Neutron star mass estimates from gamma-ray eclipses in spider millisecond pulsar binaries

TL;DR

This study uses gamma-ray eclipses observed by Fermi LAT in spider millisecond pulsar binaries to directly constrain binary inclinations and neutron star masses, providing more robust measurements than optical methods.

Contribution

It introduces a novel gamma-ray eclipse detection method to obtain model-independent neutron star mass estimates in spider systems.

Findings

Detected gamma-ray eclipses in 7 systems including PSR B1957+20.

Provided a neutron star mass estimate of 1.81 ± 0.07 solar masses for PSR B1957+20.

Showed gamma-ray eclipses can tightly constrain binary inclinations and masses.

Abstract

Reliable neutron star mass measurements are key to determining the equation-of-state of cold nuclear matter, but these are rare. "Black Widows" and "Redbacks" are compact binaries consisting of millisecond pulsars and semi-degenerate companion stars. Spectroscopy of the optically bright companions can determine their radial velocities, providing inclination-dependent pulsar mass estimates. While inclinations can be inferred from subtle features in optical light curves, such estimates may be systematically biased due to incomplete heating models and poorly-understood variability. Using data from the Fermi Large Area Telescope, we have searched for gamma-ray eclipses from 49 spider systems, discovering significant eclipses in 7 systems, including the prototypical black widow PSR B1957$+$20. Gamma-ray eclipses require direct occultation of the pulsar by the companion, and so the detection, or significant exclusion, of a gamma-ray eclipse strictly limits the binary inclination angle, providing new robust, model-independent pulsar mass constraints. For PSR B1957$+$20, the eclipse implies a much lighter pulsar ($M_{\rm psr} = 1.81 \pm 0.07\,M_{\odot}$) than inferred from optical light curve modelling.