Evidence for a Massive Neutron Star From a Radial-Velocity Study of the Companion to the Black-Widow Pulsar PSR B1957+20

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 Kobs = 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 K₂ = 353 ± 4 km s⁻¹ and a mass ratio q = MPSR/M₂ = 69.2 ± 0.8. Combined with the inclination i = 65° ± 2° inferred from models of the light curve, our best-fit pulsar mass is MPSR = 2.40 ± 0.12 M☉. 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, K₂> 343 km s⁻¹ (q > 67.3), we infer a lower limit to the pulsar mass of MPSR> 1.66 M☉