Probing General Relativity with Accreting Black Holes

TL;DR

This paper discusses how X-ray observations of accreting black holes reveal strong general relativistic effects, allowing us to probe the extreme spacetime environment near black holes through spectral and timing measurements.

Contribution

It highlights the use of X-ray reflection spectra and reverberation mapping as novel tools to test and explore general relativity in the strong gravity regime around black holes.

Findings

Detection of gravitational redshifts in reflection spectra

Measurement of light bending effects on X-ray emission

Reverberation delays constrain black hole environments

Abstract

Most of the X-ray emission from luminous accreting black holes emerges from within 20 gravitational radii. The effective emission radius is several times smaller if the black hole is rapidly spinning. General Relativistic effects can then be very important. Large spacetime curvature causes strong lightbending and large gravitational redshifts. The hard X-ray, power-law-emitting corona irradiates the accretion disc generating an X-ray reflection component. Atomic features in the reflection spectrum allow gravitational redshifts to be measured. Time delays between observed variations in the power-law and the reflection spectrum (reverberation) enable the physical scale of the reflecting region to be determined. The relative strength of the reflection and power-law continuum depends on light bending. All of these observed effects enable the immediate environment of the black hole where the effects of General Relativity are on display to be probed and explored.