Extended orbital resonance model with hump-induced oscillations

TL;DR

The paper extends the orbital resonance model to include hump-induced oscillations in accretion discs around near-extreme Kerr black holes, explaining complex QPO patterns observed in some microquasars.

Contribution

It introduces an extended resonance model (EXORM) incorporating hump-induced oscillations for Kerr black hole accretion discs, applicable to complex QPO phenomena.

Findings

EXORM can explain all QPOs in GRS 1915+105.

Hump-induced oscillations are linked to sign changes in LNRF-velocity gradients.

Model is applicable to near-extreme Kerr black hole systems.

Abstract

Change of sign of the LNRF-velocity gradient has been found for accretion discs orbiting rapidly rotating Kerr black holes with spin a > 0.9953 for Keplerian discs and a > 0.99979 for marginally stable thick discs. Such a "humpy" LNRF-velocity profiles occur just above the marginally stable circular geodesic and could be related to oscillations of accretion discs. The frequency of such "hump"-induced oscillations can be identified with the maximal rate of change of the orbital velocity within the "humpy" profile. Therefore, we introduce an extended orbital resonance model (EXORM) of quasiperiodic oscillations (QPOs) assuming non-linear resonant phenomena between oscillations with the orbital epicyclic frequencies and the humpy frequency defined in a fully general relativistic way. The EXORM is developed for both Keplerian discs and perfect-fluid tori where the approximation of oscillations with epicyclic frequencies is acceptable. Clearly, the EXORM could be applied to the near-extreme Kerr black hole systems exhibiting relatively complex QPO frequency patterns. Assuming a Keplerian disc, it can be shown that in the framework of the EXORM, all the QPOs observed in the microquasar GRS 1915+105 could be explained, while it is not possible in the case of QPOs observed in the Galactic Centre source Sgr A*.