148 research outputs found
Synchronized stationary clouds in a static fluid
The existence of stationary bound states for the hydrodynamic velocity field
between two concentric cylinders is established. We argue that rotational
motion, together with a trapping mechanism for the associated field, is
sufficient to mitigate energy dissipation between the cylinders, thus allowing
the existence of infinitely long lived modes, which we dub stationary clouds.
We demonstrate the existence of such stationary clouds for sound and surface
waves when the fluid is static and the internal cylinder rotates with constant
angular velocity . These setups provide a unique opportunity for the
first experimental observation of synchronized stationary clouds. As in the
case of bosonic fields around rotating black holes and black hole analogues,
the existence of these clouds relies on a synchronization condition between
and the angular phase velocity of the cloud.Comment: v2: 7 pages, 4 figures. Accepted for publication in Physics Letters
Scattering by stringy black holes
We study the scattering of axially incident massless scalar waves by a
charged and rotating black hole solution from heterotic string theory called
the Kerr-Sen black hole. We compute the scattering cross section using the
partial wave approach, for arbitrary incident wavelengths. We compare our
results with those of the general relativistic version of a charged and
rotating black hole, namely the Kerr-Newman black hole. We present a selection
of numerical results showing that these compact objects have similar scattering
properties.Comment: 8 pages, 4 figure
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