Probing hairy Kerr black holes through quasi-periodic oscillations II: a study based on the resonance models

Abstract

Hairy black holes (BHs), characterized by nontrivial scalar or matter fields, introduce additional degrees of freedom beyond mass, charge, and spin, providing a framework for testing deviations from the Kerr paradigm. We investigate a class of rotating hairy black holes constructed within the gravitational decoupling framework, whose spacetime satisfies the Einstein field equations with a conserved energy-momentum tensor consistent with the strong energy conditions. In our previous work (Paper I; Dasgupta et al. 2026), we studied their horizon structure and test-particle dynamics and confronted two kinematic high-frequency quasi-periodic oscillation (HFQPO) models with observations of six black hole sources. Building on that analysis, we here test nine resonance models against the same HFQPO data, enabling a systematic comparison of eleven models (two kinematic and nine resonance) within the hairy Kerr framework. A comprehensive analysis based on the two kinematic models (Paper I) and the nine resonance models reveals that the HFQPO data with the current level of precision cannot rule out the hairy Kerr scenario. By implementing the Akaike Information Criterion and by examining the consistency of the inferred spins with previous independent spin estimates, our analysis provides a systematic framework for assessing the relative performance of different HFQPO models for each source. These results highlight the potential of HFQPOs to probe black hole hair and the need for more precise observations and a better understanding of the physical origin of QPOs.