Evidence for a Massive Neutron Star from a Radial-velocity Study of the Companion to the Black-widow Pulsar PSR B1957+20

Abstract

The most massive neutron stars constrain the behavior of ultra-dense matter, with larger masses possible only for increasingly stiff equations of state. Here, we present evidence that the black-widow pulsar, PSR B1957+20, has a high mass. We took spectra of its strongly irradiated companion and found an observed radial-velocity amplitude of K _obs = 324 ± 3 km s^-1. Correcting this for the fact that, due to the irradiation, the center of light lies inward relative to the center of mass, we infer a true radial-velocity amplitude of K ₂ = 353 ± 4 km s^-1 and a mass ratio q = M _PSR/M ₂ = 69.2 ± 0.8. Combined with the inclination i = 65° ± 2° inferred from models of the light curve, our best-fit pulsar mass is M _PSR = 2.40 ± 0.12 M _sun. We discuss possible systematic uncertainties, in particular, in the light curve modeling. Taking an upper limit of i < 85° based on the absence of radio eclipses at high frequency, combined with a conservative lower limit to the motion of the center of mass, K ₂>343 km s^-1 (q>67.3), we infer a lower limit to the pulsar mass of M _PSR>1.66 M _sun.