# XSPEC model for testing the Kerr black hole hypothesis using the continuum-fitting method

## TL;DR

This paper introduces NKBB, an XSPEC model that enables testing the Kerr black hole hypothesis by analyzing the thermal spectrum of accretion disks in various spacetime geometries.

## Contribution

The paper presents a new XSPEC model, NKBB, capable of testing the Kerr black hole hypothesis using the continuum-fitting method in parametric black hole spacetimes.

## Key findings

- Model accurately reproduces relativistic effects in accretion disk spectra.
- Can be extended to various stationary, axisymmetric spacetimes.
- Facilitates testing of the Kerr hypothesis with observational data.

## Abstract

We present NKBB, an XSPEC model for the thermal spectrum of thin accretion disks in parametric black hole spacetimes. We employ the Novikov-Thorne model for the description of the accretion disk and the formalism of the transfer function proposed by Cunningham for storing all the relativistic effects of the spacetime metric. The current version of the model assumes the Johannsen metric, but it can be easily extended to any stationary, axisymmetric, and asymptotically-flat spacetime without pathological properties. The model can be used within the XSPEC package to test the Kerr nature of astrophysical black holes with the continuum-fitting method.