Supernova Fallback as Origin of Neutron Star Spins and Spin-kick Alignment

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 possibly generic spin-kick alignment has been interpreted as indication that NS spins are either induced with the NS kicks or inherited from 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. In contrast, the NS spins are only determined minutes to hours later by angular momentum associated with 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 onesided 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 spinkick alignment. We also discuss implications for natal BH kicks and the possibility of tossing the BH's spin axis during its formation.

Origin of young accreting neutron stars in high-mass X-ray binaries in supernova remnants

“…In our toy model we do not consider disc formation from fallback matter. However, this phenomenon might be quite typical (Janka et al 2021). If a disc is formed, then it might be even easier to avoid the Ejector stage and make a soft transition to the wind accretion, may be even without an intermediate propeller phase.…”

“…(Chevalier 1989). However, after some time (even tens of seconds in some models, see Janka et al 2021), fallback accretion rate eventually follows a simple power law:…”

“…With such rapidly decaying rate of fallback accretion, most part of matter is accumulated within few minutes, see Janka et al (2021) for recent numerical calculations of fallback for < 10 5 s.…”

The spins of compact objects born from helium stars in binary systems

“…A major assumption of our predictions for NS rotation rates is that of AM conservation during the core-collapse explosion. Multi-dimensional simulations (Müller et al 2018;Chan et al 2020;Stockinger et al 2020;Janka et al 2021) instead show that that asymmetric explosions and fallback accretion can significantly alter the spin rate of the NS, spinning it up to spin periods as short as milliseconds. However, those simulations may also overestimate the spin-up via fallback because of numerical gridding effects or if fallback material is inefficiently accreted (Janka et al 2021).…”

“…Multi-dimensional simulations (Müller et al 2018;Chan et al 2020;Stockinger et al 2020;Janka et al 2021) instead show that that asymmetric explosions and fallback accretion can significantly alter the spin rate of the NS, spinning it up to spin periods as short as milliseconds. However, those simulations may also overestimate the spin-up via fallback because of numerical gridding effects or if fallback material is inefficiently accreted (Janka et al 2021). Given the long NS spin periods predicted for our low-mass progenitor models, it is likely that the true spin periods for those NSs are set by fallback and are shorter than our predictions.…”