147,361 research outputs found

    Accretion column eclipses in the X-ray pulsars GX 1+4 and RX J0812.4-3114

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    Sharp dips observed in the pulse profiles of three X-ray pulsars (GX 1+4, RX J0812.4-3114 and A 0535+26) have previously been suggested to arise from partial eclipses of the emission region by the accretion column occurring once each rotation period. We present pulse-phase spectroscopy from Rossi X-ray Timing Explorer satellite observations of GX 1+4 and RX J0812.4-3114 which for the first time confirms this interpretation. The dip phase corresponds to the closest approach of the column axis to the line of sight, and the additional optical depth for photons escaping from the column in this direction gives rise to both the decrease in flux and increase in the fitted optical depth measured at this phase. Analysis of the arrival time of individual dips in GX~1+4 provides the first measurement of azimuthal wandering of a neutron star accretion column. The column longitude varies stochastically with standard deviation 2-6 degrees depending on the source luminosity. Measurements of the phase width of the dip both from mean pulse profiles and individual eclipses demonstrates that the dip width is proportional to the flux. The variation is consistent with that expected if the azimuthal extent of the accretion column depends only upon the Keplerian velocity at the inner disc radius, which varies as a consequence of the accretion rate Mdot.Comment: 7 pages, 5 figures, accepted by MNRAS. Included reference

    Determination of the internal structure of neutron stars from gravitational wave spectra

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    In this paper the internal structure of a neutron star is shown to be inferrable from its gravitational-wave spectrum. Iteratively applying the inverse scheme of the scaled coordinate logarithmic perturbation method for neutron stars proposed by Tsui and Leung [Astrophys. J. {\bf 631}, 495 (2005)], we are able to determine the mass, the radius and the mass distribution of a star from its quasi-normal mode frequencies of stellar pulsation. In addition, accurate equation of state of nuclear matter can be obtained from such inversion scheme. Explicit formulas for the case of axial ww-mode oscillation are derived here and numerical results for neutron stars characterized by different equations of state are shown.Comment: 26 pages, 14 figures, submitted to Physical Review

    Final Report: Wall Effects in Cavity Flows

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    The wall effects in cavity flows past an arbitrary two-dimensional body is investigated for both pure-drag and lifting cases based on an inviscid nonlinear flow theory. The over-all features of various theoretical flow models for inviscid cavity flows under the wall effects are discussed from the general momentum consideration in comparison with typical viscous, incompressible wake flows in a channel. In the case of pure drag cavity flows, three theoretical models in common use, namely, the open-wake, Riabouchinsky and re-entrant jet models, are applied to evaluate the solution. Methods of numerical computation are discussed for bodies of arbitrary shape, and are carried out in detail for wedges of all angles. The final numerical results are compared between the different flow models, and the differences pointed out. Further analysis of the results has led to development of several useful formulas for correcting the wall effect. In the lifting flow case, the wall effect on the pressure and hydrodynamic forces acting on arbitrary body is formulated for the choked cavity flow in a closed water tunnel of arbitrary shape, and computed for the flat plate with a finite cavity in a straight tunnel

    Cultural Influnces on Risk Tolerance and Portfolio Creation

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    We extend existing research that examines the impact of culture on risk tolerance. Using surveys completed by Chinese and American students, we find, consistent with previous studies, that Chinese students perceive themselves as more risk tolerant. However, we find that Chinese students are less consistent in matching their perceived tolerance levels with actual scores from a standard risk tolerance assessment. Further, we also examine mock portfolios created by the respondents and find no evidence that Chinese students create portfolios that are riskier than their American counterparts. Our findings suggest that differences in risk tolerance are at least partially a product of culture, but such differences may not always translate into actual investment decisions

    Smearing effect due to the spread of a probe-particle on the Brownian motion near a perfectly reflecting boundary

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    Quantum fluctuations of electromagnetic vacuum are investigated in a half-space bounded by a perfectly reflecting plate by introducing a probe described by a charged wave-packet distribution in time-direction. The wave-packet distribution of the probe enables one to investigate the smearing effect upon the measured vacuum fluctuations caused by the quantum nature of the probe particle. It is shown that the wave-packet spread of the probe particle significantly influences the measured velocity dispersion of the probe. In particular, the asymptotic late-time behavior of its zz-component, , for the wave-packet case is quite different from the test point-particle case (zz is the coordinate normal to the plate). The result for the wave-packet is \sim 1/\t^2 in the late time (\t is the measuring time), in stead of the reported late-time behavior ∼1/z2 \sim 1/z^2 for a point-particle probe. This result can be quite significant for further investigations on the measurement of vacuum fluctuations.Comment: 8 page

    Molecular line and continuum study of the W40 cloud

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    The dense cloud associated with W40, one of the nearby H II regions, has been studied in millimeter-wave molecular lines and in 1.2 mm continuum. Besides, 1280 MHz and 610 MHz interferometric observations have been done. The cloud has complex morphological and kinematical structure, including a clumpy dust ring and an extended dense core. The ring is probably formed by the "collect and collapse" process due to the expansion of neighboring H II region. Nine dust clumps in the ring have been deconvolved. Their sizes, masses and peak hydrogen column densities are: ∼0.02−0.11\sim 0.02-0.11 pc, ∼0.4−8.1M⊙\sim 0.4-8.1 M_{\odot} and ∼(2.5−11)×1022\sim (2.5-11)\times 10^{22} cm−2^{-2}, respectively. Molecular lines are observed at two different velocities and have different spatial distributions implying strong chemical differentiation over the region. The CS abundance is enhanced towards the eastern dust clump 2, while the NH3_3, N2_2H+^+, and H13^{13}CO+^+ abundances are enhanced towards the western clumps. HCN and HCO+^+ do not correlate with the dust probably tracing the surrounding gas. Number densities derived towards selected positions are: ∼(0.3−3.2)×106\sim (0.3-3.2)\times 10^6 cm−3^{-3}. Two western clumps have kinetic temperatures 21 K and 16 K and are close to virial equilibrium. The eastern clumps 2 and 3 are more massive, have higher extent of turbulence and are probably more evolved than the western ones. They show asymmetric CS(2--1) line profiles due to infalling motions which is confirmed by model calculations. An interaction between ionized and neutral material is taking place in the vicinity of the eastern branch of the ring and probably trigger star formation.Comment: 16 pages, 6 figure

    Effect of nonequilibrium phonons on hot-electron spin relaxation in n-type GaAs quantum wells

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    We have studied the effect of nonequilibrium longitudinal optical phonons on hot-electron spin relaxation in nn-type GaAs quantum wells. The longitudinal optical phonons, due to the finite relaxation rate, are driven to nonequilibrium states by electrons under an in-plane electric field. The nonequilibrium phonons then in turn influence the electron spin relaxation properties via modifying the electron heating and drifting. The spin relaxation time is elongated due to the enhanced electron heating and thus the electron-phonon scattering in the presence of nonequilibrium phonons. The frequency of spin precession, which is roughly proportional to the electron drift velocity, can be either increased (at low electric field and/or high lattice temperature) or decreased (at high electric field and/or low lattice temperature). The nonequilibrium phonon effect is more pronounced when the electron density is high and the impurity density is low.Comment: 6 pages, 3 figure
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