Pitching Cosmic Curveballs: Environmental Effects on Extreme-Mass-Ratio Inspirals with Spinning Secondaries

Just as a pitcher generates a spin when throwing a curveball to deceive a batter, a spinning secondary in an extreme-mass-ratio inspiral (EMRI) can experience Magnus and lift forces from a gaseous halo. We compute these inspirals and find that the environmental spin-coupling effects, while subdominant, induce significant deviations in the orbital trajectory over long timescales. These effects imprint a unique dephasing on the gravitational waves (GWs), producing sufficiently large mismatches and signal-to-noise ratios (SNRs) to be detectable and distinguishable by future space-based detectors like LISA, TianQin, and Taiji. We also perform a Fisher analysis and find that the inclusion of aerodynamic effects can improve spin constraints on the secondary mass due to the larger deviation in the waveform.  This new approach could provide better constraints on the secondary’s spin and a novel probe of environments around EMRIs and black holes in general.