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Hao Ding

Precise model-independent distances to Galactic neutron stars (NSs) are desirable in many ways. For instance, such distances to millisecond pulsars/double NSs are essential for testing theories of gravity. As another example, the model-independent distances to NS X-ray binaries showing photospheric radius expansion (PRE) bursts (a subset of type I X-ray bursts) can be used to probe theories of PRE bursts. Collectively, precise astrometry (measurement of parallax, proper motion and reference position) of millisecond pulsars can improve the sensitivity of the pulsar timing arrays dedicated to detecting gravitational-wave background at nano-Hz frequencies. Motivated by these scientific goals, my PhD research involves precise determination of distances, proper motions and reference positions for Galactic neutron stars, including millisecond pulsars, double neutron stars, magnetars and NS X-ray binaries. At radio frequencies, pulsar/magnetar astrometry is made with multi-epoch VLBI (very long baseline interferometry) observations spanning at least 2 years. To achieve optimal precision, different observing/data-reduction tactics are applied depending on the observing band and the calibrator environment (around the target source on the sky). At optical frequencies, Gaia data releases already provide (raw) parallax information for optically bright sources. However, due to imperfect observing setup, the Gaia parallaxes for objects at infinity (or zero-parallax point) are non-zero. Hence, as part of my PhD research, a novel method has been proposed to determine the so-called zero-parallax point.

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