Constraining Brans-Dicke Gravity with Millisecond Pulsars in Ultracompact Binaries

TL;DR

This paper explores how observations of millisecond pulsars in ultracompact binaries can be used to test and constrain deviations from general relativity, specifically within the Brans-Dicke gravity framework.

Contribution

It presents a method to use orbital period changes in ultracompact binaries to improve constraints on the Brans-Dicke parameter omega_BD beyond current limits.

Findings

Expected orbital period change rates can constrain omega_BD.

Measurements can set lower bounds on orbital inclination.

Potential to improve existing gravity tests.

Abstract

The five accreting, millisecond X-ray pulsars in ultracompact binaries that were recently discovered with the Rossi X-ray Timing Explorer provide excellent candidates for constraining the deviations from general relativity described by the Brans-Dicke parameter omega_BD. I calculate the expected rate of change of the orbital periods of these binaries and discuss the prospects of constraining omega_BD to values that are an order of magnitude larger than current constraints. Finally, I show how measurements of the orbital period derivative in ultracompact binaries can be used to place lower bounds on their orbital inclination.