Abstract Observations over the last decade have shown that neutron stars (NSs) receive a kick velocity (of order a hundred to a thousand km s−1) at birth. The physical origin of the kicks and the related supernova asymmetry is an unsolved problem. We study observational constraints on kicks for isolated pulsars and for neutron stars in binary systems. For several young pulsars, X-ray observations of compact nebulae showed that pulsar proper motion is aligned with the spin direction as defined by the symmetry axis of the nebula. Such alignment is also seen from a new analysis of radio polarization data for a large number of pulsars. On the other hand, for various NS binaries (including double NS systems, binaries with massive main-sequence star companion and binaries with massive white-dwarf companion), we find that the kick velocity is misaligned with the the NS spin axis in a number of systems, and the NS spin period (when available) in these systems is generally longer than several hundreds milliseconds. These observational constraints suggest that the kick timescale is hundreds of milliseconds to 1s, so that spin-kick alignment or misalignment can be obtained depending on the initial spin period of the NS. We discuss the implication of our result for various NS kick mechanisms.
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