MG15 - Talk detail

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Session

Accepted

Yes

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Talk

Title

Abstract

Inner part of a thin accretion disk around a Kerr black hole can serve as an important tool to study the physics of the strong gravity regime. A tilt in such a disk with respect to the black hole spin axis is particularly useful for this purpose, as such a tilt can have a significant effect on the observed X-ray spectral and timing features via Lense-Thirring precession. The inner disk was predicted to become aligned with the spin direction of the black hole by the well known Bardeen-Petterson effect. In this study, we have analytically and numerically found that the radial extent of the inner disk alignment is much smaller than what previously estimated. Moreover, for certain reasonable values of the parameters, inner disk may not be aligned at all. This makes the inner accretion disk particularly promising to probe the black hole parameters and the accretion process in strong gravity region.

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Session

Accepted

Yes

Order

Time

Talk

Title

Abstract

We consider a thin accretion disk around a spinning black hole where the outer disk is misaligned with the spin direction of the black hole. For such a scenario, the inner part of the accretion disk was predicted to become aligned with the spin axis of the black hole by the well known Bardeen-Petterson effect. However, in our recent work, where we have considered the contribution of the inner disk and torque at the inner boundary, we have shown that for certain reasonable range of the parameter values, the inner disk may not be aligned at all with the spin axis of the black hole. As the black hole exerts a torque on the accretion disk due to Lense-Thirring precession and tries to align the inner disk with its own spin direction, same torque is also applied on it due to the conservation of angular momentum. As a result of which the black hole ultimately aligns itself with the outer disk. In this work, we compute the torque on the black hole due to Lense-Thirring precession taking the contribution from the entire disk. Thus, we obtain the alignment timescale and precession timescale of the black hole and study the timescales as a function of several parameters of the system. We consider the dynamical evolution of the disk during the alignment process and explore how it affects both the timescales in persistent as well as transient accretion. We also discuss the observational implications of our results in the context of microquasars.

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