Journal of Nonlinear Science https://doi.org/10.1007/s00332-018-9527-1 Exact Solution of a Neumann Boundary Value Problem for the Stationary Axisymmetric Einstein Equations Jonatan Lenells1 · Long Pei1 Received: 4 October 2018 / Accepted: 11 December 2018 © The Author(s) 2019 Abstract For a stationary and axisymmetric spacetime, the vacuum Einstein field equations reduce to a single nonlinear PDE in two dimensions called the Ernst equation. By solving this equation with a Dirichlet boundary condition imposed along the disk, Neugebauer and Meinel in the 1990s famously derived an explicit expression for the spacetime metric corresponding to the Bardeen–Wagoner uniformly rotating disk of dust. In this paper, we consider a similar boundary value problem for a rotating disk in which a Neumann boundary condition is imposed along the disk instead of a Dirichlet condition. Using the integrable structure of the Ernst equation, we are able to reduce the problem to a Riemann–Hilbert problem on a genus one Riemann surface. By solving this Riemann–Hilbert problem in terms of theta functions, we obtain an explicit expression for the Ernst potential. Finally, a Riemann surface degeneration argument leads to an expression for the associated spacetime metric. Keywords Ernst equation · Einstein equations · Boundary value problem · Unified transform method · Fokas method · Riemann–Hilbert problem · Theta function Mathematics Subject Classification 83C15 · 37K15 · 35Q15 · 35Q76 1 Introduction Half a century ago, Bardeen and Wagoner studied the structure and gravitational field of a uniformly rotating, infinitesimally thin disk of dust in Einstein’s theory of relativity Communicated by Anthony Bloch. B Jonatan Lenells
[email protected] Long Pei
[email protected] 1 Department of Mathematics, KTH Royal Institute of Technology, 10044 Stockholm, Sweden 123 Journal of Nonlinear Science ζ D ρ0 ρ Fig.