Constraining Brans‐Dicke Gravity with Accreting Millisecond Pulsars in Ultracompact Binaries

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.