Classical spin transitions and absorptive scattering

A bstract We describe an on-shell, amplitudes-based approach to incorporating radiation absorption effects in the post-Minkowskian scattering of generic, compact, spinning bodies. Classical spinning observables are recovered by extrapolating to large spin, results calculated with finite quantum spin- s particles using the properties of spin universality and Casimir interpolation. At leading-order our results give a completely general and non-redundant parametrization of absorptive observables in terms of a finite number of Wilson coefficients associated with 3-particle mass and spin-magnitude changing on-shell amplitudes. We denote these semi-fictitious microscopic processes: classical spin transitions . Explicit results for the leading-order impulse due to the absorption of scalar, electromagnetic and gravitational radiation, for spin transitions ∆ s = 0, ±1, ±2 are given in a fully interpolated form up to $$ \mathcal{O}\left({S}^2\right) $$ O S 2 , and Casimir independent contributions given up to $$ \mathcal{O}\left({S}^4\right) $$ O S 4 . Our explicit results reveal some surprising universal patterns. We find that, up to identification of Wilson coefficients, the Casimir independent contributions to the impulse for spinning-up and spinning-down by the same magnitude |∆ s | are identical. For processes where the quantum ∆ s < 0 transition is forbidden, the corresponding classical observable is suppressed in powers of S by a predictable amount. Additionally we find that, while for generic non-aligned spin configurations there is a non-zero scattering angle at leading-order, for aligned spin, similar to non-spinning absorption, the scattering angle vanishes and the impulse is purely longitudinal. The formalism and results presented provide a significant extension of the amplitudes-based calculational pipeline for gravitational waveforms from binary black hole and neutron star systems beyond the point-particle approximation.