Two Populations of X-ray Pulsars Produced by Two Types of Supernovae

TL;DR

This study identifies two distinct sub-populations of neutron-star-hosting X-ray pulsars within Be/X-ray binaries, likely originating from two different supernova types, distinguished by their spin and orbital characteristics.

Contribution

It reveals the existence of two neutron star populations linked to different supernova mechanisms, based on their spin and orbital properties, a novel classification in astrophysics.

Findings

Two sub-populations with different spin and orbital parameters

Electron-capture supernovae linked to short spin, short orbit, low eccentricity

Distribution of log spin period shows a clear split

Abstract

Two types of supernova are thought to produce the overwhelming majority of neutron stars in the Universe. The first type, iron-core collapse supernovae, occurs when a high-mass star develops a degenerate iron core that exceeds the Chandrasekhar limit. The second type, electron-capture supernovae, is associated with the collapse of a lower-mass oxygen-neon-magnesium core as it loses pressure support owing to the sudden capture of electrons by neon and/or magnesium nuclei. It has hitherto been impossible to identify the two distinct families of neutron stars produced in these formation channels. Here we report that a large, well-known class of neutron-star-hosting X-ray pulsars is actually composed of two distinct sub-populations with different characteristic spin periods, orbital periods and orbital eccentricities. This class, the Be/X-ray binaries, contains neutron stars that accrete material from a more massive companion star. The two sub-populations are most probably associated with the two distinct types of neutron-star-forming supernovae, with electron-capture supernovae preferentially producing system with short spin period, short orbital periods and low eccentricity. Intriguingly, the split between the two sub-populations is clearest in the distribution of the logarithm of spin period, a result that had not been predicted and which still remains to be explained.