A New Method for Testing Einstein's Theory of Gravity Close to Rapidly Spinning Black Holes

TL;DR

This paper proposes a new observational method using X-ray polarimetry to test Einstein's gravity near rapidly spinning black holes, revealing extreme frame dragging effects.

Contribution

It introduces a novel spectral component caused by processes similar to Penrose interactions, enabling tests of gravity close to black holes.

Findings

Identification of a new spectral component with unique properties.

Potential for current/future X-ray polarimeters to test gravity theories.

Method to measure black hole spin via spectral signatures.

Abstract

The classical Penrose process and the collisional Penrose processes involve particles decaying or interacting very close to a spinning black hole, in which some particles acquire negative energy and fall into the black hole while others acquire that energy and can leave the system. Both processes involve an extreme form of frame dragging, i.e. the spinning black hole drags spacetime with it, and the spacetime ejects some of the particles with a large energy gain, similar to a projectile in a slingshot. Such extreme forms of frame dragging had long been believed to be unobservable as the efficiency for a black hole energizing particles in this way is very low. Here we report a new observational signature of this extreme sort of frame dragging. In rapidly spinning black holes in X-ray binaries, processes similar to collisional Penrose processes, but slightly less extreme, can give rise to a new spectral component with distinct spectral and polarimetric properties. Observations of this new spectral component with current or future broadband X-ray polarimeters will open a new window into testing Einstein's theory of gravity close to the edge of a black hole and can be used to measure the black hole spin.