THE CONTINUUM ECLIPSE OF PSR 1957+20

We have observed the eclipsing millisecond pulsar PSR 1957 + 20 as a continuum source at wavelengths of 20 and 90 cm. The average 90 cm continuum flux density of the pulsar is found to drop dramatically during pulsed eclipse. In contrast, the neutron star remains visible as a 20 cm continuum source throughout most of the pulsed eclipse, although there is evidence for a brief loss of signal immediately preceeding conjunction. The 20 cm pulsed eclipse of PSR 1957 + 20 is therefore predominantly caused by the loss of coherent pulsations rather than the diminution of the average radio flux density. These observations indicate that the density of the ionized component of the eclipsing wind does not substantially exceed 10(9) cm-3, except perhaps in a limited region directly preceeding the companion. If mass loss is responsible for the system's orbital period derivative, then the ablated material must be largely neutral or flow out away from our line of sight. Radio, optical, and infrared observations are shown to suggest that the size of the companion to PSR 1957 + 20 may approach that of its Roche lobe. This could explain the apparent ease with which the pulsar ablates its companion as well as the low plasma density of the observed evaporate. We find no evidence of 20 cm radio emission from a nebula about the pulsar.