A millisecond pulsar position determined to 0.2 milliarcsecond precision with VLBI

TL;DR

This paper introduces a new VLBI astrometry method called PINPT that significantly improves millisecond pulsar position accuracy to 0.2 milliarcseconds, aiding gravitational wave detection and reference frame linking.

Contribution

The paper presents the PINPT strategy, combining multi-frequency calibrations and core-shift corrections, to enhance VLBI pulsar position precision by a factor of five.

Findings

Achieved pulsar position uncertainty of 0.17 mas in RA and 0.32 mas in Dec.

Demonstrated the effectiveness of PINPT on PSR J2222-0137.

Projected fivefold improvement in VLBI astrometry precision.

Abstract

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 by VLBI and pulsar timing. The frame connection would provide an important pathway to examining the planetary ephemerides used in pulsar timing, and potentially enhancing the sensitivities of pulsar timing arrays used to detect stochastic gravitational-wave background at nano-Hz regime. We aim at significantly improving the VLBI-based MSP position from its current $\gtrsim1\,$mas precision level 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. We introduce a new differential astrometry strategy of using multiple calibrators observed at several widely separated frequencies, which we call PINPT (Phase-screen Interpolation plus frequeNcy-dePendent core shifT correction; read as "pinpoint") for brevity. 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 well constrained astrometrically with VLBI and pulsar timing. Using the PINPT strategy, we determined core shifts for 4 AGNs around PSR J2222-0137, and derived a VLBI-based pulsar position with uncertainty 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. The realization of the PINPT strategy promises a factor-of-5 positional precision enhancement (over conventional VLBI astrometry) for all kinds of compact radio sources observed at $\lesssim2$ GHz, including most fast radio bursts.