How rapidly do neutron stars spin at birth? Constraints from archival X-ray observations of extragalactic supernovae

TL;DR

This study uses X-ray observations of extragalactic supernovae to constrain the initial spin periods of neutron stars, providing an alternative to radio surveys and ruling out millisecond birth periods.

Contribution

It introduces a novel method based on X-ray luminosity distributions to estimate neutron star birth spins, extending beyond traditional radio survey approaches.

Findings

Millisecond birth periods are inconsistent with observed X-ray luminosities.

The method constrains initial pulsar rotational energy using supernova X-ray data.

Results support slower initial spins for neutron stars than previously hypothesized.

Abstract

Traditionally, studies aimed at inferring the distribution of birth periods of neutron stars are based on radio surveys. Here we propose an independent method to constrain the pulsar spin periods at birth based on their X-ray luminosities. In particular, the observed luminosity distribution of supernovae poses a constraint on the initial rotational energy of the embedded pulsars, via the L_X-dot{E}_{rot} correlation found for radio pulsars, and under the assumption that this relation continues to hold beyond the observed range. We have extracted X-ray luminosities (or limits) for a large sample of historical SNe observed with Chandra, XMM and Swift, that have been firmly classified as core-collapse supernovae. We have then compared these observational limits with the results of Monte Carlo simulations of the pulsar X-ray luminosity distribution, for a range of values of the birth parameters. We find that a pulsar population dominated by millisecond periods at birth is ruled out by the data.