Source velocities of fast radio bursts constrained by their dispersion measure variations

Growing evidence suggests that many fast radio bursts (FRBs) originate from extragalactic magnetars. Like all neutron stars, magnetars may be kicked at birth to large velocities, the values of which directly inform the physics governing stellar explosions and compact object formation. A subset of repeating FRBs show significant variability in their dispersion measures (i.e., electron column densities) over time, often exhibiting apparently secular trends. Here we show that all repeating FRBs with significant dispersion measure variations can be generically explained by motion of the source. We use existing measurements to constrain the minimum space velocities of the FRB sources, which are found to be 20-2800, km,s-1. We simultaneously constrain the plasma properties of the circumsource media, which are comparable to those of compact HII regions. For sources with compact radio nebulae, we directly infer the maximum source ages, and find them to be consistent with the ages of known Galactic magnetars. Our findings suggest that significant DM variability is a hallmark of young sources born with natal kicks through prompt formation channels. Applying our methods to a larger sample with continued monitoring of repeating FRB dispersion measures will reveal key physics of compact object formation.