We study observational constraints on neutron star (NS) kicks for isolatedpulsars and for neutron stars in binary systems. We are particularly interestedin the evidence of kick-spin alignment/misalignment and its dependence on theneutron star initial spin period. For several young pulsars, X-ray observationsof compact nebulae showed that pulsar proper motion is aligned with the spindirection as defined by the symmetry axis of the nebula. We also criticallyexamine the measurements of the proper motion and the projected spin axis froma large sample of pulsars with well-calibrated polarization data. We find thatamong the two dozen pulsars for which reliable measurements are available,there is a significant correlation between the spin axis and the proper motion.For various NS binaries, including double NS systems, binaries with massivemain-sequence star companion and binaries with massive white-dwarf companion,we obtain constraints on the kick magnitudes and directions from the observedorbital characteristics of the system. We find that the kick velocity ismisaligned with the the NS spin axis in a number of systems, and the NS spinperiod (when available) in these systems is generally longer than severalhundreds milliseconds. These constraints, together with spin-kick alignmentobserved in many isolated pulsars, suggest that the kick timescale is hundredsof milliseconds to 1 s, so that spin-kick alignment or misalignment can beobtained depending on the initial spin period of the NS. We discuss theimplication of our result for various NS kick mechanisms.
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