Lunch talk on Nov. 1, 2021

Effect of frame-dragging on accretion disc: The Bardeen-Petterson effect

Speaker: Chandrachur Chakraborty (IISc)

Venue: Video Conference

Time: 12:30 PM, Monday, Nov. 1, 2021

Abstract: 

The 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. However, the inner disk has been predicted to become aligned with the spin direction of the black hole by the well-known Bardeen-Petterson effect. Here we show (both analytically and numerically) that the inner disk may not be aligned at all for certain reasonable ranges of parameter values. Such a solution is timely, as a tilted inner disc has recently been inferred from X-ray spectral and timing features of the accreting black hole H1743-322. In our analysis, we consider the contribution of the inner disc, as it can stay misaligned with the black hole spin for a reasonable range of parameter values.  We find that the alignment time-scale increases with a decrease in the Kerr parameter below a critical Kerr parameter value, contrary to earlier predictions. We also find that the black hole in a typical Galactic LMXB (low-mass X-ray binary) can take a significantly longer time to align than what was estimated in the past.