# Black Hole Continuum Spectra as a Test of General Relativity: Quadratic Gravity

## TL;DR

This paper assesses whether current and future X-ray observations of black hole spectra can test modified gravity theories, finding that some theories may be constrained better than current bounds with improved systematic error control.

## Contribution

It evaluates the potential of continuum spectrum observations to constrain two specific quadratic gravity theories beyond existing bounds.

## Key findings

- Dynamical Chern-Simons gravity cannot be better constrained than current bounds.
- Einstein-dilaton-Gauss-Bonnet gravity may be constrained more tightly with next-generation telescopes.
- Systematic errors in accretion disk modeling are critical for improving constraints.

## Abstract

Observations of the continuum spectrum emitted by accretion disks around black holes allows us to infer their properties, including possibly whether black holes are described by the Kerr metric. Some modified gravity theories do not admit the Kerr metric as a solution, and thus, continuum spectrum observations could be used to constrain these theories. We here investigate whether current and next generation X-Ray observations of the black hole continuum spectrum can constrain such deviations from Einstein's theory, focusing on two well-motivated modified quadratic gravity theories: dynamical Chern-Simons gravity and Einstein-dilaton-Gauss-Bonnet gravity. We do so by determining whether the non-Kerr deviations in the continuum spectrum introduced by these theories are larger than the observational error intrinsic to the observations. We find that dynamical Chern-Simons gravity cannot be constrained better than current bounds with current or next generation continuum spectrum observations. Einstein-dilaton-Gauss-Bonnet gravity, however, may be constrained better than current bounds with next generation telescopes, as long as the systematic error inherent in the accretion disk modeling is decreased below the predicted observational error.