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Free keywords:
Astrophysics, astro-ph,General Relativity and Quantum Cosmology, gr-qc
Abstract:
We present a straightforward approach for estimating the final black hole
spin of a binary black hole coalescence with arbitrary initial masses and
spins. Making some simple assumptions, we estimate the final angular momentum
to be the sum of the individual spins plus the orbital angular momentum of a
test particle orbiting at the last stable orbit around a Kerr black hole with a
spin parameter of the final black hole. The formula we obtain is able to
reproduce with reasonable accuracy the results from available numerical
simulations, but, more importantly, it can be used to investigate what
configurations might give rise to interesting dynamics. In particular, we
discuss scenarios which might give rise to a ``flip'' in the direction of the
total angular momentum of the system. By studying the dependence of the final
spin upon the mass ratio and initial spins we find that our simple approach
suggests that it is not possible to spin-up a black hole to extremal values
through merger scenarios irrespective of the mass ratio of the objects
involved.
