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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 highmass. We took spectra of its strongly irradiated companion and found anobserved radial-velocity amplitude of K_obs=324+/-3 km/s. Correcting this forthe fact that, due to the irradiation, the center of light lies inward relativeto the center of mass, we infer a true radial-velocity amplitude of K_2=353+/-4km/s and a mass ratio q=M_PSR/M_2=69.2+/-0.8. Combined with the inclinationi=65+/-2 deg inferred from models of the lightcurve, our best-fit pulsar massis M_PSR=2.40+/-0.12 M_sun. We discuss possible systematic uncertainties, inparticular in the lightcurve modeling. Taking an upper limit of i<85 deg basedon the absence of radio eclipses at high frequency, combined with aconservative lower-limit to the motion of the center of mass, K_2>343 km/s(q>67.3), we infer a lower limit to the pulsar mass of M_PSR>1.66 M_sun.

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