretability (transparency) as attributes. For the first time, a data-driven modelling framework has been proposed designed and implemented in order to model the intricate FSW behaviours relating to AA5083 aluminium alloy, consisting of the grain size, mechanical properties, as well as internal process properties. As a result, ‘Pareto-optimal’ predictive models have been successfully elicited which, through validations on real data for the aluminium alloy AA5083, have been shown to be accurate, transparent and generic despite the conservative number of data points used for model training and testing. Compared with analytically based methods, the proposed data-driven modelling approach provides a more effective way to construct prediction models for FSW when there is an apparent lack of fundamental process knowledge

Pressure dependence of the superconducting transition and electron correlations in Na_xCoO_2 \cdot 1.3H_2O

We report T_c and ^{59}Co nuclear quadrupole resonance (NQR) measurements on the cobalt oxide superconductor Na_{x}CoO_{2}\cdot 1.3H_{2}O (T_c=4.8 K) under hydrostatic pressure (P) up to 2.36 GPa. T_c decreases with increasing pressure at an average rate of -0.49\pm0.09 K/GPa. At low pressures P\leq0.49 GPa, the decrease of T_c is accompanied by a weakening of the spin correlations at a finite wave vector and a reduction of the density of states (DOS) at the Fermi level. At high pressures above 1.93 GPa, however, the decrease of T_c is mainly due to a reduction of the DOS. These results indicate that the electronic/magnetic state of Co is primarily responsible for the superconductivity. The spin-lattice relaxation rate 1/T_1 at P=0.49 GPa shows a T^3 variation below T_c down to T\sim 0.12T_c, which provides compelling evidence for the presence of line nodes in the superconducting gap function.Comment: published on 19, Sept. 2007 on Phys. Rev.

Low temperature dielectric anomalies in HoMnO_3: The complex phase diagram

The dielectric constant of multiferroic hexagonal HoMnO_3 exhibits an unprecedented diversity of anomalies at low temperatures (1.8 K< T <10 K) and under external magnetic fields related to magnetic phase transitions in the coupled system of Ho moments, Mn spins, and ferroelectric polarization. The derived phase diagram is far more complex than previously assumed including reentrant phases, phase transitions with distinct thermal and field hysteresis, as well as several multicritical points. Magnetoelastic interactions introduce lattice anomalies at the magnetic phase transitions. The re-evaluation of the T-H phase diagram of HoMnO_3 is demanded.Comment: 12 pages, 3 figure

Strong spin-lattice coupling in multiferroic HoMnO3_{3}: Thermal expansion anomalies and pressure effect

Evidence for a strong spin-lattice coupling in multiferroic HoMnO_3 is derived from thermal expansion measurements along a- and c-axis. The magnetoelastic effect results in sizable anomalies of the thermal expansivities at the antiferromagnetic (T_N) and the spin rotation (T_{SR}) transition temperatures as well as in a negative c-axis expansivity below room temperature. The coupling between magnetic orders and dielectric properties below T_N is explained by the lattice strain induced by the magnetoelastic effect. At T_{SR} various physical quantities show discontinuities that are thermodynamically consistent with a first order phase transition

A general condition of inflationary cosmology on trans-Planckian physics

We consider a more general initial condition satisfying the minimal uncertainty relationship. We calculate the power spectrum of a simple model in inflationary cosmology. The results depend on perturbations generated below a fundamental scale, e.g. the Planck scale.Comment: 7 pages, References adde

Analytical solution of the dynamical spherical MIT bag

We prove that when the bag surface is allowed to move radially, the equations of motion derived from the MIT bag Lagrangian with massless quarks and a spherical boundary admit only one solution, which corresponds to a bag expanding at the speed of light. This result implies that some new physics ingredients, such as coupling to meson fields, are needed to make the dynamical bag a consistent model of hadrons.Comment: Revtex, no figures. Submitted to Journal of Physics