The Masses and Spins of Neutron Stars and Stellar-Mass Black Holes

TL;DR

This review summarizes current methods and findings on the masses and spins of neutron stars and stellar-mass black holes, highlighting recent advances and future prospects from electromagnetic and gravitational wave observations.

Contribution

It provides a comprehensive overview of measurement techniques, their reliability, and discusses future potential of gravitational wave data to enhance understanding of compact objects.

Findings

Mass measurements primarily from electromagnetic observations of binaries.

Black hole spin estimates have improved via iron line and continuum analysis.

Future gravitational wave detections will significantly advance mass and spin measurements.

Abstract

Stellar-mass black holes and neutron stars represent extremes in gravity, density, and magnetic fields. They therefore serve as key objects in the study of multiple frontiers of physics. In addition, their origin (mainly in core-collapse supernovae) and evolution (via accretion or, for neutron stars, magnetic spindown and reconfiguration) touch upon multiple open issues in astrophysics. In this review, we discuss current mass and spin measurements and their reliability for neutron stars and stellar-mass black holes, as well as the overall importance of spins and masses for compact object astrophysics. Current masses are obtained primarily through electromagnetic observations of binaries, although future microlensing observations promise to enhance our understanding substantially. The spins of neutron stars are straightforward to measure for pulsars, but the birth spins of neutron stars are more difficult to determine. In contrast, even the current spins of stellar-mass black holes are challenging to measure. As we discuss, major inroads have been made in black hole spin estimates via analysis of iron lines and continuum emission, with reasonable agreement when both types of estimate are possible for individual objects, and future X-ray polarization measurements may provide additional independent information. We conclude by exploring the exciting prospects for mass and spin measurements from future gravitational wave detections, which are expected to revolutionize our understanding of strong gravity and compact objects.