Very long baseline astrometry of PSR J1012+5307 and its implications on alternative theories of gravity

PSR J1012+5307 is a millisecond pulsar with a helium white dwarf (WD) companion ∼ 9.6% as massive as the pulsar. PSR J1012+5307 has been timed with high precision for about 25 years. One of the main objectives of this long-term timing is to use the large asymmetry in gravitational binding energy between the neutron star and the WD to test gravitational theories. Such tests, however, will be eventually limited by the accuracy of the distance to the pulsar. Very long baseline interferometry (VLBI) astrometric observations can complement pulsar timing by providing higher precision measurements of the parallax and hence distance, reducing the uncertainty on distance-dependent parameters in the pulsar timing model. Here, we present VLBI astrometry results spanning approximately 2.5 years for PSR J1012+5307, obtained with the Very Long Baseline Array as part of the MSPSRπ project. These provide the first proper motion and absolute position for PSR J1012+5307 measured in a quasi-inertial reference frame, allowing a comparison between timing astrometry in the solar system barycentric frame and a quasi-inertial reference frame. From the VLBI results, we measure a distance of 828^{+56}_{-18} pc for PSR J1012+5307 (all the estimates presented in the abstract are with 1σ significance), which is the most precise obtained to date. Using the new distance, we improve the uncertainty of measurements of the unmodeled contributions to orbital period decay, which, combined with three other pulsars, places new constraints on the coupling constant for dipole gravitational radiation κ = (−1.7^{+1.7}_{-1.6})E−4 and the fractional time derivative of Newton’s gravitational constant G-dot/G = −1.8^{+5.6}_{-4.8}E−13 yr^{−1} in the local universe. Both are the most precise pulsar-based constraints, consistent with the value of zero required by general theory of relativity.