Qinghuan Luo and Don Melrose have proposed a specific mechanism for the periodic eclipses observed from a specific class of pulsars orbiting about a companion star which is being evaporated by the pulsar itself.
Some pulsars are in binary systems, bound together with a companion star by their mutual gravitational attraction so that the pulsar and its companion continually orbit each other. In some binary pulsar systems the companion star has a mass only a few percent of that of the Sun. A small fraction of these systems shows two interesting phenomena: the wind from the pulsar appears to be destroying the companion star, and the radio pulses from the pulsar are periodically eclipsed - they disappear briefly once an orbit. What researchers think is happening is as follows. The wind from the pulsar, consisting of very energetic electrons and positrons, blows material off the companion producing a denser flow called the companion wind. Eventually this process will completely evaporate the companion, leaving a solitary millisecond pulsar - a rare beast. However, until the companion star evaporates its wind passes between the pulsar and the Earth once every orbit. If the wind is sufficiently dense the pulsar's radio emission is eclipsed each time this occurs, as illustrated in Figure 3.
This eclipse process provides observers with an opportunity to study the structures of both winds. The companion wind consists of a mixture of charged particles and magnetic field called a magnetized plasma. Radio waves travel marginally more slowly through a plasma than in vacuum: the denser the plasma, the slower the radio waves become - a tiny but measurable effect. Since pulsars are so regular the time of arrival of each pulse at the Earth can be predicted and any delay in its arrival can be measured. By measuring the increase in the delay time of the pulses during the approach to eclipse the density of the companion wind plasma can be deduced. The plasma also affects the two types of polarization of the radio waves, and this provides information on the magnetic field in the wind.
Figure: The line of sight from a pulsar in a
binary system varies during an orbit,
and can intersect a wind from the companion star
There are two examples of this type of eclipsing binary pulsar, PSR B1957+20 and PSR B1744-24A, which have been observed in detail. However, the actual eclipse mechanism is not well understood. One suggestion is that the increase in the plasma density along the line of sight from the pulsar as it moves behind the companion wind means that for a part of each orbit the frequency of the radio emission is below the plasma frequency. Emission below the plasma frequency cannot pass through a plasma and it is reflected, so this could explain the eclipses. However, observations of unpulsed radio emission from the eclipsing binary pulsars PSR B1957+20 and PSR B1744-24A imply that the plasma densities are many times too low to reflect the pulses, and so an alternative explanation for the eclipses is required.
Although the radio pulses are not being reflected by the wind of the companion star, it is possible that that they may be scattered as they pass through the wind. This scattering redirects radio waves initially traveling along our line of sight so that they do not reach the Earth, and hence we do not detect any radio emission. Qinghuan Luo and Don Melrose have proposed a model for these eclipsing binary pulsars in which the scattering occurs via nonlinear interactions between waves in a magnetized plasma. The beam of radio waves emitted by a pulsar is regarded as a photon beam which generates low-frequency waves in the plasma of the companion wind, referred to as plasma turbulence. The level of the plasma turbulence rises until the waves begin to scatter the photons in the beam so that they no longer propagate to the Earth but in random directions, leading to the eclipse of the pulsar. Qinghuan and Don have applied this model to both PSR B1957+20 and PSR B1744-24A, and the observed variation of the duration of the eclipse with radio frequency is consistent with that inferred from their model.