LBA observations for project V558 semester 2019APRS

PSR J0437–4715 is the nearest, brightest, and inarguably best-studied radio millisecond pulsar. Its radio pulses can be timed exceedingly accurately, making it one of the key objects in searching for low-frequency gravitational waves. The latest timing astrometry has provided a kinematic distance of 156.79+/–0.25 pc, which is five times more accurate than the existing VLBI distance and makes PSR J0437–4715 become the only known pulsar with a distance error <1 pc. To verify the kinematic distance independently and provide a stringent constraint on the time stability of the Newton's gravitational constant G by high-precision VLBI phase-referencing astrometry, we have performed 12-hour (two epochs) in-beam phase-referencing observations of the pulsar and two nearby (<1 arcmin) in-beam calibrators with the LBA+T6Km at 6.6 GHz. According to our preliminary data analysis, we have achieved a positional precision of ~25 microarcsec in the 2nd epoch. In this proposal, we request 19.5 hours (3 epochs, 6.5 hour per epoch, within about 1 year) observations of PSR J0437–4715 field with the same experiment setup. With five epochs total, we expect to measure the VLBI parallax with an accuracy of ~10 microarcsec. This would lead to a distance measurement as accurate as the kinematic distance measurement and a nearly one order of magnitude tighter constraint on the time variability of the gravitational constant G.