A pulsar's position, proper motion, and parallax are important parameters in timing equations. It is challenging to fit astrometric parameters accurately through pulsar timing, especially for pulsars that show irregular timing properties. With the fast development of related techniques, it is possible to measure astrometric parameters of more and more pulsars in a model-independent manner with very-long-baseline interferometry (VLBI). In this work we select 16 normal pulsars, whose parallax and proper motion have not been successfully fitted with timing observations or show obvious differences from the corresponding latest VLBI solutions, and do further studies on their timing properties. After updating the astrometric parameters in pulsar ephemerides with the latest VLBI measurements, we derive the latest rotation solutions of these pulsars with observation data in the S and C bands obtained from the Shanghai Tian Ma Radio Telescope. Compared with the spin frequency v inferred from previous rotation solutions, the newly fitted v show differences larger than 10(-9) Hz for most pulsars. The contribution of the Shklovsky effect to the period derivative P can be properly removed by taking advantage of the accurate proper motion and distance of target pulsars measured by VLBI astrometry. This further leads to a precise estimate of the intrinsic characteristic age tau(c). The differences between the newly measured tau(c) and corresponding previous results are as large as 2% for some pulsars. VLBI astrometric parameter solutions also lead to better measurements of timing irregularities. For PSR B0154+61, the glitch epoch (MJD 58279.5) measured with a previous ephemeris is about 13d later than the result (MJD 58266.4) obtained with VLBI astrometric parameter solutions.

pulsars: general; stars: neutron; techniques: interferometric

ShanghaiTech University; Shanghai Astronomical Observatory, Chinese Academy of Sciences; Guizhou normal university; Peking University