PSR J1012 + 5307 , a millisecond pulsar in orbit with a helium white dwarf ( WD ) , 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 .
Here , we present VLBI ( very long baseline interferometry ) astrometry results spanning approximately 2.5 years for PSR J1012 + 5307 , obtained with the Very Long Baseline Array as part of the \mathrm { MSPSR } \pi project .
These provide the first proper motion and absolute position for PSR J1012 + 5307 measured in a quasi-inertial reference frame .
From the VLBI results , we measure a distance of 0.83 ^ { +0.06 } _ { -0.02 } kpc ( all the estimates presented in the abstract are at 68 % confidence ) for PSR J1012 + 5307 , 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 \kappa _ { D } = ( -1.7 \pm 1.7 ) \times 10 ^ { -4 } and the fractional time derivative of Newton ’ s gravitational constant \dot { G } / G = -1.8 ^ { +5.6 } _ { -4.7 } \times 10 ^ { -13 } { yr ^ { -1 } } in the local universe .
As the uncertainties of the observed decays of orbital period for the four leading pulsar-WD systems become negligible in \approx 10 years , the uncertainties for \dot { G } / G and \kappa _ { D } will be improved to \leq 1.5 \times 10 ^ { -13 } { yr ^ { -1 } } and \leq 1.0 \times 10 ^ { -4 } , respectively , predominantly limited by the distance uncertainties .