Abstract
Abstract Natal kicks and spins are characteristic properties of neutron stars (NSs) and black holes (BHs). Both offer valuable clues to dynamical processes during stellar core collapse and explosion. Moreover, they influence the evolution of stellar multiple systems and the gravitational-wave signals from their inspiral and merger. Observational evidence of a possibly generic spin-kick alignment has been interpreted as an indication that NS spins are either induced with the NS kicks or inherited from the progenitor rotation, which thus might play a dynamically important role during stellar collapse. Current three-dimensional supernova simulations suggest that NS kicks are transferred in the first seconds of the explosion, mainly by anisotropic mass ejection and, on a secondary level, anisotropic neutrino emission. By contrast, the NS spins are only determined minutes to hours later by the angular momentum associated with the fallback of matter that does not become gravitationally unbound in the supernova. Here, we propose a novel scenario to explain spin-kick alignment as a consequence of tangential vortex flows in the fallback matter that is accreted mostly from the direction of the NS’s motion. For this effect the initial NS kick is crucial, because it produces a growing offset of the NS away from the explosion center, thus promoting one-sided accretion. In this new scenario conclusions based on traditional concepts are reversed. For example, pre-kick NS spins are not required, and rapid progenitor core rotation can hamper spin-kick alignment. We also discuss implications for natal BH kicks and the possibility of tossing the BH’s spin axis during its formation.
Highlights
Neutron stars are born with estimated typical rotation periods between around 10 milliseconds and hundreds of milliseconds (Chevalier 2005; Popov & Turolla 2012; Igoshev & Popov 2013; Noutsos et al 2013)
The model set is mainly based on blue supergiant (BSG) single-star and binary-merger progenitors, which were investigated for phenomenological analysis in connection to Supernova 1987A (SN 1987A), but it includes two red supergiant (RSG) progenitors, all of them with pre-supernova masses between ∼15 M and ∼24 M
The deficiencies of current 3D supernova simulations discussed in Section 3, in particular their inability to explain the systematic trend of spin-kick alignment inferred for many young pulsars from observations, motivates us to introduce a refined and revised picture of fallback accretion in supernovae
Summary
Neutron stars are born with estimated typical rotation periods between around 10 milliseconds and hundreds of milliseconds (Chevalier 2005; Popov & Turolla 2012; Igoshev & Popov 2013; Noutsos et al 2013). The hypothesis of a mechanism that delivers kick and spin to the neutron star in combination, or the assumption that a prekick rotation of the neutron star plays a crucial role, was later adopted by others, for example by Lai et al (2001), in their discussions of theoretical neutron star kick and spinup scenarios Such a picture was further elaborated by Ng & Romani (2007) in their momentum-thrust scenario based on the neutrino-cooling emission of the proto-neutron star, and it was employed by Wang et al (2006) as a framework to interpret observational spin-kick alignment of isolated pulsars and misalignment of neutron stars in binaries, respectively.
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