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As the most magnetized objects in the universe, magnetars may account for more than 12% of the neutron star population. Though their postulated link to fast radio bursts (FRBs) has been strengthened by the FRB-like bursts recently observed from a Galactic magnetar, it remains unclear whether all FRBs are originated from magnetars. On the other hand, the formation mechanism of magnetars is yet poorly understood. While almost all magnetars are identified with their soft gamma-ray and X-ray activities, a considerable fraction of them are also visible at optical/infrared and/or radio frequencies. This visibility allows precise astrometry for the magnetars. In this talk, I will explain how magnetar astrometry can refine our understanding of magnetar formation mechanism and test the link between FRBs and magnetars. Additionally, I will present recent progress of very long baseline astrometry of the fastest-spinning magnetar Swift J1818.0-1607.
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.