Neutron Star Kicks and Asymmetric Supernovae.pdf

a r X i v : a s t r o - p h / 0 0 1 2 0 4 9 v 1 3 D e c 2 0 0 0 Neutron Star Kicks and Asymmetric Supernovae Dong Lai Center for Radiophysics and Space Research, Department of Astronomy Cornell University, Ithaca, NY 14853, USA Email: dong@ Abstract. Observational advances over the last decade have left little doubt that neu- tron stars received a large kick velocity (of order a few hundred to a thousand kms?1) at birth. The physical origin of the kicks and the related supernova asymmetry is one of the central unsolved mysteries of supernova research. We review the physics of different kick mechanisms, including hydrodynamically driven, neutrino – magnetic field driven, and electromagnetically driven kicks. The viabilities of the different kick mechanisms are directly related to the other key parameters characterizing nascent neutron stars, such as the initial magnetic field and the initial spin. Recent observational constraints on kick mechanisms are also discussed. 1 Evidence for Neutron Star Kicks and Supernova Asymmetry It has long been recognized that neutron stars (NSs) have space velocities much greater (by about an order of magnitude) than their progenitors’. (e.g., Gunn Ostriker 1970). A natural explanation for such high velocities is that supernova explosions are asymmetric, and provide kicks to the nascent NSs. In the last few years, evidence for NS kicks and supernova asymmetry has become much stronger. The observational facts and considerations that support (or even re- quire) NS kicks fall into three categories: (1) Large NS Velocities (? the progenitors’ velocities ～ 30 km s?1): ? Recent studies of pulsar proper motion give 200?500 km s?1 as the mean 3D velocity of NSs at birth (e.g., Lyne and Lorimer 1994; Lorimer et al. 1997; Hansen Phinney 1997; Cordes Chernoff 1998), with possibly a significant population having velocities greater than 1000 km s?1. While velocity of ～ 100 kms?1 may in principle come from binary breakup in a supernova (with