  The birth properties of neutron stars yield important information on the
still debated physical processes that trigger the explosion and on intrinsic
neutron-star physics. These properties include the high space velocities of
young neutron stars with average values of several 100 km/s, whose underlying
"kick" mechanism is not finally clarified. There are two competing
possibilities that could accelerate NSs during their birth: anisotropic
ejection of either stellar debris or neutrinos. We here present new evidence
from X-ray measurements that chemical elements between silicon and calcium in
six young gaseous supernova remnants are preferentially expelled opposite to
the direction of neutron star motion. There is no correlation between the kick
velocities and magnetic field strengths of these neutron stars. Our results
support a hydrodynamic origin of neutron-star kicks connected to asymmetric
explosive mass ejection, and they conflict with neutron-star acceleration
scenarios that invoke anisotropic neutrino emission caused by particle and
nuclear physics in combination with very strong neutron-star magnetic fields.
