# A tilted and warped inner accretion disc around a spinning black hole: an analytical solution

## TL;DR

This paper provides an analytical solution for the tilt profile of a warped inner accretion disc around a spinning black hole, considering observational parameters and inner disc effects, relevant for interpreting X-ray observations.

## Contribution

It derives an exact analytical expression for the radial tilt profile of a warped accretion disc, including the inner disc region, which was not previously addressed.

## Key findings

- The tilt angle in 10-100 gravitational radii can be a significant fraction of the outer tilt.
- Tilt profiles can exhibit humps in 10-1000 gravitational radii, affecting X-ray features.
- The solution incorporates observable parameters like black hole mass and spin, enabling direct comparison with data.

## Abstract

Inner accretion disc around a black hole provides a rare, natural probe to understand the fundamental physics of the strong gravity regime. A possible tilt of such a disc, with respect to the black hole spin equator, is important. This is because such a tilt affects the observed spectral and timing properties of the disc X-ray emission via Lense-Thirring precession, which could be used to test the theoretical predictions regarding the strong gravity. Here, we analytically solve the steady, warped accretion disc equation of Scheurer and Feiler (1996), and find an expression of the radial profile of the disc tilt angle. In our exact solution, considering a prograde disc around a slowly spinning black hole, we include the inner part of the disc, which was not done earlier in this formalism. 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. Our tilt angle radial profile expression includes observationally measurable parameters, such as black hole mass and Kerr parameter, and the disc inner edge tilt angle $W_{\rm in}$, and hence can be ideal to confront observations. Our solution shows that the disc tilt angle in $10-100$ gravitational radii is a significant fraction of the disc outer edge tilt angle, even for $W_{\rm in} = 0$. Moreover, tilt angle radial profiles have humps in $\sim 10-1000$ gravitational radii for some sets of parameter values, which should have implications for observed X-ray features.