Pulsar astrometry with VLBI and beyond

Stars with masses about fifteen times that of the Sun end their lives in core-collapse supernovae. The outcome of the gravitational collapse of the stellar core is a compact object known as a neutron star. Typically, these stars are observable as pulsars, highly magnetized rapidly rotating neutron stars. This thesis presents the results of two multi-epoch (global) very long baseline interferometry (VLBI) campaigns, conducted to measure the parallax and proper motion of several pulsars. In addition, it also demonstrates the implementation of, and presents the first results obtained with a new phased-array observing mode developed for global VLBI observations, aiming at performing standard pulsar timing with VLBI. The first global VLBI campaign discussed, deals with seven observations of the globular cluster M15, spread out over a timespan of two years. The sensitivity of the observations allows us to put very tight constraints on the upper mass limit of the intermediate mass black hole (IMBH) proposed to reside at the core of the cluster. Our 3-sigma upper mass limit of 500 M_sun is based on a non-detection of any significant radio emission. Thus, we rule out an IMBH to be the sole central object determining cluster dynamics. In the same observations we detect two of the eight pulsars and one of the two low-mass X-ray binaries known to be hosted by M15. Moreover, we observe two further hitherto unclassified sources within the field of view of the interferometric array. We perform a parallax and proper motion analysis for all detected sources, revealing that one of the unclassified sources is a background quasar while the other one is a compact foreground source. Our astrometry of the cluster members is consistent with earlier measurements obtained from pulsar timing and improves upon the accuracy of the earlier results. For the double neutron star system M15C our data indicate that due to geodetic precession the rotational axis of the observable pulsar shifted in space moving a different part of the emission cone than that observable previously into our line of sight. Aiming at confirming or rejecting the claim that the runaway star Zeta Oph and the pulsar B1929+10 once formed a binary system in Upper Scorpius, we observed the pulsar in another multi-epoch global VLBI campaign. In the same observations we also observed the pulsars B2020+28 and B2021+51 that were claimed to have once formed a binary system in the Cygnus Superbubble. Using our astrometric measurements we perform Monte Carlo simulations tracing the trajectory of each object back in time. The results of our simulations rule out a binary origin for both pairs B1929+10/ Zeta Oph and B2020+28/ B2021+51. Finally, in collaboration with the Joint Institute for VLBI in Europe (JIVE) we develop and test a new correlation mode for global VLBI observations. Instead of cross-correlating the data, it is summed coherently, thus mimicking a single radio antenna with the joint collecting area of all participating telescopes. Using one of the observations of the globular cluster M15, we test the algorithm on the known pulsars. We are able to detect the observed pulsars, although at a lower signal-to-noise ratio than expected