A millisecond pulsar position determined to 0.2 mas precision with VLBI

Context . Precise millisecond pulsar (MSP) positions determined with very long baseline interferometry (VLBI) hold the key to building the connection between the kinematic and dynamic reference frames respectively used for VLBI and pulsar timing. A frame connection would provide an important pathway to examining the planetary ephemerides used in pulsar timing, and would potentially enhance the sensitivities of the pulsar timing arrays used to detect stochastic gravitational-wave background in the nano-Hz regime. Aims . We aim to significantly improve the precision of the VLBI-based MSP position (from >1 mas at present) by reducing the two dominant components in the positional uncertainty – the propagation-related uncertainty and the uncertainty resulting from the frequency-dependent core shifts of the reference sources. Methods . We introduce a new differential astrometry strategy called PINPT (Phase-screen Interpolation plus frequeNcy-dePendent core shifT correction; pronounced “pinpoint”), which entails the use of multiple calibrators observed at several widely separated frequencies. The strategy allows determination of the core shift and mitigates the impact of residual delay in the atmosphere. We implemented the strategy on PSR J2222-0137, an MSP that is well constrained astrometrically with VLBI and pulsar timing. Results . Using the PINPT strategy, we determined core shifts for four AGNs around PSR J2222-0137, and derived a VLBI-based pulsar position with uncertainties of 0.17 mas and 0.32 mas in Right Ascension and Declination, respectively, approaching the uncertainty level of the best-determined timing-based MSP positions. Additionally, incorporating the new observations into historical ones, we refined the pulsar proper motion and the parallax-based distance to the <10 µas yr -1 level and the subparsec level, respectively. Conclusions . The realization of the PINPT strategy promises a factor-of-five positional precision enhancement (over conventional VLBI astrometry) for all kinds of compact radio sources observed at <2 GHz, including most fast radio bursts.