The masses of neutron stars

TL;DR

This paper reviews methods for measuring neutron star masses, analyzes the observed distribution revealing a complex, possibly bimodal pattern, and discusses implications for stellar evolution and the maximum mass limits of neutron stars.

Contribution

It provides a comprehensive overview of neutron star mass measurements, highlights the complex distribution, and challenges the previous single-value paradigm.

Findings

Neutron star masses show a bimodal or complex distribution.

Some neutron stars have masses around 2 solar masses.

The field is mature enough to determine maximum and minimum mass limits.

Abstract

We present in this article an overview of the problem of neutron star masses. After a brief appraisal of the methods employed to determine the masses of neutron stars in binary systems, the existing sample of measured masses is presented, with a highlight on some very well-determined cases. We discuss the analysis made to uncover the underlying distribution and a few robust results that stand out from them. The issues related to some particular groups of neutron stars originated from different channels of stellar evolution are shown. Our conclusions are that last century's paradigm that there a single, $1.4 M_{\odot}$ scale is too simple. A bimodal or even more complex distribution is actually present. It is confirmed that some neutron stars have masses of $\sim 2 M_{\odot}$, and, while there is still no firm conclusion on the maximum and minimum values produced in nature, the field has entered a mature stage in which all these and related questions can soon be given an answer.