# Iron line spectroscopy with Einstein-dilaton-Gauss-Bonnet black holes

## TL;DR

This paper evaluates the accuracy of an approximate analytical metric for Einstein-dilaton-Gauss-Bonnet black holes by analyzing X-ray reflection spectra, showing that key features like ISCO and iron lines are reliably predicted.

## Contribution

It introduces and tests an approximate analytical metric for Einstein-dilaton-Gauss-Bonnet black holes in the context of X-ray spectroscopy analysis.

## Key findings

- ISCO increases by about 7% with dilaton charge
- Iron line shape is less sensitive to dilaton charge than expected
- Analytical metric accurately predicts key spectral features

## Abstract

Einstein-dilaton-Gauss-Bonnet gravity is a well-motivated alternative theory of gravity that emerges naturally from string theory. While black hole solutions have been known in this theory in numerical form for a while, an approximate analytical metric was obtained recently by some of us, which allows for faster and more detailed analysis. Here we test the accuracy of the analytical metric in the context of X-ray reflection spectroscopy. We analyze innermost stable circular orbits (ISCO) and relativistically broadened iron lines and find that both the ISCO and iron lines are determined sufficiently accurately up to the limit of the approximation. We also find that, though the ISCO increases by about 7% as dilaton coupling increases from zero to extremal values, the redshift at ISCO changes by less than 1%. Consequently, the shape of the iron line is much less sensitive to the dilaton charge than expected.