54,528 research outputs found
Recent X-ray measurements of the accretion-powered pulsar 4U 1907+09
X-ray observations of the accreting X-ray pulsar 4U~1907+09, obtained during
February 1996 with the Proportional Counter Array on the Rossi X-ray Timing
Experiment (RXTE), have enabled the first measurement of the intrinsic pulse
period Ppulse since 1984: Ppulse=440.341[+0.012,-0.017] s. 4U 1907+09 is in a
binary system with a blue supergiant. The orbital parameters were solved and
this enabled the correction for orbital delay effects of a measurement of
Ppulse obtained in 1990 with Ginga. Thus, three spin down rates could be
extracted from four pulse periods obtained in 1983, 1984, 1990, and 1996. These
are within 8% equal to a value of dPpulse/dt=+0.225 s/yr. This suggest that the
pulsar is perhaps in a monotonous spin down mode since its discovery in 1983.
Furthermore, the RXTE observations show transient ~18 s oscillations during a
flare that lasted about 1 hour. The oscillations may be interpreted as
Keplerian motion of an accretion disk near the magnetospheric radius. This, and
the notion that the co-rotation radius is much larger than any conceivable
value for the magnetospheric radius (because of the long spin period), renders
it unlikely that this pulsar spins near equilibrium like is suspected for other
slowing accreting X-ray pulsars. We suggest as an alternative that perhaps the
frequent occurrence of a retrograde transient accretion disk may be
consistently slowing the pulsar down. Further observations of flares can
provide more evidence of this.Comment: 26 pages, 11 figures, to be published in Astrophysical Journal part I
on March 20, 199
Correlation between the residual resistance ratio and magnetoresistance in MgB2
The resistivity and magnetoresistance in the normal state for bulk and
thin-film MgB2 with different nominal compositions have been studied
systematically. These samples show different temperature dependences of normal
state resistivity and residual resistance ratios although their superconducting
transition temperatures are nearly the same, except for the thin-film sample.
The correlation between the residual resistance ratio (RRR) and the power law
dependence of the low temperature resistivity, rho vs. T^c, indicates that the
electron-phonon interaction is important. It is found that the
magnetoresistance (MR) in the normal state scales well with the RRR, a0(MR)
proportional to (RRR)^2.2 +/- 0.1 at 50 K. This accounts for the large
difference in magnetoresistance reported by various groups, due to different
defect scatterings in the samples.Comment: 10 pages, 3 figures, submitted to Phys. Rev. B (July 6, 2001; revised
September 27, 2001); discussion of the need for excess Mg in processing and
of the power law dependence of the low temperature resistivity added in
response to referee's comment
Spectral Properties of Three Dimensional Layered Quantum Hall Systems
We investigate the spectral statistics of a network model for a three
dimensional layered quantum Hall system numerically. The scaling of the
quantity is used to determine the critical exponent for
several interlayer coupling strengths. Furthermore, we determine the level
spacing distribution as well as the spectral compressibility at
criticality. We show that the tail of decays as with
and also numerically verify the equation
, where is the correlation dimension and the
spatial dimension.Comment: 4 pages, 5 figures submitted to J. Phys. Soc. Jp
Black Holes and Photons with Entropic Force
We study entropic force effects on black holes and photons. We find that
application of an entropic analysis restricts the radial change of a
black hole of radius , due to a test particle of a
Schwartzchild radius moving towards the black hole by near
black body surface, to be given by a relation , or {\Delta R}/{\lambdabar_M} = {\Delta x}/{2 \lambdabar_m}. We
suggest a new rule regarding entropy changes in different dimensions, \Delta
S= 2\pi k D \Delta l /\lambdabar, which unifies Verlinde's conjecture and the
black hole entropy formula. We also propose to extend the entropic force idea
to massless particles such as a photon. We find that there is an entropic force
on a photon of energy , with , and therefore
the photon has an effective gravitational mass .Comment: 4 Latex pages, no figure
Depinning of three-dimensional drops from wettability defects
Substrate defects crucially influence the onset of sliding drop motion under
lateral driving. A finite force is necessary to overcome the pinning influence
even of microscale heterogeneities. The depinning dynamics of three-dimensional
drops is studied for hydrophilic and hydrophobic wettability defects using a
long-wave evolution equation for the film thickness profile. It is found that
the nature of the depinning transition explains the experimentally observed
stick-slip motion.Comment: 6 pages, 9 figures, submitted to ep
Antiferromagnetic Order in MnO Spherical Nanoparticles
We have performed unpolarized and polarized neutron diffraction experiments
on monodisperse 8 nm and 13 nm antiferromagnetic MnO nanoparticles. For the 8
nm sample, the antiferromagnetic transition temperature (114 K) is
suppressed compared to the bulk material (119 K) while for the 13 nm sample
