ORBITAL PERIOD VARIABILITY IN THE ECLIPSING PULSAR BINARY PSR B1957- EVIDENCE FOR A TIDALLY POWERED STAR(20 )

Recent observations indicate that the eclipsing pulsar binary PSR B195 7+20 undergoes alternating epochs of orbital period increase and decre ase. We apply a model developed to explain orbital period changes of a lternating sign in other close binaries to the PSR B1957+20 system and find that it fits the pulsar observations well. The novel feature of the PSR B1957+20 system is that the energy flow in the companion neede d to power the orbital period change mechanism can be supplied by tida l dissipation, making the companion the first identified tidally power ed star. The flow of energy in the companion drives magnetic activity, which underlies the observed orbital period variations. The magnetic activity and the wind driven by the pulsar irradiation results in a to rque on the spin of the companion. This torque holds the companion out of synchronous rotation, causing tidal dissipation of energy. We prop ose that the progenitor had a similar or equal to 2 hr orbital period and a companion mass of 0.1-0.2 M., and the system is evolving to long er orbital periods by mass and angular momentum loss on a timescale of 10(8) yr.