Speaker
Description
Quasi-periodic oscillations (QPOs) observed in the X-ray emission of accreting compact objects are among the most promising astrophysical messengers of strong-field gravity. Yet, despite decades of observations, their physical origin remains debated, and the most established framework to interpret them, the relativistic precession model (RPM), shows persistent observational tensions, including a systematic preference for Schwarzschild-de Sitter (SdS) geometries over standard Schwarzschild or Reissner-Nordström solutions, with no clear physical justification for the role of a cosmological constant at such scales.
We argue that this tension points to missing physics in the RPM rather than to exotic spacetime geometries. Treating accreting matter as a collection of structureless test particles is a severe oversimplification: real accretion disks carry macroscopic angular momentum and internal structure. We incorporate this ingredient into a macroscopic precession model (MPM), based on the Mathisson-Papapetrou-Dixon equations on a Schwarzschild background, which introduces a spin-curvature coupling that modifies both the Keplerian and the radial epicyclic frequencies. Crucially, this correction naturally reproduces a SdS-like behavior, offering a physical explanation for what was previously attributed to an effective cosmological constant.
We apply the MPM to eight neutron star low-mass X-ray binaries via MCMC fits to twin kHz QPOs, finding statistically competitive or superior results with respect to the SdS framework, with consistent neutron star masses, disk boundary radii in physically plausible ranges, and a natural emergence of the observed 3:2 frequency clustering. These results suggest that QPOs carry direct information about the internal structure of orbiting matter, opening a new observational window on spin-curvature effects in strong-field regimes.