Article

# Black string and velocity frame dragging

Modern Physics Letters A (Impact Factor: 1.11). 04/2007; DOI:10.1142/S0217732308026418
Source: arXiv

ABSTRACT We investigate velocity frame dragging with the boosted Schwarzschild black string solution and the boosted Kaluza-Klein bubble solution, in which a translational symmetry along the boosted \$z\$-coordinate is implemented. The velocity frame dragging effect can be nullified by the motion of an observer using the boost symmetry along the \$z-\$coordinate if it is not compact. However, in spacetime with the compact \$z-\$coordinate, we show that the effect cannot be removed since the compactification breaks the global Lorentz boost symmetry. As a result, the comoving velocity is dependent on \$r\$ and the momentum parameter along the \$z-\$coordinate becomes an observer independent characteristic quantity of the black string and bubble solutions. The dragging induces a spherical ergo-region around the black string.

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##### Article: Boosted black string bombs
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ABSTRACT: We study the formation of superradiant bound states for massive scalar fields in five-dimensional rotating black string geometries with a non-vanishing Kaluza-Klein momentum along the compact direction. Even though all Kaluza-Klein modes may form bound states in this geometry, in realistic extra-dimensional models and astrophysical black holes only the zero-mode is sufficiently light for superradiant instabilities to develop, provided the field has a small but non-vanishing mass, as for example for axion-like particles. We use analytical and numerical methods to show that, although the Kaluza-Klein momentum decreases the upper bound on the field mass for an instability to develop, it may enhance its maximum growth rate by more than 50%, thus boosting the black hole bomb mechanism. We discuss the possible observational consequences of this result and its potential as an astrophysical probe of non-trivial extra-dimensional compactifications.
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