An Optical Study of the Black Widow Population

TL;DR

This study analyzes optical data of Black Widow pulsar companions to determine neutron star masses, revealing that some neutron stars exceed 2 solar masses, challenging many equations of state.

Contribution

It provides a uniform analysis of optical observations of Black Widow systems, constraining neutron star masses and companion properties with sophisticated heating models.

Findings

Neutron star masses >2.19 M_sun at 1σ confidence

Companions are highly degenerate and inflated

Most extreme BWs have large accreted mass and low magnetic fields

Abstract

The optical study of the heated substellar companions of `Black Widow' (BW) millisecond pulsars (MSP) provides unique information on the MSP particle and radiation output and on the neutron star mass. Here we present analysis of optical photometry and spectroscopy of a set of relatively bright BWs, many newly discovered in association with Fermi $\gamma$-ray sources. Interpreting the optical data requires sophisticated models of the companion heating. We provide a uniform analysis, selecting the preferred heating model and reporting on the companion masses and radii, the pulsar heating power and neutron star mass. The substellar companions are substantially degenerate, with average densities $15-30\times$ Solar, but are inflated above their zero temperature radii. We find evidence that the most extreme recycled BW pulsars have both large $>0.8M_\odot$ accreted mass and low $<10^8$G magnetic fields. Examining a set of heavy BWs, we infer that neutron star masses larger than $2.19 M_\odot$ ($1\sigma$ confidence) or $2.08 M_\odot$ ($3\sigma$ confidence) are required; these bounds exclude all but the stiffest equations of state in standard tabulations.