Disk warping and black hole X-ray binaries: I. Tentative unification of low-frequency quasi-periodic oscillations

Context. X-ray binaries exhibit complex variability patterns studied in the power spectrum. These include the broadband noise (BBN) components and various types of narrow components called quasi-periodic oscillations (QPOs). There is currently no consensus about what determines the presence or absence of the BBN or what generates the QPOs. Many believe that QPO generation is due to framedragging effects caused by Lense–Thirring torques. Aims. We investigated the potential impact of frame-dragging effects on the accretion disk itself. In particular, we focused on its impact on the observed variability and on the presence (and types) of associated QPOs. Methods. We made analytical estimates to assess the potential presence of a geometric warp in the inner accretion disk during state transitions. Results. We show that the presence of a warp can modify the spectral-timing properties in a way that matches the observed transition between QPO types during outbursts. We also discuss the peculiar case of Cyg X-1, as well as how the hard-to-soft transition could be driven by the warp itself. Conclusions. The (expected) emergence of a warp provides a consistent explanation for the evolution of both the BBN and the QPO properties during state transitions. This offers a first path toward unifying the variability of black hole X-ray binaries.