11,612 research outputs found
Quasi Periodic Oscillations in Low Mass X-Ray Binaries and Constraints on the Equation of State of Neutron Star Matter
Recently discovered quasi periodic oscillations in the X-ray brightness of
low mass X-ray binaries are used to derive constraints on the mass of the
neutron star component and the equation of state of neutron star matter. The
observations are compared with models of rapidly rotating neutron stars which
are calculated by means of an exact numerical method in full relativity. For
the equations of state we select a broad collection of models representing
different assumptions about the many-body structure and the complexity of the
composition of super dense matter. The mass constraints differ from their
values in the approximate treatment by \sim 10%. Under the assumption that the
maximum frequency of the quasi periodic oscillations originates from the
innermost stable orbit the mass of the neutron star is in the range: . Especially the quasi periodic oscillation in the
Atoll-source 4U 1820-30 is only consistent with equations of state which are
rather stiff at high densities which is explainable, so far, only with pure
nucleonic/leptonic composition. This interpretation contradicts the hypothesis
that the protoneutron star formed in SN 1987A collapsed to a black hole, since
this would demand a maximum neutron star mass below . The recently
suggested identification of quasi periodic oscillations with frequencies around
10 Hz with the Lense-Thirring precession of the accretion disk is found to be
inconsistent with the models studied in this work, unless it is assumed that
the first overtone of the precession is observed.Comment: 12 pages including figures, to be published in MNRA
Saffman-Taylor instability in a non-Brownian suspension: finger selection and destabilization
We study the Saffman-Taylor instability in a non-Brownian suspension by
injection of air. We find that flow structuration in the Hele-Shaw cell can be
described by an effective viscosity depending on the volume fraction. When this
viscosity is used to define the control parameter of the instability, the
classical finger selection for Newtonian fluids is recovered. However, this
picture breaks down when the cell thickness is decreased below approximatively
10 grain sizes. The discrete nature of the grains plays also a determinant role
in the the early destabilization of the fingers observed. The grains produce a
perturbation at the interface proportional to the grain size and can thus be
considered as a "controlled noise". The finite amplitude instability mechanism
proposed earlier by Bensimon et al. allows to link this perturbation to the
actual values of the destabilization threshold.Comment: 4 pages, 4 figures, submitted to PR
Velocity dispersions in galaxies: 1: The SO galaxy NGC 7332
A Coude spectrum of the SO galaxy NGC 7332 with 0.9 A resolution from 4186 to 4364 A was obtained with the SEC vidicon television camera and the Hale telescope. Comparisons with spectra of G and K giant stars, numerically broadened for various Maxwellian velocity distributions, give a dispersion velocity in the line of sight of 160 + or - 20 km/sec with the best fit at G8III. The dispersion appears to be constant within + or - 35 km/sec out to 1.4 kpc (H = 100 km/sec/mpc). After correction for projection, the rotation curve has a slope of 0.16 km/sec/pc at the center and a velocity of 130 km/sec at 1.4 kpc where it is still increasing. For an estimated effective radius of 3.5 kpc enclosing half the light, the virial theorem gives a mass of 1.4 x 10 to the 11th power solar masses if the mass-to-light ratio is constant throughout the galaxy. The photographic luminosity is 8.3 x 10 to the 9th power solar luminosities so that the M/L ratio is 17
The Orbital Light Curve of Aquila X-1
We obtained R- and I-band CCD photometry of the soft X-ray transient/neutron-
star binary Aql X-1 in 1998 June while it was at quiescence. We find that its
light curve is dominated by ellipsoidal variations, although the ellipsoidal
variations are severely distorted and have unequal maxima. After we correct for
the contaminating flux from a field star located only 0.46" away, the
peak-to-peak amplitude of the modulation is ~0.25 mag in the R band, which
requires the orbital inclination to be greater than 36 degrees. The orbital
period we measure is consistent with the 18.95 h period measured by Chevalier &
Ilovaisky (1998). During its outbursts the light curve of Aql X-1 becomes
single humped. The outburst light curve observed by Garcia et al. (1999) agrees
in phase with our quiescent light curve. We show that the single humped
variation is caused by a ``reflection effect,'' that is, by heating of the side
of the secondary star facing towards the neutron star.Comment: 18 manuscript pages, 7 figures; accepted by A
Single crystal growth and anisotropy of CeRuPO
We report on the single crystal growth of the ferromagnetic Kondo lattice
system CeRuPO using a Sn flux method. Magnetic susceptibility and electrical
resistivity measurements indicate strong anisotropy of this structurally
layered compound. They evidence that the magnetic moments order
ferromagnetically along the c-direction of the tetragonal unit cell, whereas
the crystal electric field (CEF) anisotropy favors the ab-plane. Therefore,
CeRuPO presents the unusual case within rare earth systems, where the
anisotropy of the interionic exchange interaction overcomes the single ion
anisotropy due to the CEF interaction.Comment: 13 pages, 7 figures, high quality figures:
http://www.cpfs.mpg.de/~krellner
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