X-RAYS FROM THE ECLIPSING PULSAR 1957+20

Two examples of eclipsing pulsars, PSRs 1957 + 20 and 1744 - 25, have been discovered and studied1-7. In these systems, radiation from a pulsar drives a particle wind from a low-mass degenerate companion, creating an outflow which eclipses the radio pulsar once each orbital period. Ultimately the secondaries in both of these systems may be eroded away entirely, but the process by which the radiative wind ablates the smaller star remains controversial. Here we report the detection of soft X-rays, of approximately 1 keV energy, from PSR1957 + 20. This high-energy radiation, also observed by Kulkarni et al.8, should be a valuable diagnostic of the wind in this recycled pulsar system. Possible sources of the X-ray emission are the interstellar nebula driven by the pulsar wind, the interaction between the pulsar and its evaporating companion, and the pulsar itself. The small apparent size of the X-ray object argues against the first of these possibilities, and suggests that the X-rays are produced within the binary.