Is PSR1957 + 20 eclipsed by a comet, magnetosphere or particulate cloud?

Abstract

The extraordinary eclipsing binary pulsar PSR1957 + 20 is in eclipse over eight per cent of its orbit¹. This is more than twice the extent of the companion's Roche lobe, the maximum stable synchronous configuration. A possible explanation² for the large eclipse is that it is caused by a plasma wind driven from the companion and forming a comet-like tail³. But an ionized cometary tail implies a refractive eclipse, which would produce large variations in pulse arrival times; this is inconsistent with the small time delays observed. Here we suggest two alternative explanations. One possibility is that the companion has a typical white-dwarf magnetic field (∼ 10⁵ gauss), and that the eclipse is then due to the plasma-filled magnetosphere which is confined and blown back sharply by the pulsar wind, with the time delays produced by a much less dense tail. In the other limit of negligible magnetization of the companion, an eclipse could be caused by a particulate cloud (comprising particles of size ∼ 10 cm) which should form about the companion. We assume that the companion is the remnant of an evolved companion star that is now being excited by the 622-Hz pulsar radiation. Such excitation could either supply plasma to the magnetosphere (for the magnetized case) or pull off neutral gas and deposit it in the vicinity to sustain a particulate cloud (for the non-magnetized case). Near-infrared radiation from this variable system might test the cloud hypothesis, whereas time delays in a magnetized tail suggest radio-frequency polarization tests for the magnetosphere model.