147,361 research outputs found
Accretion column eclipses in the X-ray pulsars GX 1+4 and RX J0812.4-3114
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
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 -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
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
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
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 -component, , for the wave-packet case is quite different from the test
point-particle case ( 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 for a point-particle probe. This result can be quite significant
for further investigations on the measurement of vacuum fluctuations.Comment: 8 page
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Experimental and Numerical Investigation on Progressive Collapse Resistance of Post-tensioned Precast Concrete Beam-Column Sub-assemblages
In this paper, four 1/2 scaled precast concrete (PC) beam-column sub-assemblages with high performance connection were tested under push-down loading procedure to study the load resisting mechanism of PC frames subjected to different column removal scenarios. The parameters investigated include the location of column removal and effective prestress in tendons. The test results indicated that the failure modes of unbonded post-tensioned precast concrete (PTPC) frames were different from that of reinforced concrete (RC) frames: no cracks formed in the beams and wide opening formed near the beam to column interfaces. For specimens without overhanging beams, the failure of side column was eccentric compression failure. Moreover, the load resisting mechanisms in PC frames were significantly different from that of RC frames: the compressive arch action (CAA) developed in concrete during column removal was mainly due to actively applied pre-compressive stress in the concrete; CAA will not vanish when severe crush in concrete occurred. Thus, it may provide negative contribution for load resistance when the displacement exceeds one-beam depth; the tensile force developed in the tendons could provide catenary action from the beginning of the test. Moreover, to deeper understand the behavior of tested specimens, numerical analyses were carried out. The effects of concrete strength, axial compression ratio at side columns, and loading approaches on the behavior of the sub-assemblages were also investigated based on validated numerical analysis
Molecular line and continuum study of the W40 cloud
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: pc, and cm, 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 NH, NH, and
HCO 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: cm. 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
We have studied the effect of nonequilibrium longitudinal optical phonons on
hot-electron spin relaxation in -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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