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Having spin periods of only up to dozens of milliseconds, millisecond pulsars (MSPs) exhibit unparalleled spin stability, and hence serve as precise probes of abundant astrophysical effects. Previous studies show that the tangential space velocity distribution of MSPs is around 100 km/s, which suggests that only a small number of MSPs born in globular clusters (GCs) would remain in the GCs, given the typical GC escape velocity of only 50 km/s. However, this expectation is not met with the relatively large number of MSPs counted in GCs. This disagreement is known as the globular cluster retention problem. Additionally, undercounted MSPs are believed to a prime source of the observed gamma-ray excess at the Galactic centre, under the premise that a large fraction of Galactic centre MSPs have near-zero space velocities. This premise is again inconsistent with the historical space velocity distribution of MSPs. In this talk, I will present a new tangential space velocity distribution unveiled by 15 MSPs with precise proper motion and parallax-based distances. The astrometric results were obtained with the MSPSRPI project --- a VLBI astrometric program focusing on MSPs. Interestingly, our new tangential space velocity distribution is on the small side of the previous studies, and has a tentative multi-modal feature, where the low-space-velocity mode is consistent with zero. Such a tangential space velocity distribution of MSPs, if confirmed with a larger sample of MSPs precisely measured astrometrically, may potentially address the globular cluster retention problem and solve the mystery of Galactic centre gamma-ray excess.
Dr. Hao Ding has recently started an EACOA fellowship at Shanghai Astronomical Observatory. He completed his PhD at Swinburne University of Technology with the thesis entitled “Enhancing the use of Galactic neutron stars as physical laboratories with precise astrometry”. His research involves different aspects of high-precision astrometry (including Gaia astrometry), with special focus on the development and application of advanced phase referencing VLBI (very long baseline interferometry) techniques.