Anomalous hydrodynamics kicks neutron stars

TL;DR

This paper proposes that chiral anomaly-induced hydrodynamic effects, influenced by magnetic fields and vorticity, can explain the high velocities observed in newborn neutron stars through asymmetric neutrino emission.

Contribution

It introduces a systematic effective-field-theory framework linking microscopic anomalies to macroscopic neutron star kicks, highlighting the role of chiral transport effects.

Findings

Chiral transport effects can produce asymmetric neutrino emission.

Magnetic field-induced effects are sufficient to explain neutron star kicks.

Mixed gauge-gravitational anomalies have notable implications.

Abstract

Observations show that, at the beginning of their existence, neutron stars are accelerated briskly to velocities of up to $1000$ km/s. We discuss possible mechanisms contributing to these kicks in a systematic effective-field-theory framework. Anomalies of the underlying microscopic theory result in chiral transport terms in the hydrodynamic description, and we identify these as explanation for the drastic acceleration. In the presence of vorticity or a magnetic field, the chiral transport effects cause neutrino emission along the respective axes. In typical scenarios, the transport effect due to the magnetic field turns out to be strong enough to explain the kicks. Mixed gauge-gravitational anomalies enter in a distinct way, and we also discuss their implications.