title: Precessional Instability in Binary Black Holes with Aligned Spins.
abstract: Binary black holes on quasicircular orbits with spins aligned with their orbital angular momentum have been test beds for analytic and numerical relativity for decades, not least because symmetry ensures that such configurations are equilibrium solutions to the spin-precession equations. In this work, we show that these solutions can be unstable when the spin of the higher-mass black hole is aligned with the orbital angular momentum and the spin of the lower-mass black hole is antialigned. Spins in these configurations are unstable to precession to large misalignment when the binary separation r is between the values r(ud±)=(√(χ(1))±√(qχ(2)))(4)(1-q)(-2)M, where M is the total mass, q≡m(2)/m(1) is the mass ratio, and χ(1) (χ(2)) is the dimensionless spin of the more (less) massive black hole. This instability exists for a wide range of spin magnitudes and mass ratios and can occur in the strong-field regime near the merger. We describe the origin and nature of the instability using recently developed analytical techniques to characterize fully generic spin precession. This instability provides a channel to circumvent astrophysical spin alignment at large binary separations, allowing significant spin precession prior to merger affecting both gravitational-wave and electromagnetic signatures of stellar-mass and supermassive binary black holes.
authors.affiliation: Department of Applied Mathematics and Theoretical Physics, Centre for Mathematical Sciences, University of Cambridge, Wilberforce Road, Cambridge CB3 0WA, United Kingdom.
authors.forename: Davide
authors.initials: D
authors.lastname: Gerosa
authors.affiliation: Department of Physics, The University of Texas at Dallas, Richardson, Texas 75080, USA.
authors.forename: Michael
authors.initials: M
authors.lastname: Kesden
authors.affiliation: Center for Computational Relativity and Gravitation, Rochester Institute of Technology, Rochester, New York 14623, USA.
authors.forename: Richard
authors.initials: R
authors.lastname: O'Shaughnessy
authors.affiliation: Department of Physics and Astronomy, The University of Mississippi, University, Mississippi 38677, USA.
authors.forename: Antoine
authors.initials: A
authors.lastname: Klein
authors.affiliation: Department of Physics and Astronomy, The University of Mississippi, University, Mississippi 38677, USA.|CENTRA, Departamento de Física, Instituto Superior Técnico, Universidade de Lisboa, Avenida Rovisco Pais 1, 1049 Lisboa, Portugal.
authors.forename: Emanuele
authors.initials: E
authors.lastname: Berti
authors.affiliation: Department of Applied Mathematics and Theoretical Physics, Centre for Mathematical Sciences, University of Cambridge, Wilberforce Road, Cambridge CB3 0WA, United Kingdom.|Department of Physics and Astronomy, The University of Mississippi, University, Mississippi 38677, USA.|California Institute of Technology, Pasadena, California 91125, USA.
authors.forename: Ulrich
authors.initials: U
authors.lastname: Sperhake
authors.affiliation: Department of Physics and Astronomy, The University of Mississippi, University, Mississippi 38677, USA.|Dipartimento di Fisica E. Fermi, Università di Pisa, Pisa 56127, Italy.
authors.forename: Daniele
authors.initials: D
authors.lastname: Trifirò