SPH simulations of irradiation-driven warped accretion discs and the long periods in X-ray binaries

We present three dimensional smoothed particle hydrodynamics (SPH) calculations of irradiation-driven warping of accretion discs. Initially unwarped planar discs are unstable to the radiation reaction when the disc is illuminated by a central radiation source. The disc warps and tilts and precesses slowly in a retrograde direction; its shape continuously flexes in response to the changing orientation of the Roche potential. We simulate ten systems: eight X-ray binaries, one cataclysmic variable (CV), and a `generic' low mass X-ray binary (LMXB). We adopt system parameters from observations and tune a single parameter: our model X-ray luminosity (\(L_{*}\)) to reproduce the observed or inferred super-orbital periods. Without exception, across a wide range of parameter space, we find an astonishingly good match between the observed \(L_{X}\) and the model \(L_{*}\). We conclude irradiation-driven warping is the mechanism underlying the long periods in X-ray binaries. Our Her X-1 simulation simultaneously reproduces the observed \(L_{X}\), the "main-" and "short-high" X-ray states and the orbital inclination. Our simulations of SS 433 give a maximum warp angle of \(18.6^{\circ}\), a good match to the cone traced by the jets, but this angle is reached only in the outer disc. In all cases, the overall disc tilt is less than \(\degrees{13}\) and the maximum disc warp is less than and or equal to \(\degrees{21}\).