The relativistic pulsar-white dwarf binary PSR J1738+0333 - II. The most stringent test of scalar-tensor gravity

We report the results of a 10-year timing campaign on PSR J1738+0333, a 5.85-ms pulsar in a low-eccentricity 8.5-h orbit with a low-mass white dwarf companion. We obtained 17376 pulse times of arrival with a stated uncertainty smaller than s and weighted residual rms of s. The large number and precision of these measurements allow highly significant estimates of the proper motion μ α, δ= (+7.037±0.005, +5.073±0.012)masyr -1, parallax π x=(0.68±0.05)mas and a measurement of the apparent orbital decay, (all 1σ uncertainties). The measurements of μ α, δ and π x allow for a precise subtraction of the kinematic contribution to the observed orbital decay; this results in a significant measurement of the intrinsic orbital decay: This is consistent with the orbital decay from the emission of gravitational waves predicted by general relativity, , i.e. general relativity passes the test represented by the orbital decay of this system. This agreement introduces a tight upper limit on dipolar gravitational wave emission, a prediction of most alternative theories of gravity for asymmetric binary systems such as this. We use this limit to derive the most stringent constraints ever on a wide class of gravity theories, where gravity involves a scalar-field contribution. When considering general scalar-tensor theories of gravity, our new bounds are more stringent than the best current Solar system limits over most of the parameter space, and constrain the matter-scalar coupling constant to be below the 10 -5 level. For the special case of the Jordan-Fierz-Brans-Dicke, we obtain the 1σ bound , which is within a factor of 2 of the Cassini limit. We also use our limit on dipolar gravitational wave emission to constrain a wide class of theories of gravity which are based on a generalization of Bekenstein's Tensor-Vector-Scalar gravity, a relativistic formulation of modified Newtonian dynamics. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.