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 K obs = 324 $\pm$ 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 $\pm$ 4 km s-1 and a mass ratio q = M PSR/M 2 = 69.2 $\pm$ 0.8. Combined with the inclination i = 65$\,^{\circ}$ $\pm$ 2$\,^{\circ}$ inferred from models of the light curve, our best-fit pulsar mass is M PSR = 2.40 $\pm$ 0.12 M sun. We discuss possible systematic uncertainties, in particular, in the light curve modeling. Taking an upper limit of i <85$\,^{\circ}$ 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.