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 +/- 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 degrees +/- 2 degrees inferred from models of the light curve, our best-fit pulsar mass is M-PSR = 2.40 +/- 0.12 M-circle dot. We discuss possible systematic uncertainties, in particular, in the light curve modeling. Taking an upper limit of i < 85 degrees 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 > 1.66M(circle dot).