Neutron star kicks and implications for their rotation at birth

TL;DR

This paper explores how off-center natal kicks imparted during neutron star birth can induce rotation, analyzing their correlation with observed pulsar velocities and periods to understand neutron star birth dynamics.

Contribution

It introduces a model linking off-center natal kicks to neutron star rotation and constrains the kick location using pulsar data, highlighting challenges and potential for core-collapse simulations.

Findings

Estimated kick location at $2.03^{+3.74}_{-1.69}$ km with 90% confidence.

Model robust across pulsars with different periods and velocities.

Challenges in fitting data at low transverse velocities.

Abstract

Neutron stars are born out of core-collapse supernovae, and they are imparted natal kicks at birth as a consequence of asymmetric ejection of matter and possibly neutrinos. Unless the force resulting from the kicks is exerted exactly at their center, it will also cause the neutron star to rotate. In this paper, we discuss the possibility that neutron stars may receive off-center natal kicks at birth, which imprint a natal rotation. In this scenario, the observed pulsar spin and transverse velocity in the Galaxy are expected to correlate. We develop a model of the natal rotation imparted to neutron stars and constrain it by the observed population of pulsars in our Galaxy. When considering a single-kick position parameter, we find that the location of the off-center kick is $R_{\rm kick}=2.03^{+3.74}_{-1.69}$\,km at $90\%$ confidence, and is robust when considering pulsars with different observed periods, transverse velocities, and ages. Nonetheless, the model encounters challenges in effectively fitting the data, particularly at small transverse velocities, prompting the exploration of alternative models that include more complex physics. Our framework could be used as a guide for core-collapse simulations of massive stars.