2011ApJ...728...95V

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. 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 and a mass ratio q = M_PSR/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 M_PSR= 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 (q>67.3), we infer a lower limit to the pulsar mass of M_PSR>1.66 M_☉.