Pulsar-black hole binaries: prospects for new gravity tests with future radio telescopes

TL;DR

Future radio telescopes like FAST and SKA could enable precise tests of black hole physics and gravity theories by timing pulsars orbiting black holes, revealing black hole properties and constraining alternative gravity models.

Contribution

This paper demonstrates the potential of upcoming radio telescopes to measure black hole parameters and test gravity theories using pulsar-black hole systems through simulated observations.

Findings

A few years of timing can measure black hole mass and spin with high precision.

Extreme system configurations are needed to measure the black hole quadrupole moment.

Next-generation telescopes are crucial for discovering and studying pulsar-black hole binaries.

Abstract

The anticipated discovery of a pulsar in orbit with a black hole is expected to provide a unique laboratory for black hole physics and gravity. In this context, the next generation of radio telescopes, like the Five-hundred-metre Aperture Spherical radio Telescope (FAST) and the Square Kilometre Array (SKA), with their unprecedented sensitivity, will play a key role. In this paper, we investigate the capability of future radio telescopes to probe the spacetime of a black hole and test gravity theories, by timing a pulsar orbiting a stellar-mass-black-hole (SBH). Based on mock data simulations, we show that a few years of timing observations of a sufficiently compact pulsar-SBH (PSR-SBH) system with future radio telescopes would allow precise measurements of the black hole mass and spin. A measurement precision of one per cent can be expected for the spin. Measuring the quadrupole moment of the black hole, needed to test GR's no-hair theorem, requires extreme system configurations with compact orbits and a large SBH mass. Additionally, we show that a PSR-SBH system can lead to greatly improved constraints on alternative gravity theories even if they predict black holes (practically) identical to GR's. This is demonstrated for a specific class of scalar-tensor theories. Finally, we investigate the requirements for searching for PSR-SBH systems. It is shown that the high sensitivity of the next generation of radio telescopes is key for discovering compact PSR-SBH systems, as it will allow for sufficiently short survey integration times.