(120 K) is comparable to the bulk. The neutron diffraction data of the
nanoparticles is well described using the bulk MnO magnetic structure but with
a substantially reduced average magnetic moment of 4.20.3 /Mn for
the 8 nm sample and 3.90.2 /Mn for the 13 nm sample. An analysis of
the polarized neutron data on both samples shows that in an individual MnO
nanoparticle about 80 of Mn ions order. These results can be explained by a
structure in which the monodisperse nanoparticles studied here have a core that
behaves similar to the bulk with a surface layer which does not contribute
significantly to the magnetic order.Comment: 7 pages, 5 figure
Analyzing X-Ray Pulsar Profiles: Geometry and Beam Pattern of Her X-1
We report on our analysis of a large sample of energy dependent pulse
profiles of the X-ray binary pulsar Hercules X-1. We find that all data are
compatible with the assumption of a slightly distorted magnetic dipole field as
sole cause of the asymmetry of the observed pulse profiles. Further the
analysis provides evidence that the emission from both poles is equal. We
determine an angle of 20 deg between the rotation axis and the local magnetic
axis. One pole has an offset of 5 deg from the antipodal position of the other
pole. The beam pattern shows structures that can be interpreted as pencil- and
fan-beam configurations. Since no assumptions on the polar emission are made,
the results can be compared with various emission models. A comparison of
results obtained from pulse profiles of different phases of the 35-day cycle
indicates different attenuation of the radiation from the poles being
responsible for the change of the pulse shape during the main-on state. These
results also suggest the resolution of an ambiguity within a previous analysis
of pulse profiles of Cen X-3, leading to a unique result for the beam pattern
of this pulsar as well. The analysis of pulse profiles of the short-on state
indicates that a large fraction of the radiation cannot be attributed to the
direct emission from the poles. We give a consistent explanation of both the
evolution of the pulse profile and the spectral changes with the 35-day cycle
in terms of a warped precessing accretion disk.Comment: 24 pages, 12 figures. To appear in ApJ 529 #2, 1 Feb 200
Hydrodynamic afterburner for the CGC at RHIC
Firstly, we give a short review about the hydrodynamic model and its
application to the elliptic flow phenomena in relativistic heavy ion
collisions. Secondly, we show the first approach to construct a unified model
for the description of the dynamics in relativistic heavy ion collisions.Comment: 15 pages, 7 figures, invited talk presented at "Hot Quarks 2004",
July 18-24, 2004, Taos Valley, NM, US
X-ray scattering study of two length scales in the critical fluctuations of CuGeO3
The critical fluctuations of CuGeO have been measured by synchrotron
x-ray scattering, and two length scales are clearly observed. The ratio between
the two length scales is found to be significantly different along the
axis, with the axis along the surface normal direction. We believe that
such a directional preference is a clear sign that surface random strains,
especially those caused by dislocations, are the origin of the long length
scale fluctuations.Comment: 5 pages, 4 figures, submitted to PR
Heavy-to-light baryonic form factors at large recoil
We analyze heavy-to-light baryonic form factors at large recoil and derive
the scaling behavior of these form factors in the heavy quark limit. It is
shown that only one universal form factor is needed to parameterize Lambda_b to
p and Lambda_b to Lambda matrix elements in the large recoil limit of light
baryons, while hadronic matrix elements of Lambda_b to Sigma transition vanish
in the large energy limit of Sigma baryon due to the space-time parity
symmetry. The scaling law of the soft form factor eta(P^{\prime} \cdot v),
P^{\prime} and v being the momentum of nucleon and the velocity of Lambda_b
baryon, responsible for Lambda_b to p transitions is also derived using the
nucleon distribution amplitudes in leading conformal spin. In particular, we
verify that this scaling behavior is in full agreement with that from
light-cone sum rule approach in the heavy-quark limit. With these form factors,
we further investigate the Lambda baryon polarization asymmetry alpha in
Lambda_b to Lambda gamma and the forward-backward asymmetry A_{FB} in Lambda_b
to Lambda l^{+} l^{-}. Both two observables (alpha and A_{FB}) are independent
of hadronic form factors in leading power of 1/m_b and in leading order of
alpha_s. We also extend the analysis of hadronic matrix elements for Omega_b to
Omega transitions to rare Omega_b to Omega gamma and Omega_b to Omega l^{+}
l^{-} decays and find that radiative Omega_b to Omega gamma decay is probably
the most promising FCNC b to s radiative baryonic decay channel. In addition,
it is interesting to notice that the zero-point of forward-backward asymmetry
of Omega_b to Omega l^{+} l^{-} is the same as the one for Lambda_b to Lambda
l^{+} l^{-} to leading order accuracy provided that the form factors
\bar{\zeta}_i (i=3, 4, 5) are numerically as small as indicated from the quark
model.Comment: 19 page
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