# Probing the space time around a black hole with X-ray variability

## TL;DR

This paper reviews how X-ray variability in black-hole binaries can test General Relativity in strong gravitational fields, highlighting recent models and the potential of upcoming observations for breakthroughs.

## Contribution

It synthesizes current observational and theoretical developments, emphasizing the role of X-ray variability studies in testing fundamental physics around black holes.

## Key findings

- Models connect X-ray signals to GR fundamental frequencies.
- Current observations support black holes as laboratories for GR tests.
- Future facilities may enable significant breakthroughs in understanding strong-field gravity.

## Abstract

In the past decades, the phenomenology of fast time variations of high-energy flux from black-hole binaries has increased, thanks to the availability of more and more sophisticated space observatories, and a complex picture has emerged. Recently, models have been developed to interpret the observed signals in terms of fundamental frequencies connected to General Relativity, which has opened a promising way to measure the prediction of GR in the strong-field regime. I review the current standpoint both from the observational and theoretical side and show that these systems are the most promising laboratories for testing GR and the observations available today suggest that the next observational facilities can lead to a breakthrough in the field.