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Viscous Stability of Relativistic Keplerian Accretion Disks
We investigate the viscous stability of thin, Keplerian accretion disks in
regions where general relativistic (GR) effects are essential. For gas pressure
dominated (GPD) disks, we show that the Newtonian conclusion that such disks
are viscously stable is reversed by GR modifications in the behaviors of
viscous stress and surface density over a significantly large annular region
not far from the innermost stable orbit at r=\rms. For slowly-rotating
central objects, this region spans a range of radii 14\lo r\lo 19 in units of
the central object's mass . For radiation pressure dominated (RPD) disks,
the Newtonian conclusion that they are viscously unstable remains valid after
including the above GR modifications, except in a very small annulus around
, which has a negligible influence. Inclusion of the stabilizing
effect of the mass-inflow through the disk's inner edge via a GR analogue of
Roche-lobe overflow adds a small, stable region around \rms~for RPD disks, but
leaves GPD disks unchanged. We mention possible astrophysical relevance of
these results, particularly to the high-frequency X-ray variabilities observed
by the .Comment: 18 pages, 3 figures, accepted by The Astrophysical Journal Letter
Elliptic Flow and Dissipation in Heavy-Ion Collisions at E_{lab} = (1--160)A GeV
Elliptic flow in heavy-ion collisions at incident energies
(1--160)A GeV is analyzed within the model of 3-fluid dynamics (3FD). We show
that a simple correction factor, taking into account dissipative affects,
allows us to adjust the 3FD results to experimental data. This single-parameter
fit results in a good reproduction of the elliptic flow as a function of the
incident energy, centrality of the collision and rapidity. The experimental
scaling of pion eccentricity-scaled elliptic flow versus
charged-hadron-multiplicity density per unit transverse area turns out to be
also reasonably described. Proceeding from values of the Knudsen number,
deduced from this fit, we estimate the upper limit the shear
viscosity-to-entropy ratio as at the SPS incident energies.
This value is of the order of minimal observed in water and liquid
nitrogen.Comment: 10 pages, 7 figures, version accepted by Phys. Rev.
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