black-widow pulsars

black-widow pulsars

A class of binary millisecond pulsars in which the radiation from the pulsar is destroying (‘evaporating’) the companion star. The first system, discovered 1988, was PSR 1957+20. The pulsar has a spin period of 0.0016 seconds and the orbital period is 9.2 hours. The mass of the companion has already been reduced to about 0.02 solar masses, and the system is likely to become a single millisecond pulsar in about 20–30 million years. The system shows long radio eclipses lasting for some 10% of the orbital period. This implies that the material that causes the eclipses is outside the Roche lobe of the companion (see equipotential surfaces) and therefore provides direct observational evidence for an ‘irradiation-driven’ evaporative wind. The pulsar PSR 1744–24A in the globular cluster Terzan 5 provides another example of this phenomenon.
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This was just the first of an exotic class of objects now known as black-widow pulsars (November issue, page 16), which eat away at their companion stars with their vicious winds.