Probing Hairy Kerr Black Holes through Quasi-Periodic Oscillations I: A study based on the kinematic models
Black holes endowed with nontrivial scalar or matter fields, known as hairy black holes, possess additional parameters beyond mass, charge, and spin, leading to richer phenomenology. Constraining their parameter space is therefore essential, particularly in view of current electromagnetic and gravitational-wave observations. In this work, we study rotating hairy black hole solutions inspired by the gravitational decoupling method, which satisfy the Einstein field equations with a conserved energy-momentum tensor obeying the strong energy conditions. We explore in detail the horizon structure of such black holes and report for the first time certain unique features not observed in Kerr black holes. We examine how the hair parameters influence the fundamental frequencies governing particle motion in the hairy Kerr spacetime and compare these effects with the Kerr case. Since theoretical models of high-frequency quasi-periodic oscillations (HFQPOs) are directly linked to these fundamental frequencies, this provides a powerful observational probe of hairy black holes. By confronting several kinematic HFQPO models with observations from six black hole sources, we report that for most sources, the HFQPO data at the current level of precision cannot distinguish between the Kerr and hairy Kerr scenarios. However, based on agreement with previous spin estimates, our analysis provides a systematic framework to assess the relative suitability of different HFQPO models for each source. Notably, even with current observational precision, we find that both the Relativistic Precession Model and the Tidal Disruption Model appear unsuitable for the sources GRO J1655-40 and GRS 1915+105. The broader implications of these findings are discussed.