The eclipsing millisecond pulsar PSR 1957 + 20

Information obtained over the past year on the eclipsing millisecond pulsar PSR 1957 + 20 and its orbiting companion is discussed. The pulsar is found to be similar in many ways to other millisecond pulsars: its spin parameters are extremely stable, its period derivative is very small, its profile has a strong interpulse, and its radio spectrum has a steep power-law index. The orbit is nearly circular, and the mass function implies a companion mass not much greater than 0.022 solar. Eclipses last for approximately 56 and 50 min at 318 and 430 MHz, respectively, corresponding to a nu exp - 0.41 + or - 0.09 dependence of eclipse duration on frequency. The available evidence points strongly toward a system in which the radiation from the pulsar heats the companion to the point of ablation, thereby driving a stellar wind that trails outward and behind the companion. 28 refs.