Lateral spreading effects on VLBI radio images of neutron star merger jets

ABSTRACT Very long baseline interferometry radio images recently proved to be essential in breaking the degeneracy ejecta model for neutron star merger GW170817. We discuss properties of synthetic jet afterglows by using semi-analytical models laterally spreading or non-spreading jets. The image centroid initially moves away from explosion point sky with apparent superluminal velocity. After reaching a maximum displacement, its motion is reversed. This behaviour line that found full hydrodynamic simulations. show evolution shift and size are significantly different when lateral considered. For Gaussian plausible parameters, morphology much closer circular. displacement occurrence time smaller/earlier factor few Our results indicate it crucial include effects analysing also obtain viewing angle ?obs provided ratio core ?c determined afterglow light curves. simple method based on point-source approximation provides reasonable angular estimates ($10{-}20{{\ \rm per\ cent}}$ errors at most). By taking sample structured jets, we ? 0.32 GW170817, consistent previous studies.